U.S. patent application number 13/015277 was filed with the patent office on 2012-08-02 for temperature correcting current-controlled ring oscillators.
This patent application is currently assigned to NXP B.V.. Invention is credited to Martin BUGBEE, Andrew BURTT.
Application Number | 20120194278 13/015277 |
Document ID | / |
Family ID | 45655278 |
Filed Date | 2012-08-02 |
United States Patent
Application |
20120194278 |
Kind Code |
A1 |
BUGBEE; Martin ; et
al. |
August 2, 2012 |
TEMPERATURE CORRECTING CURRENT-CONTROLLED RING OSCILLATORS
Abstract
A thermally-compensated oscillator has a current reference with
an output current which relates to an ambient temperature with a
first relationship, a ring oscillator having an operating frequency
which relates to the ambient temperature with a second
relationship, and which receives the output current of the current
reference and outputs an oscillator signal, and a level shifter
which receives the oscillator signal from the ring oscillator and
outputs a corresponding voltage-regulated clock signal.
Inventors: |
BUGBEE; Martin;
(Southampton, GB) ; BURTT; Andrew; (Southampton,
GB) |
Assignee: |
NXP B.V.
Eindhoven
NL
|
Family ID: |
45655278 |
Appl. No.: |
13/015277 |
Filed: |
January 27, 2011 |
Current U.S.
Class: |
331/8 |
Current CPC
Class: |
H03K 3/0315 20130101;
H03K 3/011 20130101 |
Class at
Publication: |
331/8 |
International
Class: |
H03L 7/00 20060101
H03L007/00 |
Claims
1. A thermally-compensated oscillator, comprising: a current
reference having an output current which has a first relationship
to an ambient temperature; at least one ring oscillator having an
operating frequency which has a second relationship to the ambient
temperature, and which receives the output current of the current
reference and in response outputs an oscillator signal; and a level
shifter which receives the oscillator signal from the ring
oscillator and outputs a corresponding voltage-regulated clock
signal.
2. A thermally-compensated oscillator according to claim 1, wherein
the current reference includes a first grounded MOSFET and a second
grounded MOSFET, the first MOSFET and the second MOSFET being
gate-coupled, and a source of the second MOSFET being grounded via
a resistance which has a third relationship to the ambient
temperature.
3. A thermally-compensated oscillator according to claim 2, wherein
the third relationship has a value such that the first relationship
corresponds to a proportional increase in the output current over a
temperature range.
4. A thermally-compensated oscillator according to claim 3, wherein
the first relationship has a value such that the second
relationship is constant over the temperature range.
5. A thermally-compensated oscillator according to claim 4, wherein
a frequency of the oscillator signal is constant over the
temperature range.
6. A thermally-compensated oscillator according to claim 2, the
resistance comprising least two resistors.
7. A thermally-compensated oscillator according to claim 2, further
comprising: a third MOSFET and a fourth MOSFET transistor of the
current reference, the third and the fourth MOSFETs being
gate-coupled and supplying the output current to the ring
oscillator, wherein the first and second MOSFETs are of a same
channel type, and the third and fourth MOSFETs are of an opposite
channel type to the first and second MOSFETS.
8. A method of regulating a ring oscillator, comprising: providing
a current which has a first relationship to an ambient temperature;
applying the current to a ring oscillator having an operating
frequency which has a second relationship to the ambient
temperature, the ring oscillator in response outputting an
oscillator signal; and using the oscillator signal to generate a
corresponding voltage-regulated clock signal.
9. A method according to claim 8, wherein the providing step
comprises using an electrical circuit having a transistor which is
grounded via a resistance which has a third relationship to the
ambient temperature.
10. A method according to claim 9, wherein the third relationship
has a value such that the first relationship corresponds to a
proportional increase in the current over a temperature range.
11. A method according to claim 10, wherein the first relationship
has a value such that the second relationship is constant over the
temperature range.
12. A method according to claim 11, wherein a frequency of the
oscillator signal is constant over the temperature range.
13. A method according to claim 9, wherein the resistance includes
at least two resistors.
14. A method according to claim 8, wherein the using step is
performed by a level shifter.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to current-controlled ring
oscillators, and, more specifically, to regulating a frequency of a
current-controlled ring oscillator's output signal in accordance
with ambient temperature.
BACKGROUND OF THE INVENTION
[0002] Many integrated circuit devices require stable oscillators
to provide the reference clocks used in the system. Since virtually
all integrated circuit devices operate in environments whose
temperature fluctuates, such a reference clock should exhibit a
stable oscillating frequency over a range of operating
temperatures. Furthermore, it is desirable for a reference clock to
exhibit a stable oscillating frequency over a range of operating
voltages, and also not be affected by process variations which
might arise during device manufacture.
[0003] Many applications necessitate that the device's power
consumption be made as small as possible; for example, battery
powered devices and/or RFID (radio frequency identification) tags
benefit from a reduction in power consumption. In the case of
battery powered devices, decreasing power consumption can increase
the time required until battery recharging or replacement is
required, and/or permit the use of a battery storing less power
(which, typically, is a smaller, lighter and less costly
battery).
[0004] In the case of an RFID device, power consumption and
operating range vary inversely, so if power consumption decreases,
the operating range increases.
[0005] The oscillator also should consume as little area as
possible on silicon, at least in part to reduce the cost of
manufacture of the device, to allow greater design flexibility, and
to provide a more compact device.
[0006] Ring oscillators are one type of device suitable for use as
clocks in integrated circuit devices. A ring oscillator can consist
of a odd number of NOT gates which are connected in series such
that the output of the last NOT gate is fed back to the input of
the first NOT gate. Since each NOT gate cannot switch output state
instantaneously, time is required for a signal to propagate from
the input of the first NOT gate through the series of gates and
thereby change the state at the output of the last NOT gate. This
looping behaving contributes to the operating frequency of the ring
oscillator.
[0007] It is known that the performance of ring oscillators is
temperature dependent, as discussed by Datta et al. in the article
"Analysis of a Ring Oscillator Based on Chip Thermal Sensor in 65
nm Technology" (2005).
[0008] Many consumer and professional electronic devices can be
used at operating temperatures typically ranging from -40.degree.
C. to 125.degree. C., particularly if those devices are portable.
It is therefore likely that a device using a ring oscillator to
provide a clock signal will experience temperature fluctuations,
resulting in temperature-induced variations in the clock signal by
that ring oscillator. Such clock signal variations could
undesirably affect device performance, or require the inclusion of
compensating components or logic in the device to cope with those
variations.
[0009] Current solutions to this problem are not satisfactory. For
example, one option, the RC oscillator, requires the use of a
more-than-minimal silicon area.
[0010] Accordingly, there is a need for a ring oscillator whose
performance is substantially independent of operating
temperature.
SUMMARY OF THE INVENTION
[0011] This invention proposes a scheme for temperature correcting
a current controlled ring oscillator, preferably to thereby provide
a stable clock reference over temperature fluctuations (and,
possibly (by careful design), supply voltage variations). The
proposed solution has the advantage that it requires minimal
silicon area as compared to other solutions, such as an RC
oscillator, operates at low power supply levels and is relatively
temperature independent.
[0012] One aspect of this invention is a thermally-compensated
oscillator having a current reference with an output current which
has a first relationship to an ambient temperature, a ring
oscillator having an operating frequency which has a second
relationship to the ambient temperature, and which receives the
output current of the current reference and outputs an oscillator
signal, and a level shifter which receives the oscillator signal
from the ring oscillator and outputs a corresponding
voltage-regulated clock signal.
[0013] A further aspect of this invention is that the first
relationship can be selected to have a value such that the second
relationship is constant over a temperature range.
[0014] Another aspect of this invention is a method of regulating a
ring oscillator by providing a current with a value which has a
first relationship to an ambient temperature, applying the current
to a ring oscillator having an operating frequency which has a
second relationship to the ambient temperature, the ring oscillator
in response outputting an oscillator signal, and using the
oscillator signal to generate a corresponding voltage-regulated
clock signal.
[0015] Yet another aspect of this invention is selecting the first
relationship with a value such that the second relationship is
constant over a temperature range.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic diagram showing an exemplary RFID
device which can employ an oscillator in accordance with this
invention;
[0017] FIG. 2 is a schematic diagram showing a clock device in
accordance with this invention;
[0018] FIGS. 3A through 3D are graphs depicting relationships
between (1) various oscillator currents and frequencies and (2)
temperature;
[0019] FIG. 4 is a circuit diagram depicting a clock device in
accordance with this invention;
[0020] FIG. 5 is a schematic diagram showing an embodiment in which
one reference circuit can drive several oscillators; and
[0021] FIG. 6 is a schematic diagram of a modification of a portion
of the clock device of FIG. 4 which can facilitate trimming of
resistors to minimize temperature dependence.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0022] FIG. 1 depicts an exemplary RFID device 2. Device 2 is
powered in a known manner by an antenna 5, which, during device
operation, is located within the RF field emitted by transmitter 4
("within" is used generally and means that the RF field is
sufficiently strong to result in power generation in the RFID
device 2). The amount of power required by the device 2 for
operation directly affects the maximum distance from the
transmitter 4 that the device 2 can operate at. The oscillator 1 in
the device 2 supplies the clock signals(s) used by the digital
logic 3 in the device 2.
[0023] A simple very low power oscillator could be made by using a
current starved ring oscillator and trimming the bias current
required so that the oscillator operates at a specific frequency.
However the operating frequency of that ring oscillator will be
temperature dependent when the ring oscillator is biased with a
fixed reference current.
[0024] A proportional to absolute temperature (PTAT) current source
can have its temperature coefficient adjusted to compensate for the
temperature coefficient of the current-controlled oscillator (CCO).
This can be done by providing the PTAT with one or more resistors
having different temperature coefficients, which thereby change the
temperature dependence of the PTAT.
[0025] FIG. 2 shows a proposed oscillator device 12 in accordance
with this invention, consisting of a reference current supply 9, a
current-controlled ring oscillator 10, and a level shifter 11 which
converts the output of the ring oscillator to full logic level
swing so that the output can be used as a clock signal. Typically
the bias current supplied by the reference current supply 9 to the
ring oscillator 10 will be trimmed to allow the frequency of
oscillation to be accurately defined. Such trimming is accomplished
in known ways.
[0026] In principle, bipolar devices could also be used to generate
the PTAT current source, since such devices also exhibit a PTAT Vbe
difference. However, presently, this configuration is not thought
to be preferred.
[0027] FIG. 4 shows one possible implementation for the oscillator
12. Oscillator 12 includes a PTAT current reference section 12a, a
ring oscillator section 12b, and a level shifter section 12c, which
are all powered by supply voltage Vcc. Startup circuit 13 inputs an
initial signal to the oscillator's PTAT current reference 12a, and
level shifter 12c outputs a clock signal CLK, which can be used in
known fashion.
[0028] The PTAT current reference 12a is constructed from n-type
MOSFET transistors N1, N2, P1, P2 and resistors R1 and R2. In
theory, the same circuit topology could be employed with other
types of transistors, i.e., bipolar, although the particular
characteristics of the type of transistors used will be a factor in
the circuit's design.
[0029] In FIG. 4, the source of transistor N1 is grounded directly,
and the source of transistor N2 is grounded via resistors R1 and
R2. The gates of transistors N1 and N2 are connected to each other
and also to the drain of transistor N1. The gates of transistors P1
and P2 are connected to each other and also to the drain of
transistor P2. The sources of transistors P1 and P2 are connected
to voltage supply Vcc to power the PTAT current reference.
[0030] As shown in FIG. 3A, if the resistors R1 and R2 have no
temperature dependence then the output current of the current
reference Iptat will be directly proportional to the temperature,
as shown by line a.
[0031] If instead one uses a combination of resistors R1 and R2
having different temperature coefficients, by suitably selecting R1
and R2, the resulting temperature coefficient of the current
reference can be varied, as is shown by lines a-d in FIG. 3B. Line
a corresponds to FIG. 3A, wherein the resistors R1 and R2 have no
temperature dependence so that the output current Iptat is directly
proportional to temperature, line b corresponds to resistors R1 and
R2 have a direct temperature dependence such that Iptat is more
dependent upon temperature than when R1 and R2 have no temperature
dependence (line a), line c corresponds to R1 and R2 being chosen
such that Iptat has a constant (temperature invariant) value, and
line d corresponds to R1 and R2 chosen to result in a negative
temperature dependence for Iptat. It should be understood that the
results depicted in FIG. 3B assume resistors with positive and
negative temperature coefficients are available in the process
technology used to manufacture the oscillator 12. The manner in
which R1 and R2 (at least one of which has a resistance that varies
in relationship to the ambient temperature) can be chosen to
control Iptat will be discussed in detail hereafter.
[0032] While FIG. 3A and 3B show linear relationships between Iptat
and temperature, it should be understood that if the resistances of
at least some of the resistors used do not vary linearly with
temperatures, these Iptat-temperature relationships also will not
be linear. This in fact may be useful in, for example, situations
where the current which the ring oscillator needs to maintain a
constant frequency is not linear with temperature. By carefully
selecting resistor types and absolute resistances, one can then
obtain adequate temperature compensation.
[0033] If required, R1 and R2 can optionally be trimmed during
production testing to minimize the temperature dependence of the
oscillator after manufacture. An implementation of an example
trimming scheme is shown in FIG. 6, which depicts a modification of
the PTAT current reference section 12a of the oscillator 12 of FIG.
4. In this configuration, rather than provide only two calibration
resistors R1 and R2 (as in FIG. 4), resistors R1A, R1B, R2A and R2B
are provided along with associated n-type MOSFET transistors N1B,
N1A, N2B, N2A. Transistors N1B, N1A, N2B and N2A can be driven
selectively (by actuating circuitry and logic not shown) to allow
the resistors R1B, R1A, R2B and R2A to be switched into or out of
the circuit leg between the source of transistor N2 and ground as
desired so that the absolute PTAT current can be trimmed (a total
of 16 possible curves are possible). This permits adjustment of the
effective value of R1 and R2 and therefore changes the temperature
characteristic of the reference (depending upon their relative
resistances when compared with R1 and R2, the family of Iptat v.
Temp curves may not show a simple offset difference, but also a
small temperature dependent offset). Furthermore, trimming could be
performed during the operational life of the oscillator, if
necessary or desired.
[0034] Returning to FIG. 4, the reference current Iptat from PTAT
12a is mirrored from the gate of transistor P2 to the gate of
transistor P8, which is part of the ring oscillator 12b. Ring
oscillator 12b is constructed from transistors N3, N4, N5 and P3,
P4, P5 (ring oscillator 12b is shown as a three stage ring
oscillator by way of example only and not limitation, and the
present invention is suitable for use with ring oscillators having
any number of stages). The current mirrored from the PTAT to the
ring oscillator is trimmed using transistor P8, which acts as a
current DAC (by switching in different sized devices to mirror the
bias current), this allows the frequency of the ring oscillator to
be production trimmed to the required value. It will be appreciated
that this arrangement is particularly beneficial because it can
compensate for process, variations that might affect the properties
of one or more components used in the oscillator 12.
[0035] FIG. 3C shows the temperature dependence of the current Iosc
supplied by PTAT current reference 12a, that current Iosc being
supplied to the sources of transistors P3, P4, P5. As explained
previously, the temperature dependence (or independence) of the
current output by PTAT current reference 12a depends upon the
properties of resistors R1 and R2; resistors R1 and R2 can be
chosen so that the current Iosc varies positively with temperature
(line a), negatively with temperature (line c), or remains
temperature invariant (line b).
[0036] The current required to keep ring oscillator 12b operating
at a fixed frequency will vary with temperature. Typically if the
voltage at the drain of transistor P8 is >> Vtn, Vtp
(Threshold voltages of N3-N5, P3-P5) then the current required will
increase with temperature. As this voltage is lowered the
temperature dependency will decrease.
[0037] The present invention compensates for the temperature
dependence of ring oscillator 12b by providing a PTAT current
reference 12a which has a temperature dependence complementary to
the temperature dependence of the ring oscillator 12b. That is, the
ring oscillator can be matched to the current reference, or the
current reference can be matched to the ring oscillator. By
matching the temperature dependency of the current reference to the
current required for the ring oscillator 12 b to operate at a fixed
frequency, a temperature-independent oscillator 12 can be
constructed, as shown in FIG. 3D.
[0038] This invention takes advantage of the fact that the output
frequency, fosc, of the ring oscillator 12b varies inversely with
the temperature dependence of the current Iosc applied thereto
(this follows at least from FIGS. 3C and 3D).
[0039] Selecting resistors R1 and R2 to obtain a particular
temperature-reference current Iptat relationship, leading to a
particular temperature-current Iosc relationship, as discussed
above, will result in a ring oscillator whose output frequency, as
shown in FIG. 3D, can be made constant (line b), can be made to
vary directly with temperature (line c), or can be made to vary
inversely with temperature (line a).
[0040] That is, if the current Iptat output by PTAT reference 12a
has a particular direct relationship to temperature (FIG. 3B, line
b) through the use of R1 and R2 at least one of which has a
resistance that increases directly with temperature, the current
Iosc supplied by the oscillator 12 will be temperature-invariant
(FIG. 3C, line b), and the operating frequency of ring oscillator
12b also will be temperature invariant (FIG. 3D, line b).
[0041] In the same way, if the current Iptat output by PTAT
reference 12a has a direct relationship to temperature (FIG. 3B,
line a) through the use of R1 and R2 at least one of which has a
resistance that does not vary with temperature, the current Iosc
output by oscillator 12 will vary directly with temperature (FIG.
3C, line a), and the operating frequency of oscillator 12 will vary
inversely with temperature (FIG. 3D, line a).
[0042] If the current Iptat output by PTAT reference 12a does not
vary with temperature (FIG. 3B, line c) through the use of R1 and
R2 at least one of which has a resistance that varies inversely
with temperature, the current Iosc output by the oscillator 12 will
vary inversely with temperature, and the operating frequency of the
oscillator 12 will vary directly with temperature (FIG. 3D, line
c).
[0043] Returning to FIG. 4, the output of the ring oscillator 12b
is level shifted to digital switching levels by level shifter 12c,
using transistors N6, P6 and that output is buffered with
transistors N7, P7. The level shifter output is then available as a
clock signal CLK, which can be supplied to devices which employ
such a clock signal. Other level shifting schemes are well known
and can be used.
[0044] FIG. 5 depicts a further configuration of this invention in
which portions corresponding to portions already described above
are identified with the same reference numbers incremented by 100.
Here, a biased PTAT reference 112a comparable to PTAT reference 12a
described above can be used to drive multiple ring oscillators
112b, 112b', respectively having outputs CLK1 and CLK2, and which
together are part of oscillator section 114. More than two ring
oscillators could be provided. If desired, the ring oscillators can
have different operating frequencies, resulting in an
multi-frequency temperature invariant oscillator 112.
[0045] This invention allows the use of a minimal number of
components and minimal silicon in comparison to a conventional
compared to an RC-based oscillator, which can require large-sized
passive components. Additionally, it is predicted that, for a 140
nm, 1.2-2.0V design, this invention can operate at relatively low
voltages, around 2Vt, and consumes a relatively small amount of
power (less than approximately 700 nA in this example).
[0046] The accuracy of the oscillator has been shown by both
prediction and testing to be +/-3% over 125.degree. C. in a 140 nm
process for a 1 MHz oscillator, this being worst case performance
over process and temperature without trimming of temperature
dependence of the PTAT (as shown in FIG. 6).
[0047] An additional feature of the implementation is that the
frequency of oscillation can be made linear (or at least
substantially linear) with temperature (FIG. 3D, lines A or C) by
an appropriate choice of R1 and R2 in the PTAT current reference
12a.
[0048] Various exemplary embodiments are described in reference to
specific illustrative examples. The illustrative examples are
selected to assist a person of ordinary skill in the art to form a
clear understanding of, and to practice the various embodiments.
However, the scope of systems, structures and devices that may be
constructed to have one or more of the embodiments, and the scope
of methods that may be implemented according to one or more of the
embodiments, are in no way confined to the specific illustrative
examples that have been presented. On the contrary, as will be
readily recognized by persons of ordinary skill in the relevant
arts based on this description, many other configurations,
arrangements, and methods according to the various embodiments may
be implemented.
[0049] The present invention has been described with respect to
particular embodiments and with reference to certain drawings, but
the invention is not limited thereto, but rather, is set forth only
by the claims. The drawings described are only schematic and are
non-limiting. In the drawings, for illustrative purposes, the size
of various elements may be exaggerated and not drawn to a
particular scale. It is intended that this invention encompasses
inconsequential variations in the relevant tolerances and
properties of components and modes of operation thereof Imperfect
practice of the invention is intended to be covered.
[0050] Where the term "comprising" is used in the present
description and claims, it does not exclude other elements or
steps. Where an indefinite or definite article is used when
referring to a singular noun, e.g. "a" "an" or "the", this includes
a plural of that noun unless something otherwise is specifically
stated. Hence, the term "comprising" should not be interpreted as
being restricted to the items listed thereafter; it does not
exclude other elements or steps, and so the scope of the expression
"a device comprising items A and B" should not be limited to
devices consisting only of components A and B. This expression
signifies that, with respect to the present invention, the only
relevant components of the device are A and B.
* * * * *