U.S. patent application number 10/406415 was filed with the patent office on 2004-10-07 for fast, accurate and low power supply voltage booster using a/d converter.
Invention is credited to Chen, Pau-Ling, Cleveland, Lee, Le, Binh Quang, Ly, Cathy Thuvan.
Application Number | 20040196093 10/406415 |
Document ID | / |
Family ID | 32990272 |
Filed Date | 2004-10-07 |
United States Patent
Application |
20040196093 |
Kind Code |
A1 |
Le, Binh Quang ; et
al. |
October 7, 2004 |
FAST, ACCURATE AND LOW POWER SUPPLY VOLTAGE BOOSTER USING A/D
CONVERTER
Abstract
Flash memory array systems and methods are disclosed for
producing a regulated boosted word line voltage for read
operations. The system comprises a multi-stage voltage boost
circuit operable to receive a supply voltage and one or more output
signals from a supply voltage detection circuit to generate the
boosted word line voltage having a value greater than the supply
voltage. The voltage boost circuit comprises a precharge circuit
and a plurality of boost cells connected to a common node of the
boosted word line, and a timing control circuit. The stages of the
plurality of boost cells are coupled in series for charge sharing
between the stages, and couple a predetermined number of boost
cells to the boosted word line common node to provide an
intermediate voltage to the boosted word line during the pre-boost
timing, thereby anticipating a final boosted word line voltage
provided during the boost timing. The voltage boost circuit is
operable to receive the one or more output signals from the supply
voltage detection circuit and alter a boost gain of the multi-stage
voltage boost circuit based on the one or more output signals,
thereby causing the boosted word line voltage to be substantially
independent of the supply voltage value.
Inventors: |
Le, Binh Quang; (San Jose,
CA) ; Ly, Cathy Thuvan; (Santa Clara, CA) ;
Cleveland, Lee; (Santa Clara, CA) ; Chen,
Pau-Ling; (Saratoga, CA) |
Correspondence
Address: |
ESCHWEILER & ASSOCIATES, LLC
NATIONAL CITY BANK BUILDING
629 EUCLID AVE., SUITE 1210
CLEVELAND
OH
44114
US
|
Family ID: |
32990272 |
Appl. No.: |
10/406415 |
Filed: |
April 3, 2003 |
Current U.S.
Class: |
327/536 |
Current CPC
Class: |
G11C 5/145 20130101;
G11C 8/08 20130101 |
Class at
Publication: |
327/536 |
International
Class: |
H03K 003/01 |
Claims
What is claimed is:
1. A system for generating a regulated boosted voltage, comprising:
a multi-stage voltage boost circuit operable to receive a supply
voltage and one or more output signals from a supply voltage
detection circuit to generate the boosted word line voltage having
a value greater than the supply voltage, the voltage boost circuit
comprising: a precharge circuit; a plurality of boost cells each
having a plurality of boost stages and a stage precharge switch,
the boost cells and the precharge circuit connected to a common
node; a timing control circuit operable to initiate and control the
precharge circuit and the stage precharge switch to initially
charge the stages of the plurality of boost cells to the supply
voltage during a precharge timing, to initiate and control voltage
boosting of the boost cells during a pre-boost timing and a boost
timing, wherein the stages of the plurality of boost cells are
coupled in series for charge sharing between the stages, and to
couple a predetermined number of boost cells to the common node to
provide an intermediate voltage to the common node during the
pre-boost timing, thereby anticipating a final boosted voltage at
the common node, which is provided during the boost timing; and
wherein the voltage boost circuit is operable to receive the one or
more output signals from the supply voltage detection circuit and
alter the boost gain of the multi-stage voltage boost circuit
during the boost timing based on the one or more output signals,
thereby causing the boosted voltage at the common node to be
substantially independent of the supply voltage value.
2. The system of claim 1, wherein the altering of the boost gain of
the voltage boost circuit alters an effective boost capacitance and
an effective load capacitance, thereby resulting in a boosted
voltage that is substantially independent of variations in the
supply voltage.
3. The system of claim 1, wherein the timing control circuit
comprises: a precharge gate booster operable to provide a boosted
gate signal to the precharge circuit and the stage precharge switch
to initiate charging the stages of the plurality of boost cells to
the supply voltage during a precharge timing; a boost timer
operable to initiate and control voltage boosting of the boost
cells during a pre-boost timing and a boost timing, wherein the
stages of the plurality of boost cells are coupled in series for
charge sharing between the stages; and wherein the timing control
circuit is further operable to couple a predetermined number of
boost cells to the boosted common node to provide an intermediate
voltage to the boosted common node during the pre-boost timing,
thereby anticipating the final boosted common node voltage value
during the boost timing.
4. The system of claim 1, wherein the precharge circuit comprises a
switch operable to couple the supply voltage to the plurality of
boost cells to initially charge the stages of the plurality of
boost cells to the supply voltage during a precharge timing as
directed by the precharge gate booster.
5. The system of claim 1, wherein one or more boost cells of the
plurality of boost cells comprises: a plurality of boost stages
having a stage capacitor, wherein each stage is operable to
contribute a voltage to the boosted word line output voltage by a
value associated with the applied supply voltage; a stage precharge
switch operable to couple the supply voltage to the plurality of
boost stages of the boost cell to initially charge the stages to
the supply voltage during a precharge timing as directed by the
precharge gate booster; and a coupling switch operable to series
couple the plurality of stages of the boost cell for charge sharing
between the stage capacitors, wherein the boost cell is operable to
receive the precharge from the precharge circuit at the common node
during the precharge timing.
6. The system of claim 5, wherein one or more boost cells of the
plurality of boost cells further comprises: a logic circuit
operable to determine whether the boost cell is configured to
provide an intermediate voltage to the boosted common node during
the pre-boost timing based on an input from the boost timer, and
operable to receive one of the output signals from the supply
voltage detection circuit to determine whether the boost cell has
been selected to contribute voltage boosting during the boost
timing, or whether the stage capacitor is to act as a load
capacitor, and further operable to provide an intermediate voltage
to the boosted common node during the pre-boost timing, thereby
anticipating a final boosted common node voltage which is provided
during the boost timing in response to the output signal from the
supply voltage detection circuit.
7. The system of claim 5, wherein the boost cells further comprise
a precharge circuit to initially charge the stages of the plurality
of boost cells to the supply voltage during a precharge timing as
directed by the precharge gate booster.
8. The system of claim 1, wherein each boost cell of the plurality
of boost cells comprises: a first stage capacitor having a first
terminal selectively coupled to the supply voltage through a first
stage precharge switch and a second terminal coupled to a boost
signal; a second stage capacitor having a first terminal coupled to
the boosted common node which forms an output of the voltage boost
cell, and a second terminal selectively coupled through a second
stage precharge switch to a circuit ground potential; a coupling
switch to provide series coupling between the first and second
stage capacitors, the coupling switch having a first terminal
coupled to the first terminal of the first stage capacitor, a
second terminal coupled to the second terminal of the second stage
capacitor, and a third terminal coupled to a boost timing input
terminal from the boost timer; the first stage precharge switch
having a first terminal coupled to the supply voltage, a second
terminal coupled to the first terminal of the first stage capacitor
and the first terminal of the coupling switch, and a third terminal
coupled to a first stage precharge timing input terminal from the
precharge gate booster; the second stage precharge switch having a
first terminal coupled to the second terminal of the second stage
capacitor and the second terminal of the coupling switch, a second
terminal coupled the circuit ground potential, and a third terminal
coupled to a second stage boost timing input terminal from the
boost timer; a logic circuit operable to determine whether the
boost cell was predetermined to provide an intermediate voltage to
the boosted common node during the pre-boost timing based on an
input signal from the boost timer, and operable to receive one of
the output signals from the supply voltage detection circuit to
determine whether the boost cell has been selected to contribute
voltage boosting during the boost timing, or whether the stage
capacitor is to act as a load capacitor in a nonselected boost
cell; and wherein when the input signal from the boost timer is low
and the boost capacitors of the boost cell and the load capacitors
of the nonselected boost cells charge to a voltage value which is
greater than the supply voltage value, and when the switch is open,
the boost signal is at a high level equal to about the supply
voltage, and the boost capacitors and the load capacitors
experience charge sharing, thereby causing the first terminals
thereof to increase to a boost voltage value greater than the
supply voltage value, wherein the boost voltage value is a function
of the capacitance of the boost capacitor and the load capacitor,
respectively.
9. The system of claim 8, wherein the voltage boost circuit
comprises: a plurality of boost cells each having a first terminal
coupled to the first terminals of the stage capacitors, and each
having a second terminal which is selectively coupleable to a
circuit ground potential or a voltage potential approximately equal
to the supply voltage based on the one or more output signals from
the supply voltage detection circuit, thereby causing one or more
of the plurality of stage capacitors to contribute three times the
supply voltage along with the default boost cell when the boost
cells are selected, or two times the supply voltage to the boosted
word line when the boost cells are not selected by the output
signals from the supply voltage detection circuit, thereby
adjusting the boost gain based on the supply voltage value.
10. The system of claim 1, wherein the one or more output signals
from the supply voltage detection circuit comprises: a plurality of
voltages associated with the supply voltage, and which collectively
form a digital word which reflects the supply voltage value.
11. The system of claim 1, wherein the bits of the digital word
have a binary weighting across a range of voltage detection, along
with a corresponding binary weighting of their respective boost
cell stage capacitors used for boost compensation.
12. A method of generating a boost voltage that is substantially
independent of variations in a supply voltage, comprising: varying
a boost gain of a voltage boost circuit used to generate the boost
voltage in response to a detected supply voltage value, wherein the
boost gain variation causes the boost voltage to be substantially
independent of variations in the supply voltage, wherein varying
the boost gain occurs in two phases, wherein in a pre-boost phase
the gain is varied a predetermined amount while the supply voltage
is being detected, and wherein in a boost phase that follows the
pre-boost phase the gain is varied a further amount based on the
detected supply voltage value.
13. The method of claim 12, wherein detecting the supply voltage
value comprises: inputting the supply voltage value into an
analog-to-digital converter; and generating a multi-bit digital
word which is related to the supply voltage value.
14. The method of claim 12, wherein detecting the supply voltage
value comprises: generating a plurality of voltage values
associated with the supply voltage value; comparing each of the
plurality of voltage values to a reference voltage value; and
generating a digital output value associated with each of the
comparisons, thereby generating a multi-bit digital word and
reflecting the supply voltage value.
15. The method of claim 12, wherein the varying of the boost gain
of the voltage boost circuit comprises altering an effective boost
capacitance and an effective load capacitance, thereby resulting in
a boosted word line voltage which is substantially independent of
variations in the supply voltage.
16. The method of claim 15, wherein the voltage boost circuit
comprises a plurality of boost cells, each cell having a plurality
of boost capacitors, the boost cells having a first terminal
coupled to an output node of the voltage boost circuit and a
precharge circuit, a second terminal coupled to a boost signal, and
a third terminal coupled to one or more output signals from the
detected supply voltage value, and wherein the voltage boost
circuit further comprises a load capacitor having a first terminal
coupled to a circuit ground potential and a second terminal coupled
to the output node, wherein varying the boost gain comprises: while
precharging the boost capacitors of the boost cells by coupling a
first terminal of a plurality of boost capacitors to the supply
voltage, and a second terminal of the plurality of boost capacitors
to a ground potential, enable the supply level detection to provide
the one or more output signals from the detected supply voltage
value to an input terminal of the voltage booster circuit; allowing
the output node of the voltage boost circuit to transition to a
level approximately equal to the supply voltage level; coupling the
plurality of boost capacitors in series within each boost cell;
coupling the second terminal of the one or more predetermined boost
cells and a fixed boost cell to the supply voltage level and
allowing the series capacitors to charge share to preboost the
output node of the voltage boost circuit; applying the supply
voltage level detection signals to the boost cells when the supply
voltage level detection signal data is valid; coupling the second
terminal of one or more selected boost cells of the plurality of
boost cells and the fixed boost cell to the supply voltage level;
and allowing the boost capacitors to charge share with the supply
voltage to boost the output node of the voltage boost circuit to a
final boost voltage based on the one or more output signals from
the detected supply voltage value, thereby altering an effective
boost capacitance and an effective load capacitance associated with
the boost capacitor and the load capacitor based on the detected
supply voltage value.
17. The method of claim 12, wherein one or more predetermined boost
cells are temporarily forced to the ON state during the pre-boost
timing interval to anticipate a final boost voltage.
18. The method of claim 12, wherein all the boost cells are
temporarily forced to the ON state during the pre-boost timing
interval to anticipate a final boost voltage.
19. A system for generating a regulated boost voltage, comprising:
a multi-stage voltage boost circuit operable to receive a supply
voltage and one or more output signals from a supply voltage
detection circuit and to generate the boosted voltage having a
value greater than the supply voltage; and a voltage boost
compensation circuit operably coupled to the supply voltage
detection circuit and the voltage boost circuit, and operable to
receive the one or more output signals from the supply voltage
detection circuit and alter a boost gain of the multi-stage voltage
boost circuit based on the one or more output signals, thereby
causing the boosted word line voltage to be substantially
independent of the supply voltage value, wherein the boost gain is
altered in two phases, wherein in a pre-boost phase the gain is
altered a predetermined amount while the supply voltage is being
detected, and in a boost phase that follows the pre-boost phase the
gain is altered a further amount based on the detected supply
voltage value.
Description
FIELD OF INVENTION
[0001] The present invention relates generally to memory systems
and in particular, to flash memory array systems and methods for
producing a voltage boost circuit, wherein a voltage detection
circuit may be used to measure the V.sub.CC applied to a voltage
boost circuit, along with boost compensation circuitry to regulate
the boost voltage output from V.sub.CC variations. The boost
voltage may be applied to a wordline for read mode operations of
memory cells.
BACKGROUND OF THE INVENTION
[0002] Flash and other types of electronic memory devices are
constructed of thousands or millions of memory cells, adapted to
individually store and provide access to data. A typical memory
cell stores a single binary piece of information referred to as a
bit, which has one of two possible states. The cells are commonly
organized into multiple cell units such as bytes which comprise
eight cells, and words which may include sixteen or more such
cells, usually configured in multiples of eight. Storage of data in
such memory device architectures is performed by writing to a
particular set of memory cells, sometimes referred to as
programming the cells. Retrieval of data from the cells is
accomplished in a read operation. In addition to programming and
read operations, groups of cells in a memory device may be erased,
wherein each cell in the group is programmed to a known state.
[0003] The individual cells are organized into individually
addressable units or groups such as bytes or words, which are
accessed for read, program, or erase operations through address
decoding circuitry, whereby such operations may be performed on the
cells within a specific byte or word. The individual memory cells
are typically comprised of a semiconductor structure adapted for
storing a bit of data. For instance, many conventional memory cells
include a metal oxide semiconductor (MOS) device, such as a
transistor in which a binary piece of information may be retained.
The memory device includes appropriate decoding and group selection
circuitry to address such bytes or words, as well as circuitry to
provide voltages to the cells being operated on in order to achieve
the desired operation.
[0004] The erase, program, and read operations are commonly
performed by application of appropriate voltages to certain
terminals of the cell MOS device. In an erase or program operation
the voltages are applied so as to cause a charge to be stored in
the memory cell. In a read operation, appropriate voltages are
applied so as to cause a current to flow in the cell, wherein the
amount of such current is indicative of the value of the data
stored in the cell. The memory device includes appropriate
circuitry to sense the resulting cell current in order to determine
the data stored therein, which is then provided to data bus
terminals of the device for access to other devices in a system in
which the memory device is employed.
[0005] Flash memory is a type of electronic memory media which can
be rewritten and hold its content without power. Flash memory
devices generally have life spans from 100 K to 1 MEG write cycles.
Unlike dynamic random access memory (DRAM) and static random access
memory (SRAM) memory chips, in which a single byte can be erased,
flash memory is typically erased in fixed multi-bit blocks or
sectors. Conventional flash memories are constructed in a cell
structure wherein a single bit of information is stored in each
flash memory cell. In such single bit memory architectures, each
cell typically includes a MOS transistor structure having a source,
a drain, and a channel in a substrate or P-well, as well as a
stacked gate structure overlying the channel. The stacked gate may
further include a thin gate dielectric layer (sometimes referred to
as a tunnel oxide) formed on the surface of the P-well. The stacked
gate also includes a polysilicon floating gate overlying the tunnel
oxide and an interpoly dielectric layer overlying the floating
gate. The interpoly dielectric layer is often a multilayer
insulator such as an oxide-nitride-oxide (ONO) layer having two
oxide layers sandwiching a nitride layer. Lastly, a polysilicon
control gate overlies the interpoly dielectric layer.
[0006] The control gate is connected to a word line associated with
a row of such cells to form sectors of such cells in a typical NOR
configuration. In addition, the drain regions of the cells are
connected together by a conductive bit line. The channel of the
cell conducts current between the source and the drain in
accordance with an electric field developed in the channel by the
stacked gate structure. In the NOR configuration, each drain
terminal of the transistors within a single column is connected to
the same bit line. In addition, each flash cell associated with a
given bit line has its stacked gate terminal coupled to a different
word line, while all the flash cells in the array have their source
terminals coupled to a common source terminal. In operation,
individual flash cells are addressed via the respective bit line
and word line using peripheral decoder and control circuitry for
programming (writing); reading or erasing functions.
[0007] Such a single bit stacked gate flash memory cell is
programmed by applying a relatively high voltage to the control
gate and connecting the source to ground and the drain to a
predetermined potential above the source. A resulting high electric
field across the tunnel oxide leads to a phenomena called
"Fowler-Nordheim" tunneling. During this process, electrons in the
core cell channel region tunnel through the gate oxide into the
floating gate and become trapped in the floating gate since the
floating gate is surrounded by the interpoly dielectric and the
tunnel oxide. As a result of the trapped electrons, the threshold
voltage of the cell increases. This change in the threshold voltage
(and thereby the channel conductance) of the cell created by the
trapped electrons is what causes the cell to be programmed.
[0008] In order to erase a typical single bit stacked gate flash
memory cell, a relatively high voltage is applied to the source,
and the control gate is held at a negative potential, while the
drain is allowed to float. Under these conditions, a strong
electric field is developed across the tunnel oxide between the
floating gate and the source. The electrons that are trapped in the
floating gate flow toward and cluster at the portion of the
floating gate overlying the source region and are extracted from
the floating gate and into the source region by way of
Fowler-Nordheim tunneling through the tunnel oxide. As the
electrons are removed from the floating gate, the cell is
erased.
[0009] For a read operation, a certain voltage bias is applied
across the drain to source of the cell transistor. The drain of the
cell is the bit line, which may be connected to the drains of other
cells in a byte or word group. The voltage at the drain in
conventional stacked gate memory cells is typically provided at
between 0.5 and 1.0 volts in a read operation. A voltage is then
applied to the gate (e.g., the word line) of the memory cell
transistor in order to cause a current to flow from the drain to
source. The read operation gate voltage is typically applied at a
level between a programmed threshold voltage (V.sub.T) and an
unprogrammed threshold voltage. The resulting current is measured,
by which a determination is made as to the data value stored in the
cell.
[0010] More recently, dual bit flash memory cells have been
introduced, which allow the storage of two bits of information in a
single memory cell. The bit line voltage required to read dual bit
memory cells is typically higher than that of single bit, stacked
gate architecture memory cells, due to the physical construction of
the dual bit cell. For example, some dual bit memory cell
architectures require between 1.5 and 2.0 volts to properly bias
the bit line or drain of such cells in a read operation. Because
the voltage applied to the bit line or drain of the memory cell is
derived from the memory device supply voltage (V.sub.CC), the
ability to provide the higher bit line voltage required to read the
newer dual bit memory cells may be impaired when the supply voltage
is at or near lower rated levels. In addition, low power
applications for memory devices, such as cellular telephones,
laptop computers, and the like, may further reduce the supply
voltage available.
[0011] In a prior art flash memory device, boosted voltage circuits
apply a boosted word line voltage for the read mode operations of
memory cells. V.sub.CC variations are typically reflected in the
output of the boost voltage circuit that is supplied to the word
line of the flash memory array, during a read operation. Such
variations in word line voltages from the boost circuit degrades
the ability in the read mode circuitry to discriminate accurately
whether or not a cell is programmed. In addition, as device
densities and memory speed requirements continue to increase, the
speed requirement of the voltage booster circuit may need to
increase to keep pace with the remainder of the memory circuit.
Further, as supply voltage levels decrease with the higher density
architectures, a single stage voltage booster circuit may be
inadequate to supply the required boost voltage. Accordingly, there
is a need for a means of compensation for the variations in the
V.sub.CC supply applied to a multi-stage boosted voltage circuit,
and for fast boost voltage regulation.
SUMMARY OF THE INVENTION
[0012] The following presents a simplified summary of the invention
in order to provide a basic understanding of some aspects of the
invention. This summary is not an extensive overview of the
invention. It is intended neither to identify key or critical
elements of the invention nor to delineate the scope of the
invention. Its sole purpose is to present some concepts of the
invention in a simplified form as a prelude to the more detailed
description that is presented later.
[0013] In the present invention of flash memory array systems and
methods for producing a multi-stage voltage boost circuit, one or
more output signals from a voltage detection circuit used to
measure the value of V.sub.CC is applied to a voltage boost circuit
that is used to generate a boosted word line voltage for the read
mode operations of memory cells. V.sub.CC variations are typically
reflected in the output of the boost voltage circuit that is
supplied to the word line of the flash memory array. By
compensating for the variations in the V.sub.CC supply applied to
the voltage boost circuit, the boost voltage is regulated, thereby
enabling a more consistent read voltage on the word line.
[0014] According to one aspect of the present invention, a voltage
value associated with the V.sub.CC supply voltage is ascertained,
for example, using an A/D converter. The determined voltage value
is then used to compensate or otherwise adjust a single or a
multi-stage voltage boost circuit. For example, a digital word
representing the V.sub.CC voltage value is used to vary effective
boost capacitance and load capacitance values within the voltage
boost circuit, thereby resulting in an output boost voltage that is
substantially independent of variations in V.sub.CC. Consequently,
the present invention provides a generally constant boost voltage,
for example, a boosted word line voltage, which facilitates an
accurate reading of flash memory cells despite fluctuations in the
V.sub.CC.
[0015] In another aspect of the present invention, the final
regulated output voltage of the voltage boost circuit (voltage
booster) is anticipated by selecting a set of predetermined boost
cells to provide a boosted voltage level during a pre-boost timing
preceding the boost timing. The pre-boost timing occurs before the
A/D converter has completed determining and latching an output of
the digital word representing the V.sub.CC voltage that is then
followed by the actual boost operation during the boost timing.
[0016] In still another aspect of the present invention, the
voltage booster circuit comprises a plurality of boost cells having
one or more boost stage capacitors in each boost cell, a precharge
circuit, and a timing control circuit comprising a precharge gate
booster and a boost timer.
[0017] To the accomplishment of the foregoing and related ends, the
invention comprises the features hereinafter fully described and
particularly pointed out in the claims. The following description
and the annexed drawings set forth in detail certain illustrative
embodiments of the invention. These embodiments are indicative,
however, of but a few of the various ways in which the principles
of the invention may be employed. Other objects, advantages and
novel features of the invention will become apparent from the
following detailed description of the invention when considered in
conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a plan view schematically illustrating an
exemplary layout of a memory device;
[0019] FIG. 2 is a schematic diagram illustrating an exemplary core
portion of a memory circuit;
[0020] FIG. 3 is a partial cross-sectional view of a conventional
stacked gate memory cell;
[0021] FIG. 4 is a distribution plot illustrating a programmed cell
threshold voltage distribution and an unprogrammed cell threshold
voltage distribution of a number of core cells of an exemplary
prior art flash memory array, and a typical read margin between the
distribution plots;
[0022] FIG. 5 is a simplified schematic illustration of an
exemplary prior art voltage booster circuit for reading a memory
cell;
[0023] FIG. 6 is a system level functional block diagram
illustrating an exemplary regulated voltage booster system in which
various aspects of the invention may be carried out;
[0024] FIG. 7A is a simplified schematic illustration of an
exemplary equivalent circuit of a single stage voltage booster
circuit similar to that of FIG. 6 in accordance with another aspect
of the invention;
[0025] FIG. 7B is a simplified schematic illustration of an
exemplary equivalent circuit of the single stage voltage booster
circuit of FIG. 7A in accordance with another aspect of the
invention;
[0026] FIG. 8 is a simplified schematic illustration of an
exemplary equivalent circuit of a two-stage voltage booster circuit
similar to that of FIG. 6 in accordance with another aspect of the
invention;
[0027] FIG. 9 is a system level functional block diagram
illustrating an exemplary multi-bit two stage regulated voltage
booster using a digital multi-bit input in which various aspects of
the invention may be carried out;
[0028] FIG. 10 is another system level functional block diagram
illustrating an exemplary four bit two stage regulated voltage
booster using a digital four bit input, including a precharge gate
booster and a boost timer, in which various aspects of the
invention may be carried out;
[0029] FIG. 11 is a simplified timing diagram illustrating
exemplary read mode timings and output of the voltage booster of
FIG. 10;
[0030] FIG. 12 is a schematic illustration of a two-stage voltage
boost cell for the least significant bits of an exemplary regulated
voltage booster using digital supply voltage compensation in
accordance with an aspect of the invention;
[0031] FIG. 13 is a schematic illustration of a two-stage voltage
boost cell for the most significant bits of an exemplary regulated
voltage booster using digital supply voltage compensation in
accordance with an aspect of the invention;
[0032] FIG. 14 is a schematic illustration of a two-stage default
voltage boost cell of an exemplary regulated voltage booster using
digital supply voltage compensation in accordance with an aspect of
the invention;
[0033] FIG. 15 is a schematic illustration of a precharge gate
booster of an exemplary regulated voltage booster in accordance
with an aspect of the invention;
[0034] FIG. 16 is a schematic illustration of a multi-stage voltage
boost cell of an exemplary regulated voltage booster using digital
supply voltage compensation in accordance with an aspect of the
invention; and
[0035] FIG. 17 is a flow diagram illustrating an exemplary method
for a regulated booster operation in association with an aspect of
the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The present invention will now be described with reference
to the drawings, wherein like reference numerals are used to refer
to like elements throughout. The present invention relates to a
flash memory array circuit for producing a boosted voltage which is
substantially independent of V.sub.CC fluctuations, and which may
be used as a boosted word line voltage for the read mode operations
of memory cells. The invention comprises a multi-stage voltage
boost circuit that provides a boosted voltage which is greater than
the supply voltage, and finds particular usefulness in low supply
voltage applications. The V.sub.CC power supply is applied to the
voltage boost circuit to supply power for the boost operation.
V.sub.CC variations that were conventionally reflected in the
output of the boost voltage circuit are identified and compensation
for such variations are generated to thereby generate word line
voltages during a read mode which are substantially independent of
variations in V.sub.CC.
[0037] According to an exemplary aspect of the present invention,
the system receives one or more output signals from a voltage
detection circuit (e.g., analog to digital converter, digital
thermometer) used to measure the V.sub.CC that is applied to a
voltage boost circuit. The voltage boost circuit comprises a
plurality of boost cells having one or more boost stages per boost
cell. The voltage boost circuit may, for example, be used to
generate a boosted word line voltage for the read mode operations
of memory cells. The detected V.sub.CC value is then employed in
the boost cells providing compensation to vary a manner in which
the boost circuit output voltage is generated. By compensating for
these variations in the V.sub.CC supply that is applied to the
voltage boost circuit, the boost voltage can be regulated, enabling
a more stable word line read voltage. This allows proper read
operations with respect to the memory cell of interest in the flash
memory, even where the supply voltage varies.
[0038] Another feature of the present invention relates to the
reduction or elimination of the slow response time typical of
voltage regulation circuits. Feedback, or other types of regulation
response delays are of major concern in memory devices where word
line rise times under about 20 ns are desired. The inventors of the
present invention have devised a method of compensation that has an
advantage of eliminating the cycle of: waiting for the regulation
circuit elements to respond to their own outputs, feeding these
outputs back to their input circuit elements, waiting for another
output, then attempting to correct for subsequent outputs and
inputs in an iterative fashion. Additionally, the present
invention, through the use of multiple stages of voltage boost,
permits the use of low supply voltages, for example, memory
operations at a supply voltage of about 1.6 v.
[0039] To minimize response delays in the exemplary compensation
circuit and methodology of the present invention, the voltage boost
circuit (voltage booster) anticipates the final regulated output
voltage of the voltage boost circuit. Predetermined most
significant bit (MSB) boost cells are forced to an "ON state" to
begin "pre-boosting" until the digital word from the A/D converter
becomes available during the actual boost timing. Thus, in this
method, an "intermediate voltage" which is roughly between (e.g.,
midway) the highest and the lowest boost voltage that could be
selected by the digital word is forced to the boost voltage output
until the final selection can be made by the A/D converter.
Subsequently or concurrently, the A/D converter determines the
digital word representing the V.sub.CC voltage and latches the word
to the A/D output connected to the boost circuit input. The digital
word and latch enable signal LATCH_EN at the boost circuit input
selects and enables the boost cells to effectively trim the boost
voltage up or down to the final boost voltage. Thus, substantially
less or virtually no feedback time is required in this method.
[0040] In accordance with another aspect of the present invention,
all the boost cells may be temporarily forced to the ON state
during the pre-boost timing if, for example, the word line has a
high load capacitance or must be assured of reaching a higher boost
voltage more quickly. As the load capacitance requires a finite
time to charge to the required voltage, forcing the boost cells
during a brief pre-boost interval further mitigates a charge
delay.
[0041] In contrast to a prior art voltage boost circuit, the
exemplary fast compensation methodology further minimizes response
delays and circuit complexity by eliminating the need of high
voltage output stage switches that may be used to isolate the boost
cells from one another. Instead, the outputs of all the boost cells
are tied directly together, thus the "OFF state" boost cells (via
selection of the A/D) along with the word line path, will act as a
load to the "ON state" boost cells.
[0042] When the V.sub.CC sample is taken or otherwise determined
and a certain number of A/D comparison outputs are turned ON, a
corresponding number of boost cells are added to the boost circuit
or to the load circuit in relation to the value of the V.sub.CC.
The amount of compensation provided to the boost circuit output
V.sub.BOOST is therefore regulated to the V.sub.CC in an iterative
fashion based on the number of voltage detection and compensation
elements desired. The resolution of the compensation desired may be
adjusted to fit the specific requirements of the boosted voltage
usage, for example, by increasing the A/D converter from an 8 bit
to a 16 bit A/D converter.
[0043] In another aspect of the invention, the bits of the digital
word, may also be weighted (e.g., evenly, binarly, exponentially),
or weighted in any other suitable fashion across the range of
voltage detection, along with a weighting of their respective boost
cell stage capacitors used for boost compensation as may be
desired.
[0044] Referring initially to prior art FIGS. 1 and 2,
semiconductor memory devices typically include multiple individual
components formed on or within a substrate. Such devices often
comprise a high density section and a low density section. For
example, as illustrated in prior art FIG. 1, a memory device such
as a flash memory 10 comprises one or more high density core
regions 12 and a low density peripheral portion 14 on a single
substrate 16. The high density core regions 12 typically include at
least one M.times.N array of individually addressable,
substantially identical memory cells and the low density peripheral
portion 14 typically includes input/output (I/O) circuitry and
circuitry for selectively addressing the individual cells (such as
decoders for connecting the source, gate and drain of selected
cells to predetermined voltages or impedances to enable designated
operations of the cell such as programming, reading or
erasing).
[0045] The memory cells within the core portion 12 are coupled
together in a circuit configuration, such as a NOR configuration
illustrated in FIG. 2. Each memory cell 20 in such an exemplary
configuration has a drain 22, wherein the drains of more than one
cell are connected to a common bit line, a source 24, and a stacked
gate 26. Each stacked gate 26 is coupled to a word line (WL.sub.0,
WL.sub.1, . . . , WL.sub.N) while each drain 22 is coupled to a bit
line (BL.sub.0, BL.sub.1, . . . , BL.sub.N). Lastly, each source 24
is coupled to a common source line CS. Using peripheral decoder and
control circuitry (not shown), each memory cell 20 may be addressed
for programming, or reading functions, in a fashion known in the
art.
[0046] FIG. 3 provides a cross-sectional illustration of a typical
memory cell 20 in the core region 12 of FIGS. 1 and 2. Such a
memory cell 20 typically includes the source 24, the drain 22 and a
channel 28 in a substrate 30; and the stacked gate structure 26
overlying the channel 28. The stacked gate 26 includes a thin gate
dielectric layer 32 (commonly referred to as the tunnel oxide)
formed on the surface of the substrate 30. The tunnel oxide layer
32 coats a portion of the top surface of the silicon substrate 30
and serves to support an array of different layers directly over
the channel 28. The stacked gate 26 includes a first film layer 38,
such as a doped polycrystalline silicon (polysilicon or poly I)
layer that serves as a floating gate 38 that overlies the tunnel
oxide 32. Note that the various portions of the transistor 20
highlighted above are not drawn to scale in FIG. 3, but rather are
illustrated as such for ease of illustration and to facilitate an
understanding of the device operation.
[0047] Above the poly I layer 38 is an interpoly dielectric layer
40. The interpoly dielectric layer 40 is often a multilayer
insulator such as an oxide-nitride-oxide (ONO) layer having two
oxide layers sandwiching a nitride layer, or in the an alternative
can be another dielectric layer such as tantalum pentoxide.
Finally, the stacked gate 26 includes an upper or second
polysilicon layer (poly II) 44 which serves as a polysilicon
control gate overlying the ONO layer 40. The control gates 44 of
the respective cells 20 that are formed in a given row share a
common word line (WL) associated with the row of cells (see, e.g.,
FIG. 2). In addition, as highlighted above, the drain regions 22 of
the respective cells in a vertical column are connected together by
a conductive bit line (BL). The channel 28 of the cell 20 conducts
current between the source 24 and the drain 22 in accordance with
an electric field developed in the channel 28 by the stacked gate
structure 26.
[0048] The memory cell 20 is programmed by applying a relatively
high gate voltage V.sub.G to the control gate 38 and a moderately
high drain voltage V.sub.D to the drain 22 in order to produce
"hot" (high energy) electrons in the channel 28 near the drain 22.
The hot electrons accelerate across the tunnel oxide 32 and into
the floating gate 34, which become trapped in the floating gate 38
because the floating gate 38 is surrounded by insulators (the
interpoly dielectric 40 and the tunnel oxide 32). As a result of
the trapped electrons, a threshold voltage (V.sub.T) of the memory
cell 20 increases. This change in the threshold voltage (and
thereby the channel conductance) of the memory cell 20 created by
the trapped electrons is what causes the memory cell 20 to be
programmed.
[0049] To read the memory cell 20, a predetermined gate voltage
greater than the threshold voltage of an unprogrammed memory cell,
but less than the threshold voltage of a programmed memory cell, is
applied to the control gate 44. If the memory cell 20 conducts
(e.g., a sensed current in the cell exceeds a minimum value), then
the memory cell 20 has not been programmed (the memory cell 20 is
therefore at a first logic state, e.g., a one "1"). Conversely, if
the memory cell 20 does not conduct (e.g., the current through the
cell does not exceed a threshold value), then the memory cell 20
has been programmed (the memory cell 20 is therefore at a second
logic state, e.g., a zero "0"). Thus, each memory cell 20 may be
read in order to determine whether it has been programmed (and
therefore identify the logic state of the data in the memory cell
20).
[0050] In order to erase the memory cell 20, a relatively high
source voltage V.sub.S is applied to the source 24 and the control
gate 44 is held at a negative potential (V.sub.G<0 volts), while
the drain 22 is allowed to float. Under these conditions, a strong
electric field is developed across the tunnel oxide 32 between the
floating gate 38 and the source region 24. The electrons that are
trapped in the floating gate 38 flow toward and cluster at the
portion of the floating gate 38 overlying the source region 24 and
are extracted from the floating gate 38 and into the source region
22 by way of tunneling through the tunnel oxide 32. Consequently,
as the electrons are removed from the floating gate 38, the memory
cell 20 is erased.
[0051] It is thus seen that appropriate voltages must be applied to
the various terminals (e.g., source, drain, and gate) of the cells
20 in the memory device 10 in order to perform various operations
(e.g., program, erase, read) associated with the device 10.
However, as stated above, the applied voltages have heretofore been
derived from the supply voltage to which the device 10 is
connected. Where such a supply voltage is not high enough to supply
the voltages required to perform such operations, however, the
device 10 may be rendered inoperative or inapplicable in certain
systems. This condition may result in low power applications of the
memory device 10, for instance, in portable device applications
wherein the supply voltage may be low. Alternatively, the memory
cells in a memory device may comprise dual bit architectures
requiring higher bit line voltages at the drain of the individual
cells in order to properly perform read operations. Thus, a voltage
boosting circuit is needed to boost the bit line voltage in
conditions where the supply voltage is insufficient to allow proper
read operations. Also, as the V.sub.CC supply voltage changes over
time, with temperature, or with the application of various loads,
the boost voltage will reflect the V.sub.CC changes. The present
invention overcomes or minimizes these problems by providing a
voltage boost, and compensation for the reflected V.sub.CC
variations in the voltage boost circuit, enabling a word line boost
voltage which is substantially independent of V.sub.CC variations,
thus providing more reliability in read operations.
[0052] FIG. 4 illustrates the need for a widely separated
distribution 200 of unprogrammed 250 and programmed 260 memory cell
threshold voltages. In the read mode operation, a read mode word
line voltage 230 is selected somewhere midway within the read
margin 240. This word line voltage 230 is then applied to the
specified word line to see whether or not the flash cell of
interest is conducting and thus provide a determination as to
whether the cell threshold is above the word line voltage and
therefore the cell is programmed, or below the word line voltage
and therefore the cell is unprogrammed.
[0053] If the boosted word line voltage that is applied to a cell
for this analysis is subject to variations with the V.sub.CC
supply, then the determination as to whether or not the cell is
programmed will also be subject to uncertainty, because the word
line voltage may exit the read margin 240 of FIG. 4. In a read mode
operation that uses a reference cell, although the absolute value
of the boost voltage can be outside the read margin window 240, the
variation of the wordline voltage can still cause a read margin
reduction due to gm-degradation of the core cells after cycling.
Adding additional uncertainty to the cell read mode determination,
the reference voltages applied to the boosted voltage boost circuit
voltage will also, as discussed above, reflect some function of the
V.sub.CC supply variations.
[0054] FIG. 5 illustrates a prior art voltage booster circuit 300
for feeding the word line in a memory cell read operation. During
an address transition pulse (ATD) time period (not shown), a BOOST
signal 312 goes low, and a BOOSTHV signal which is generated with a
high voltage inverter 327, goes high. A V.sub.BOOST potential 325
on the high voltage inverter 327 causes saturated conduction of an
n-mos transistor 330, for example, whereby the V.sub.CC conducts
substantially through transistor 330 to precharge the boost
capacitor C.sub.B at 320 and load capacitor C.sub.L at 335 to
V.sub.CC while the BOOST terminal 315 is held at ground. At the end
of the ATD time period, the BOOST signal 312 commands the
transistor 330 to turn-off by going high, and the BOOST terminal
315 is switched from ground to V.sub.CC. Therefore, the charge
voltage on the boost capacitor is now added to the V.sub.CC voltage
to force a charge sharing between C.sub.B and C.sub.L such that a
new voltage is generated at the V.sub.BOOST terminal 310 which is
greater than V.sub.CC but less than twice V.sub.CC. The actual
V.sub.BOOST terminal 310 voltage can be calculated as follows:
From: Q=CV
therefore; Q.sub.B=C.sub.BV.sub.CC and Q.sub.L=V.sub.CC
after V.sub.BOOST has settled, the total charge will be:
Q.sub.TOTAL(final)=Q.sub.TOTAL(initial)
Q.sub.TOTAL(final)=(V.sub.BOOST-V.sub.CC)C.sub.B+V.sub.BOOSTC.sub.L
therefore:
(V.sub.BOOST=V.sub.CC)C.sub.B+V.sub.BOOSTC.sub.L=(C.sub.B+C.sub.L)V.sub.CC
solving for V.sub.BOOST:
V.sub.BOOST=((2C.sub.B+C.sub.L)/(C.sub.B+C.sub.L)- )V.sub.CC
as a simple example, where C.sub.B=C.sub.LC, we have:
V.sub.BOOST=(3C/2C)V.sub.CC
V.sub.BOOST=(3/2)V.sub.CC
[0055] Intuitively, then, we prove that V.sub.BOOST would result in
a voltage mid-way between V.sub.CC and 2V.sub.CC for the prior art
voltage booster. Note, however, that V.sub.BOOST is a function of
V.sub.CC as well as the values of C.sub.B and C.sub.L. Therefore,
as V.sub.CC varies, the boost voltage output V.sub.BOOST will also
vary. As discussed above, such variations in V.sub.BOOST are
undesirable since they can lead to read errors.
[0056] In accordance with several aspects of the invention, FIG. 6
illustrates a system level functional block diagram of an exemplary
regulated voltage booster system 340, for example, for feeding the
word line in a memory cell read operation. An A/D converter 342
measures the supply voltage V.sub.DD 343, determines and outputs a
digital code comprising "n bits" which forms a digital word 345
associated with the level of the supply voltage V.sub.DD 343. The
digital word 345 is latched to the output of the A/D converter 342
with the arrival of an A/D latch signal AD_LT 346, which further
latches digital word 345 to the inputs of a voltage boost circuit
347 signaling that the A/D digital word 345 is available.
[0057] Voltage boost circuit 347 then provides a boosted voltage at
a V.sub.BOOST output terminal 348 that is compensated according to
the detected V.sub.DD 343 and the bits of the digital word 345.
Each bit of the digital word 345 may be used to control a separate
boost cell within the boost circuit 347 to provide compensation to
the boost circuit output V.sub.BOOST 348. Therefore, the output
V.sub.BOOST 348 is regulated to the V.sub.DD 343 in an iterative
fashion based on the number of voltage detection and compensation
elements (e.g., boost capacitors, boost cells, boost cell stages)
desired. The resolution of the compensation desired may be adjusted
to fit the specific requirements of the boosted voltage usage, for
example, by increasing from a 4 bit to an 8 bit A/D converter.
[0058] FIG. 7A illustrates a simplified schematic diagram of an
exemplary equivalent circuit for a single stage voltage boost
circuit 400 similar to that of the regulated voltage booster 340 of
FIG. 6, and functioning similar to that generally described in
association with FIG. 6. The voltage boost circuit 400 comprises a
precharge circuit, a plurality of boost cells, and a timing control
circuit (not shown). The V.sub.BOOST compensated output 402 is a
function of fixed boost circuit components comprised of a fixed
boost cell 405 having a fixed boost capacitor C.sub.B 406, and a
load capacitor C.sub.L 407 (e.g., representative of the word line
load capacitance), and is supplemented with a plurality of boost
cells 409 having boost capacitors C.sub.0 . . . n-1 410 which
provide voltage boost compensation. The plurality of boost cells
409 take their inputs from the AD0 thru ADn-1 digital word inputs
411 from, for example, the A/D converter 342 of FIG. 6.
Collectively, the plurality of boost cells 409 which have A/D bit
inputs from the digital word 411 may also be referred to as a
compensation circuit 412, since the boost cells 409, unlike the
fixed boost cell 405, are operable to provide compensation to the
voltage boost output 402 from the digital word (e.g., 345 of FIG.
6).
[0059] The plurality of boost cells 409 are individually selected
for boost compensation by a respective input of the AD0 thru ADn-1
digital word 411 from the stable latched A/D output. The
compensation circuit 412 is operable to switch the boost capacitors
C.sub.0 . . . n-1 410 between V.sub.DD and ground depending on the
detected supply level. During the ATD time period, B/P.sub.G
switches 415 and 416 close on the precharge (P.sub.G) side and open
on the boost (B) side. The V.sub.DD voltage precharges the load
capacitor C.sub.L 407 and the fixed boost capacitor C.sub.B 406,
along with the boost capacitors C.sub.0 . . . n-1 410, which are
also grounded by the B/P switches 415 and 416 in the P.sub.G
selection. Further, each fixed boost capacitor C.sub.B 406, or
boost capacitor C.sub.0 . . . n-1 410, has a pull-up precharge
switch P.sub.V (e.g., a transistor) coupleable to V.sub.DD, a
grounding precharge switch P.sub.G (e.g., a transistor) coupleable
to a ground potential, and a boost switch B coupleable to a boost
potential (e.g., V.sub.DD).
[0060] At the end of the ATD time period, a Pre-BOOST time period
begins wherein the B/P.sub.G switch 415 and a predetermined set of
the B/P.sub.G switches 416 open on the precharge P.sub.G side and
close on the boost B side. During this Pre-BOOST time period the
fixed boost capacitor C.sub.B 406 is switched back to V.sub.DD,
along with the predetermined set of the boost cells 410. The A/D
digital word data is not yet available during this time, but the
fixed boost capacitor C.sub.B 406, along with the predetermined set
of boost capacitors C.sub.0 . . . n-1 410, which are also now
switched to V.sub.CC, begin to boost the voltage to the V.sub.BOOST
compensated output 402. At this point, if these precharged
capacitors were not connected to the load capacitors, V.sub.BOOST
would be elevated to 2V.sub.DD, however, load capacitor C.sub.L
407, is still held at ground, and the nonselected
(nonpredetermined) boost capacitors of 410 are now switched to
ground. This forces all the precharge stored in C.sub.B, C.sub.L,
and the selected C.sub.0 . . . n-1 capacitors to charge share among
all the capacitors on the V.sub.BOOST output 402 bringing the
boosted voltage to an intermediate level between V.sub.DD and
2V.sub.DD.
[0061] When the A/D word data is subsequently made available,
during the BOOST time period, the selected bits of AD0 thru ADn-1
411 enable selected boost cells 409 with boost capacitors C.sub.0 .
. . n-1 410 to trim up or trim down the V.sub.BOOST output 402
bringing the boosted voltage to a final level between V.sub.DD and
2V.sub.DD corresponding to the V.sub.DD as measured by the A/D,
thereby resulting in a boosted word line voltage which is
substantially independent of variations in the supply voltage.
[0062] FIG. 7B is a schematic illustration of an equivalent circuit
of an exemplary voltage booster 430 in accordance with one aspect
of the invention, and as described in FIG. 7A for the single stage
voltage boost circuit 400. The equivalent circuit 430 demonstrates
the result of the final selection made by the A/D converter in the
voltage boost circuit 400 in accordance with an aspect of the
present invention. The voltage boost circuit 400 of FIG. 7A adjusts
or alters an effective boost capacitance C.sub.Beff and an
effective load capacitance C.sub.Leff, thereby resulting in a
boosted word line voltage V.sub.BOOST which is substantially
independent of variations in the supply voltage V.sub.DD. As this
altering provides an adjustment of the output voltage based on the
ratio established by selection between the effective boost
capacitance and the effective load capacitance, an effective
altering of a voltage "boost gain" is also established for the
voltage boost circuit.
[0063] C.sub.Beff is the effective total boost capacitance 435 as
seen by the boost circuit 430, comprising C.sub.B, plus all the A/D
selected capacitors C.sub.0+ . . . C.sub.n-2. C.sub.Leff is the
effective total load capacitance 440, comprising C.sub.L, plus all
the voltage detector nonselected capacitors C.sub.1+ . . .
C.sub.n-1 as seen by the boost circuit 430 and impressed on the
V.sub.BOOST 442 boosted voltage output line. Therefore the
effective boost capacitance C.sub.Beff and effective load
capacitance C.sub.Leff is a function of V.sub.DD. Note that FIG. 7B
illustrates a set of arbitrary examples for C.sub.Beff and
C.sub.Leff.
[0064] Therefore, the effective V.sub.BOOST terminal voltage 442 of
FIG. 7B for one arbitrary example of the present invention
becomes:
From:
V.sub.BOOST=((2C.sub.B+C.sub.L)/(C.sub.B+C.sub.L))V.sub.DD
We have:
V.sub.BOOST=((2C.sub.Beff+C.sub.Leff)/(C.sub.Beff+C.sub.Leff))V.s-
ub.DD
where: C.sub.Beff=C.sub.B+C.sub.0+ . . . C.sub.n-2 (of selected
boost capacitors)
and where: C.sub.Leff=C.sub.L+C.sub.1+ . . . C.sub.n-1 (of
nonselected boost capacitors)
[0065] It should be noted, that the total number of capacitors used
in this exemplary method remains constant.
[0066] FIG. 8 illustrates a simplified schematic diagram of an
exemplary equivalent circuit for a two stage voltage boost circuit
450 similar to that of the regulated voltage booster 340 of FIG. 6
in which various aspects of the invention may be carried out. Boost
circuit 450 also functions similar to that generally described in
association with FIG. 7A, except that the two stage booster permits
boosting to a maximum final level of 3V.sub.DD rather than the
final level of between V.sub.DD and 2V.sub.DD as described for FIG.
7A. Voltage boost circuit 450 comprises a precharge circuit, a
plurality of, for example, two stage boost cells and a timing
control circuit (not shown). The V.sub.BOOST compensated output 452
is again, a function of fixed boost circuit components comprised of
a fixed two stage boost cell 455, and the load capacitor C.sub.L
407, and is supplemented with a plurality of boost cells 459 to
provide double stage voltage boost compensation. The fixed two
stage boost cell 455 comprises fixed boost capacitors C.sub.BA 456a
and C.sub.BB 456b, while the plurality of boost cells 459 comprise
boost capacitors C.sub.0A . . . (n-1)A 460a and C.sub.0B . . .
(n-1)B 460b which are series coupled by coupling switches or
transistors (DB) 468 to provide double stage voltage boost
compensation. The plurality of boost cells 459 take their inputs
from the AD0 thru ADn-1 digital word inputs 411 from, for example,
the A/D converter 342 of FIG. 6. Collectively, the plurality of
boost cells 459 which have A/D bit inputs from the digital word 411
may also be referred to as a boost compensation circuit 462, since
the boost cells 459, unlike the fixed boost cell 455, are operable
to provide compensation to the voltage boost output 452 from the
digital word (e.g., 345 of FIG. 6).
[0067] Again, the plurality of boost cells 459 are individually
selected for boost compensation by a respective input of the AD0
thru ADn-1 digital word 411, from the stable latched A/D output.
The boost compensation circuit 462 is operable to switch the boost
capacitors C.sub.0A . . . (n-1)A 460a between V.sub.DD and ground
depending on the detected supply level, while C.sub.0B . . . (n-1)B
460b are switched between ground and the output voltage that is
generated by C.sub.0A . . . (n-1)A 460a. During the ATD time
period, B/P.sub.G switches 465 and 466 close on the precharge
P.sub.G side and open on the boost B side. The V.sub.DD voltage
precharges the load capacitor C.sub.L 407 and the fixed boost cell
455, along with the boost capacitors C.sub.0A . . . (n-1)A 460a and
C.sub.0B . . . (n-1)B 460b, which are also grounded by the
B/P.sub.G switches 465 and 466 in the P.sub.G selection. Further,
each fixed boost cell 455, or boost cell 459, has a pull-up
precharge switch P.sub.V (e.g., a transistor) coupleable to
V.sub.DD, a grounding precharge switch P.sub.G (e.g., a transistor)
coupleable to a ground potential, and a boost switch B coupleable
to a boost potential (e.g., V.sub.DD).
[0068] At the end of the ATD time period, a Pre-BOOST time period
begins wherein the B/P.sub.G switch 465 and a predetermined set of
the B/P.sub.G switches 466 open on the precharge P.sub.G side and
close on the boost B side. During this Pre-BOOST time period the
fixed boost capacitance C.sub.B 406 is switched back to V.sub.DD,
along with the predetermined set of the boost cells 410. The A/D
digital word data is not yet available during this time, but the
fixed boost capacitors C.sub.BA 456a and C.sub.BB 456b, along with
the predetermined set of boost capacitors C.sub.0A . . . (n-1)A
460a and C.sub.0B . . . (n-1)B 460b are series coupled by coupling
transistors DB 468, while the C.sub.0A . . . (n-1)A 460a set of
boost capacitors are switched to V.sub.CC, to begin the double
stage voltage boost compensation to the V.sub.BOOST output 452. At
this point, if these precharged capacitors were not connected to
the load capacitors, V.sub.BOOST would be elevated to a maximum of
3V.sub.DD (e.g., V.sub.DD+V.sub.C0A . . . (n-1)A+V.sub.C0B . . .
(n-1)B), however, load capacitor C.sub.L 407, is still held at
ground, and the nonselected (nonpredetermined) boost capacitors of
C.sub.0A . . . (n-1)A 460a and C.sub.0B . . . (n-1)B 460b are now
switched to ground. This forces all the precharge stored in
C.sub.BA and C.sub.BB, C.sub.L, and the selected C.sub.0A . . .
(n-1)A and C.sub.0B . . . (n-1)B capacitors to charge share among
all the capacitors on the V.sub.BOOST output 452 bringing the
boosted voltage to an intermediate level between V.sub.DD and
3V.sub.DD.
[0069] When the A/D word data is subsequently made available,
during the BOOST time period, the selected bits of AD0 thru ADn-1
411 enable selected boost cells 459 to trim up or trim down the
V.sub.BOOST output 452 bringing the boosted voltage to the final
level corresponding to the V.sub.DD as measured by the A/D, thereby
resulting in a double stage boosted word line voltage which is
substantially independent of variations in the supply voltage.
Thus, for the same reasons that the single stage boost circuit 400
of FIG. 7A has the equivalent circuit 430 of FIG. 7B, the double
stage boost circuit 450 of FIG. 8 similarly corresponds to the
equivalent circuit 430 of FIG. 7B, wherein adjusting or altering an
effective boost capacitance C.sub.Beff and an effective load
capacitance C.sub.Leff, results in a boosted word line voltage
V.sub.BOOST which is substantially independent of variations in the
supply voltage V.sub.DD. Therefore, the ratio between the effective
boost capacitance and the effective load capacitance may be altered
corresponding to the V.sub.DD to further alter a voltage "boost
gain" of the voltage boost circuit 450.
[0070] FIGS. 9 and 10 illustrate a system level functional block
diagram of an exemplary multi-bit two stage regulated voltage
booster 500, and an exemplary four bit two stage regulated voltage
booster 600, respectively, in which various aspects of the
invention may be carried out. Both voltage boost circuits 500 and
600 function similar to that generally described in association
with FIG. 8, wherein using, for example, a two stage booster
permits boosting to a maximum final level of 3V.sub.DD, wherein the
ratio of the effective boost capacitance to the effective load
capacitance may be altered corresponding to a measured V.sub.DD
value. Although a two stage voltage boost circuit is illustrated,
it is also appreciated that a multi-stage and a multi-bit regulated
booster is anticipated in the scope of the present invention.
[0071] In FIG. 9, voltage boost circuit 500 comprises a timing
control circuit 510, a plurality of, for example, two stage boost
cells (e.g., cell(0)-cell(n-1) 514 and a fixed two stage boost cell
515), a precharge circuit 522, and inputs AD0 thru ADn-1 525 from,
for example, a digital word 345. The circuit 500 is operable to
generate a V.sub.BOOST compensated output 532 corresponding to a
measurement of the V.sub.DD value. The V.sub.BOOST compensated
output 532 is again, a function of fixed boost circuit components
comprised of the fixed two stage boost cell 515, and the load
capacitor C.sub.L (not shown) as reflected by the word line
capacitance at V.sub.BOOST. V.sub.BOOST compensated output 532 is
supplemented with a plurality of double stage boost cells 514 to
provide double stage voltage boost compensation. Each of the two
stage boost cells 514 and 515 comprise two boost capacitors which
are series coupled by a coupling transistor to provide double stage
voltage boost compensation to a common node at the V.sub.BOOST
compensated output 532. The plurality of boost cells 514 take their
inputs from the AD0 thru ADn-1 digital word inputs 525 from, for
example, the A/D converter 342 of FIG. 6. The inputs 525 of the
digital word are recognized as presently available for use by the
voltage booster circuit 500 by the presence of an A/D latch signal
AD_LT input signal 526. Collectively, the plurality of boost cells
514 which have A/D bit inputs 525 from, for example, the digital
word 345 may also be referred to as a boost compensation circuit
540 (or, e.g., the selectable boost cells), since the boost cells
514, unlike the fixed boost cell 515, are operable to provide
compensation to the voltage boost output 532 from the digital word
(e.g., 345 of FIG. 6).
[0072] The timing control circuit 510 of the voltage boost circuit
500 FIG. 9, controls the precharge and boost timings previously
described in a general context. The timing control 510 comprises a
precharge gate booster 550 for providing a boosted gate signal
BOOST_H1 555 for a precharge switch within the compensation circuit
540 boost cells 514 and within the fixed boost cell 515, and a
boosted gate signal BOOST_H2 556 of the precharge circuit 522.
Initialized by a BOOST signal 560, the timing control 510 further
comprises a boost timer 562 for enabling three BOOST(B/D/DB) timing
signals 565 to control a PREBOOST timing and a BOOST timing in
boost cells 514 and 515, and two BOOST(B1/D) timing signals 567
used to synchronize the precharge gate booster 550. The PREBOOST
timing of the timing control circuit 510 provides the control for
initializing the predetermined set of boost cells to begin boosting
before the digital inputs 525 are acknowledged as available by the
AD_LT input signal 526 providing anticipation of the final
V.sub.BOOST compensated output 532. For example, one or more of the
cells(n-1:0) may be predetermined by internal logic or other
external logic circuits to provide the anticipatory preboost as
needed before the arrival of the AD_LT input signal 526 which
denotes the beginning of the BOOST timing.
[0073] As discussed, FIG. 10 illustrates an exemplary four bit two
stage regulated voltage booster 600 using a corresponding four bit
digital input, in one aspect of the present invention. As voltage
booster 600 is one exemplary implementation of voltage booster 500
of FIG. 9, booster 600 may not be described again in full detail
for the sake of brevity. The voltage boost circuit 600 of FIG. 10
also comprises a timing control circuit 610, a plurality of, for
example, two stage boost cells (e.g., (cell(0)-cell(3) 614 and
cell(4) 615), a precharge circuit 622 (e.g., a switch transistor),
and inputs AD0 thru ADn-1 625 from, for example, a digital word 345
are operable to direct a V.sub.BOOST compensated output 632
corresponding to a measurement of the V.sub.DD value. The
V.sub.BOOST compensated output 632 is again, a function of fixed
boost circuit components comprised of a fixed two stage boost cell
615, and the load capacitor C.sub.L (not shown) as reflected by the
word line capacitance at V.sub.BOOST. V.sub.BOOST compensated
output 632 is supplemented with a plurality of double stage boost
cells 614 to provide double stage voltage boost compensation.
[0074] The inputs 625 of the digital word are made available for
use by the voltage booster circuit 600 with the presence of an A/D
latch AD_LT input signal 626. Collectively, the plurality of boost
cells 614 having A/D bit inputs 625 provide a boost compensation
circuit 640, and provide compensation to the voltage boost output
632.
[0075] The timing control circuit 610 of FIG. 10, controls the
precharge and boost timings and comprises a precharge gate booster
650 for providing a boosted gate signal BOOST_H1 to a precharge
transistor in the compensation circuit 640 boost cells 614 and to
the fixed boost cell 615, and a boosted gate signal BOOST_H2 to the
precharge circuit transistor 622. Initialized by a BOOST signal
660, the timing control 610 further comprises a boost timer 662 for
enabling a BOOSTB, a BOOSTD, and a BOOSTDB timing signal to control
a PREBOOST timing and a BOOST timing in boost cells 614 and 615,
and a BOOSTB1, and a BOOSTD timing signals 667 used to synchronize
the precharge gate booster 650.
[0076] The PREBOOST timing of the timing control circuit 610
provides, according to one aspect of the present invention, control
for initializing a predetermined set of boost cells to begin
boosting before the digital inputs 625 are acknowledged as
available by the AD_LT input signal 626 providing anticipation of
the final V.sub.BOOST compensated output 632. For example, boost
cell3 and cell2 (e.g., cells(3:2)) may be predetermined by logic
circuits internal to the boost cell to provide the anticipatory
preboost as needed before the arrival of the AD_LT input signal 626
which denotes the beginning of the BOOST timing, as will be shown
in greater detail in connection with FIG. 11.
[0077] FIG. 11 demonstrates an exemplary timing diagram 700 for the
read mode timings and output of the exemplary two stage voltage
booster circuits of FIGS. 9 and 10. Portions of the timing diagram
of FIG. 11 will be used to describe the operation of FIGS. 8 and 9,
and other portions of the timing diagram of FIG. 11 will be used as
a reference to explain the operations of an exemplary system of
FIGS. 10 and following, according to the invention.
[0078] The timing of the voltage booster of FIGS. 9 and 10 is
relative to that of the read access timing 705 that is about 50 ns
as depicted in timing diagram 700 of FIG. 11. For a new access, an
address transition pulse (ATD) 710 is generated when a new address
715 is applied at time t.sub.p (720) of FIG. 11. The address
transition pulse ATD 710, goes high for about 10 ns during which
time all boosting capacitors (e.g., C.sub.0-(n-1)A, and
C.sub.0-(n-1)B of FIG. 8) of the boost cells (e.g., 455 and 459 of
FIG. 8, or cell0-cell3 (614) and cell4 (615) of FIG. 10) precharge.
The ATD 710 time interval is further used to decode the new address
715 at time t.sub.p (720) to a common node of the word line (e.g.,
632 of FIG. 10) which also precharges the word line load
capacitance C.sub.L (e.g., 407 of FIG. 8) to V.sub.DD bringing the
final V.sub.BOOST output 632 from about 0 volts to about V.sub.DD,
shown along the V.sub.BOOST charge curve segment 730 of FIG. 11.
Also during the ATD interval 710, the A/D converter block (e.g.,
342 of FIG. 6) and the fast reference circuit (not shown) are
enabled from the rising edge of the ATD pulse to measure the
V.sub.DD. A/D latch signal AD_LT (735) is also high during the
entire A/D converter measurement.
[0079] At time t.sub.0 (740), ATD goes low again to end the boost
capacitor precharge, while the BOOST signal (e.g., 560 of FIG. 10)
goes high initiating a preboost period during the initial portion
of the BOOST interval 745. During the preboost period at time to
(740) a few predetermined boost cells (e.g., boost cells(3:2) of
FIG. 10), which have logic circuits wired to make this
predetermination, begin voltage boosting the word line. In the case
of the two stage boost cells, this means that both of the boost
capacitors (e.g., C.sub.(n-1)A 460a and C.sub.(n-1)B 460b of FIG.
8) are coupled in series, while the noncoupled terminal of
C.sub.(n-1)A (460a) is switched to V.sub.DD, forcing boost
capacitor C.sub.(n-1)B (460b) to share its charge plus the V.sub.DD
supply voltage with load capacitor C.sub.L (407) so that the
C.sub.(n-1)A, C.sub.(n-1)B, and C.sub.L charge share to a value
between V.sub.DD and 3V.sub.DD, as shown along the V.sub.BOOST
charge curve segment 747 between to (740) and t.sub.2 (750). Thus a
predetermined set of boost cells begin boosting before the digital
inputs 625 are made available with the AD_LT input signal 735 to
anticipate the final V.sub.BOOST compensated output (e.g., 632 of
FIG. 10).
[0080] Since speed is a high priority during the read operations,
the inventors have also taken advantage of the PREBOOST portion of
the BOOST signal timing interval 745 of the present invention, to
detect the V.sub.DD using the A/D converter, so that time is not
wasted separately measuring the V.sub.DD and charging the
compensation capacitors of the boost cells. The initial PREBOOST
timing is therefore used to preboost the boost capacitors and load
capacitors to anticipate the final V.sub.BOOST compensated output,
and is also used to detect the value of V.sub.DD.
[0081] As the A/D converter finishes measuring the V.sub.DD value,
the A/D comparator outputs are latched at t.sub.2 (750) to ensure a
stable output voltage wherein the outputs of the comparators (e.g.,
the digital word 625 of FIG. 10) are then enabled with an enable
comparators signal EN_COMP 760 for about 6 ns, for example, between
times t.sub.1 (755) and t.sub.2 (750).
[0082] At time t.sub.2 (750), AD_LT (735) (e.g., or 626 of FIG. 10)
goes low as the A/D conversion measurement of the V.sub.DD data
present on the A/D converter is latched to the output of the A/D
converter and to the inputs (e.g., the inputs AD0 thru ADn-1 of
digital word 625 of FIG. 10) and enabled by AD_LT (735). Thus, at
time t.sub.2 (750), the final selection of the boost cells (e.g.,
cell(3:0)) may be made and these boost cells may further alter the
final compensated V.sub.BOOST output voltage 632 as shown by the
V.sub.BOOST curve segments 770, 770a and 770b. For example,
V.sub.BOOST curve segment 770 illustrates that no final change of
compensation was required to the digital word 625 from that of the
predetermined set of selected boost cells. V.sub.BOOST curve
segment 770a indicates that further boosting was required, while
V.sub.BOOST curve segment 770b indicates that further loading was
required to reduce V.sub.BOOST with a corresponding greater or
lesser digital word 625 value, respectively. As previously
discussed, the range here of the output V.sub.BOOST voltage value
for a typical two stage voltage booster is in the range of between
V.sub.DD and 3V.sub.DD.
[0083] By time t.sub.3 (780), approximately 3-4 ns after time
t.sub.2 (750), the final V.sub.BOOST value has been achieved by the
compensation circuit 640 according to the digital word 625. The
digital word value dictates which cells 514, 614 are ultimately
employed to provide the boost to the final value. At time t.sub.4
(785), BOOST (745) goes low again and the read drive output DRIVE
OUTPUT 790 goes high for about 8ns during the actual memory read
operation until the end of the read access time 705 at time t.sub.5
(795).
[0084] FIGS. 12-14 are schematic illustrations of exemplary two
stage voltage booster cells 614 for various bits of the exemplary
regulated voltage booster circuit 600 of FIG. 10 for supply voltage
compensation, in accordance with an aspect of the invention and the
timing diagram 700 of FIG. 11. The booster cells 800 of FIG. 12,
and 900 of FIG. 13, are each selectively controlled by one input
bit AD0-AD3 of the digital word 625, thus may be also be referred
to as "selectable boost cells" (e.g., cell0-cell3, 614). The logic
circuitry 810, 910 differ based on what cells will be employed in
the pre-boost function, as will be further appreciated below.
Booster cell 1000 of FIG. 14 is not controlled by a digital word
input bit, because it is always used for boosting to provide a
default or minimum boosted voltage output V.sub.BOOST, and as such
may be referred to as the "fixed boost cell" (e.g., cell4, 615).
Although the booster cells are illustrated and labeled for
application to a specific exemplary input bit (e.g., AD(0) or
AD(3)), the booster cell schematics and functionality may apply to
any number of input bits of the digital word 625.
[0085] For example, boost cell 800 of FIG. 12 comprises logic
circuit 810 for selection of the boost cell, a digital input bit,
two boost capacitors C0a and C0b for double boosting the
V.sub.BOOST 632 word line, precharge transistors 820 and 821 to
precharge the boost capacitors C0a and C0b, respectively, and
coupling transistor 868 for series coupling boost capacitors C0a
and C0b. Booster cell 800 of FIG. 12, is selectively controlled
using logic circuit 810, by the least significant bits (LSB's)
(e.g., AD(0) or AD(1)) of the digital word 625 by providing an
input bit AD(0) which is enabled by AD_LT (626 of FIG. 10, and 735
at t.sub.2 (750) of FIG. 11). Logic circuit 810 is further enabled
by input BOOSTD from the boost timer 662 of the timing control
circuit 610 of FIG. 10 when boost cell 800 is to begin boosting the
V.sub.BOOST word line voltage, if also selected by, for example,
AD(0).
[0086] Initially, however, boost capacitor C0a of FIG. 12, is
precharged to V.sub.DD when precharge transistor 820 is enabled by
boosted gate signal BOOST_H1 from the precharge gate booster 650
with buffer 812 held low. Boost capacitor C0b is precharged to
V.sub.DD when precharge circuit transistor 622 is enabled by
another boosted gate signal BOOST_H2 (656) from the precharge gate
booster 650 via a common node of the V.sub.BOOST 632 word line, and
when precharge transistor 821 is enabled by BOOSTB from the boost
timer 662 of timing control circuit 610 pulling the lower terminal
of C0b to ground.
[0087] When the boost cell is selected to boost and is enabled by
BOOSTDB, coupling transistor 868 couples boost capacitors C0a and
C0b in series. About the same time buffer 812 is instructed to go
high applying V.sub.DD to the lower terminal of the series pair of
capacitors. Boost capacitors C0a and C0b are thus forced to charge
share adding their 2V.sub.DD of charge to the V.sub.DD supplied by
buffer 812, thereby boosting the V.sub.BOOST 632 word line to a
maximum final level of 3V.sub.DD. However, as discussed previously,
as all the boost cells are wired in parallel at a common node, the
final boost voltage achieved to the V.sub.BOOST 632 word line
becomes a function of the ratio of the effective boost capacitance
to the effective load capacitance selected corresponding to the
measured V.sub.DD. Booster cell 900 of FIG. 13 and fixed booster
cell 1000 of FIG. 14 function similar to that of booster cell 800
of FIG. 12, and as such may not be fully described again for the
sake of brevity.
[0088] Similarly, booster cell 900 of FIG. 13, is also selectively
controlled using logic circuit 910, by the most significant bits
(MSB's) (e.g., AD(2) or AD(3)) of the digital word 625 by providing
an input bit AD(3) which is enabled by AD_LT (626 of FIG. 10, and
735 at t.sub.2 (750) of FIG. 11). Logic circuit 910 is further
enabled by input BOOSTD from the boost timer 662 of the timing
control circuit 610 of FIG. 10, which times and provides a boosted
gate voltage to the precharge transistors when boost cell 900 is to
begin boosting the V.sub.BOOST word line voltage, if also selected
by, for example, AD(3). By contrast, however, to the logic circuit
810 of FIG. 12, logic circuit 910 may be prewired in accordance
with an aspect of the present invention as shown, to force the
boost cell to boost during the initial PREBOOST timing portion of
the BOOST time interval (e.g., between to (740) to t.sub.2 (750) of
FIG. 11), to anticipate the final V.sub.BOOST word line voltage. It
should also be noted at this point that even the nonpredetermined
boost cells during the preboost interval will at least provide
2V.sub.DD to the boosted word line voltage, as the output of the
buffer 912 will be low and C3a and C3b will be series coupled to
charge share their precharge of V.sub.DD each.
[0089] As previously discussed, fixed booster cell 1000 of FIG. 14
(e.g., cell4, 615) is not controlled by a digital word input bit,
because it is always used for boosting to provide a default or
minimum boosted voltage output V.sub.BOOST. The fixed booster cell
1000 is similar to the bit selectable boost cells, except that no
logic circuit or digital input bit is present.
[0090] FIG. 15 illustrates further details of a precharge gate
booster 1100 and the booster 650 of the exemplary regulated voltage
booster circuit 600 of FIG. 10 in accordance with an aspect of the
invention. Precharge gate booster 1100 provides two boosted gate
voltage signals BOOST_H1 (1101) and BOOST_H2 (1102). As the voltage
booster of the present invention is able to operate at very low
supply voltages, the gate-drive to the transistors used to
precharge the boosting capacitors may be small and require high
subthreshold leakage transistors. Thus, to insure full conduction
and turn-off of these precharge transistors at low supply voltages,
a boosted voltage may need to be supplied to the gate of the
precharge transistors (e.g., 820 of FIG. 12, and 622 of FIG.
10).
[0091] Boosted gate voltage signal BOOST_H1 (1101) is supplied by a
single stage voltage boost circuit 1105, while boosted gate voltage
signal BOOST_H2 (1102) is supplied by a double stage voltage boost
circuit 1115. Single stage boost circuit 1105 supplies boosted gate
voltage signal BOOST_I11 (1101) to the precharge transistors (e.g.,
820 of FIG. 12) in all the boost cells. Double stage boost circuit
1115 supplies boosted gate voltage signal BOOST_H2 (1102) to the
precharge circuit (e.g., 522 of FIG. 9, and 622 of FIG. 10).
Portions of the timing and control of the precharge for the boost
cells is thus supplied through these boost circuits.
[0092] To switch a boosting cell in or out, a switching circuit may
be employed at the output of each boost cell. Since it is desired
to have high speed, a high voltage switch is not employed. Since
each of the boosting cells are tied together, if some cells are off
and other cells are on, then the off cells, along with the wordline
path, will act as the load seen by the "on cells." Therefore the
size of the various capacitors within the boost cells (C.sub.BA,
C.sub.BB, C.sub.0A, C.sub.0B . . . C(n-1)A, C.sub.(n-1)B) may, in
one example, be sized to take such effect into account. This may be
accomplished by solving concurrently five equations and five
unknowns (since the present example has 1 pre-boost cell and 4
boost cells coupled to the output), with equation written for a
different V.sub.DD voltage values (e.g., 1.6V, 1.7V, 1.8V, 1.9V and
2.0V. For each equation, the charge on the capacitors before and
after the charge is transferred to the capacitive loading is
equated. Since 2 capacitors are coupled in series within each boost
cell, their equivalent capacitance is C/2. The five equations may
be formulated as follows:
V.sub.BOOST(C.sub.3/2+C.sub.2/2+C.sub.1/2+C.sub.0/2+C.sub.LOAD)+(V.sub.BOO-
ST-2)(C.sub.4/2)=2(C.sub.4+C.sub.3+C.sub.2+C.sub.1+C.sub.0+C.sub.LOAD)
(1)
V.sub.BOOST(C.sub.2/2+C.sub.1/2+C.sub.0/2+C.sub.LOAD)+(V.sub.BOOST-1.9)(C.-
sub.4/2+C.sub.3/2)=1.9(C.sub.4+C.sub.3+C.sub.2+C.sub.1+C.sub.0+C.sub.LOAD)
(2)
V.sub.BOOST(C.sub.1/2+C.sub.0/2+C.sub.LOAD)+(V.sub.BOOST-1.8)(C.sub.4/2+C.-
sub.3/2+C.sub.2/2)=1.8(C.sub.4+C.sub.3+C.sub.2+C.sub.1+C.sub.0+C.sub.LOAD)
(3)
V.sub.BOOST(C.sub.0/2+C.sub.LOAD)+(V.sub.BOOST-1.7)(C.sub.4/2+C.sub.3/2+C.-
sub.2/2+C.sub.1/2)=1.7(C.sub.4+C.sub.3+C.sub.2+C.sub.1+C.sub.0+C.sub.LOAD)
(4)
V.sub.BOOST(C.sub.LOAD)+(V.sub.BOOST-1.6)(C.sub.4/2+C.sub.3/2+C.sub.2/2+C.-
sub.1/2+C.sub.0/2)=1.6(C.sub.4+C.sub.3+C.sub.2+C.sub.1+C.sub.0+C.sub.LOAD)
(5)
[0093] FIG. 16 illustrates an equivalent circuit of an exemplary
multi-stage voltage boost cell 1200 of an exemplary regulated
voltage booster (e.g., 600 of FIG. 10) using digital supply voltage
compensation in accordance with another aspect of the invention.
The multi-stage voltage boost cell 1200 is an extension of the two
stage voltage booster 450 of FIG. 8, and as such functions and
operates similarly and need not be fully described again for the
sake of brevity. Multi-stage voltage boost cell 1200 comprises
stages (1, 2 . . . m) 1210, 1220, and 1230 respectively, for
boosting the V.sub.BOOST word line 1235 up to a potential
(m+l)V.sub.DD voltage value. Each boost stage comprises a boost
capacitor C.sub.1-m, a pull-up precharge switch P.sub.V (e.g., a
transistor) coupleable to V.sub.DD, a grounding precharge switch
P.sub.G (e.g., a transistor) coupleable to a ground potential.
Additionally, each multi-stage voltage boost cell 1200, has a boost
switch B coupleable to a boost potential (e.g., V.sub.DD) for the
preboost and boost time intervals to apply V.sub.DD to the C.sub.1
(bottom) boost capacitor of the series of capacitors.
[0094] Further, all but stage 1 (1210) would have a coupling switch
DB for series coupling all the boost capacitors C.sub.1-m, while
stage 1 of each of the boost cells, except for the default fixed
boost cell, would have a single bit input AD(0-(n-1)) of a digital
word (e.g., 625 of FIG. 10) from, for example, an A/D converter.
Particular to stage m, the pull-up precharge switch P.sub.V (e.g.,
622 of FIG. 10 and external to the boost cell) is coupled to the
common node of the V.sub.BOOST word line 1235 which is common to
all the boost cells, and also coupleable to V.sub.DD.
[0095] Another aspect of the invention provides a methodology for
regulating the boost operation in a memory device, which may be
employed in association with the memory devices illustrated and
described herein, as well as with other memory devices. Referring
now to FIG. 17, an exemplary method 1300 is illustrated for
regulating the boost operation in a memory device. While the
exemplary method 1300 is illustrated and described herein as a
series of acts or events, it will be appreciated that the present
invention is not limited by the illustrated ordering of such acts
or events, as some steps may occur in different orders and/or
concurrently with other steps apart from that shown and described
herein, in accordance with the invention. In addition, not all
illustrated steps may be required to implement a methodology in
accordance with the present invention. Moreover, it will be
appreciated that the method 1300 may be implemented in association
with the apparatus and systems illustrated and described herein as
well as in association with other systems not illustrated.
[0096] The method 1300 comprises precharging a plurality of boost
cells of a multi-stage voltage booster circuit while starting a
supply voltage level detection (e.g., an A/D converter, digital
thermometer), in order to control one or more boost cells used in a
boost voltage compensation circuit corresponding to the supply
error, and to correct for the supply level error reflected in the
output of a voltage boost circuit. The regulated boost operation
method begins at step 1302. At 1304 the boost capacitors within
each boost cell are initially precharged to the supply voltage
(e.g., V.sub.DD) while the supply level detection circuit is
enabled to begin sampling and measuring the supply voltage level.
At 1306, the V.sub.BOOST output node (e.g., 632 of FIG. 10) of the
voltage booster circuit is allowed to transition to the supply
voltage level. At 1308, the boost capacitors within each of the
boost cells are coupled in series (e.g., by switch DB 468 of FIG.
8, or 868 of FIG. 12). At 1310, a preboost of the V.sub.BOOST
output node begins by coupling one or more of a set of
predetermined (e.g., by a prewired logic circuit 910 of FIG. 13)
boost cells (e.g., 900 of FIG. 13) along with a fixed boost cell
(e.g., 1000 of FIG. 14) to the supply voltage, and allowing the
capacitors of the boost cells to charge share to the V.sub.BOOST
output node of the voltage booster circuit. This pre-boost occurs
while the supply voltage value is still in the process of being
ascertained.
[0097] When the data is made available, for example, by the A/D
latch signal AD_LT (e.g. 346 of FIG. 6, 626 of FIG. 10), the supply
voltage level detection circuit generates one or more supply
voltage level detection signals (e.g., 345 associated in FIG. 6
with A/D 342, or 625 in FIG. 10), in response to the A V.sub.DD
compared to a target value set by a reference voltage, in order to
apply the supply voltage level detection signals to a boost
compensation circuit at 1312, wherein the boosted voltage is
greater than the supply voltage.
[0098] At 1314, the supply level detection signals are used to
select one or more boost cells which are coupled along with the
fixed boost cell to the supply voltage and allowed to charge share
with each other and the supply voltage applied to the boost cells,
and with the V.sub.BOOST output node of the voltage booster
circuit. Further, during this boost period, some cells may be
deselected to provide compensation (adjustment). Thereafter, a
regulated boost voltage V.sub.BOOST resulting from the applied
compensation is produced at step 1316 in order to ascertain the
data value stored in a memory cell. The regulated boost operation
thereafter ends at 1318, and the method 1300 may be repeated for
subsequent voltage boost and read operations of the memory device.
The methodology 1300 thus provides for quick, accurate voltage
boosting in a voltage boost circuit that uses an A/D converter to
compensate for V.sub.DD voltage variations, which may be applied to
a word line of memory core cells during read operations of flash
memory arrays. Therefore the method 1300 generates a V.sub.BOOST
voltage that is substantially independent of variations in
V.sub.DD. Other variants of methodologies may be provided in
accordance with the present invention, whereby compensation or
regulation of a boosted voltage is accomplished.
[0099] Although the invention has been shown and described with
respect to one or more implementations, equivalent alterations and
modifications will occur to others skilled in the art upon the
reading and understanding of this specification and the annexed
drawings. In particular regard to the various functions performed
by the above described components (assemblies, devices, circuits,
etc.), the terms (including a reference to a "means") used to
describe such components are intended to correspond, unless
otherwise indicated, to any component which performs the specified
function of the described component (i.e., that is functionally
equivalent), even though not structurally equivalent to the
disclosed structure which performs the function in the herein
illustrated exemplary implementations of the invention. In
addition, while a particular feature of the invention may have been
disclosed with respect to only one of several implementations, such
feature may be combined with one or more other features of the
other implementations as may be desired and advantageous for any
given or particular application. Furthermore, to the extent that
the term "includes" is used in either the detailed description and
the claims, such term is intended to be inclusive in a manner
similar to the term "comprising."
* * * * *