U.S. patent application number 09/773655 was filed with the patent office on 2001-10-04 for cmos low battery voltage detector.
Invention is credited to Andersson, Olle, Ohlsson, Tony.
Application Number | 20010026226 09/773655 |
Document ID | / |
Family ID | 9885138 |
Filed Date | 2001-10-04 |
United States Patent
Application |
20010026226 |
Kind Code |
A1 |
Andersson, Olle ; et
al. |
October 4, 2001 |
CMOS low battery voltage detector
Abstract
A system and method for detecting a low battery voltage supplied
to a battery operated integrated circuit. A stable reference
voltage provided by a bangap reference is compared with the battery
voltage. A switched capacitor circuit is used instead of the more
conventional resistor combination to supply a scaled representation
of the battery voltage. Power requirements are reduced by combining
the bandgap reference and the comparator into a single
component.
Inventors: |
Andersson, Olle; (Carlsbad,
CA) ; Ohlsson, Tony; (Johanneshou, SE) |
Correspondence
Address: |
LAW OFFICE OF LAWRENCE E LAUBSCHER, JR
1160 SPA RD
SUITE 2B
ANNAPOLIS
MD
21403
US
|
Family ID: |
9885138 |
Appl. No.: |
09/773655 |
Filed: |
February 1, 2001 |
Current U.S.
Class: |
340/636.15 ;
340/663 |
Current CPC
Class: |
G01R 19/16552 20130101;
G01R 19/16542 20130101 |
Class at
Publication: |
340/636 ;
340/663 |
International
Class: |
G08B 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 9, 2000 |
GB |
0002830.8 |
Claims
1. A detector for providing a low battery voltage indication
comprising: a stable voltage reference and comparator circuit for
comparing the battery voltage and the voltage reference; switch
means to switch between a first operational state and a second
operational state; a switched capacitor circuit to store charges
related to the battery voltage and the reference voltage in each
operational state; and a clock to initiate switching between the
first and second states; wherein an output from the comparator
during the second state indicates whether the battery voltage is
below a preset threshold.
2. A detector as defined in claim 1 wherein said stable voltage
reference is a bandgap reference circuit.
3. A detector as defined in claim 2 wherein said bandgap reference
circuit comprises a pair of bipolar transistors supplied with a
first current density in said first state and a second greater
current density in said second state.
4. A detector as defined in claim 1 wherein said first operational
state is a precharge phase and said second operational state is an
evaluation phase.
5. A detector as defined in claim 1 fabricated on an integrated
circuit (IC) employing CMOS technology.
6. A detector as defined in claim 5 wherein said comparator is an
operational amplifier.
7. A detector as defined in claim 6 wherein offset voltages due to
first and second inputs to said operational amplifier are cancelled
by said switched capacitor circuit.
8. A detector as defined in claim 1 wherein capacitors in said
switched capacitor circuit define a weighting factor for
establishing said threshold.
9. A detector as defined in claim 1 wherein said threshold (Vmin)
is set such that:
Vmin>V.sub.BE+K.sub.1.DELTA.V.sub.BE-K.sub.2BAT where: Vmin is
the threshold value V.sub.BE is the base to emitter voltage across
a first transistor of the bandgap reference; .DELTA.V.sub.BE is the
difference between base to emitter voltages in two bandgap
transistors having different current densities; V.sub.BAT is the
battery voltage; K.sub.1 is a first scaling factor; and K.sub.2 is
a second scaling factor.
10. A detector as defined in claim 9 having three capacitors
(C.sub.1, C.sub.2 and C.sub.3) wherein K.sub.1 and K.sub.2 are
determined by said capacitors according to the ratios
K.sub.1=C.sub.1+C.sub.2/C.sub.3 and K.sub.2=C.sub.1/C.sub.3.
11. A detector as defined in claim 1 wherein said bandgap reference
circuit comprises three bipolar transistors with a first switchable
current source is supplied to one transistor and a second current
source is supplied to the other two transistors.
12. A method of detecting a low battery voltage supplied to an
integrated circuit comprising: providing a stable voltage reference
and comparator circuit for comparing battery voltage against the
reference voltage; providing a capacitor circuit for storing
charges associated with the battery voltage and the reference
voltage; providing switching means to switch between a first phase
wherein said capacitors are charged to a first voltage level and a
second phase wherein said capacitors are charged to a second level;
and comparing said first and second levels to determine whether
said battery voltage is above or below a preset threshold.
13. The method as defined in claim 12 wherein said switching means
are MOS devices.
14. The method of claim 12 wherein said preset threshold (Vmin) is
such that: Vmin=Vref-K*Vbat where: Vref is the reference voltage
Vbat is the battery voltage; and K is a scaling factor
15. The method of claim 14 wherein said stable voltage reference is
a bandgap cell having a value of approximately 1.2 volts and Vmin
is approximately 1.2/K.
Description
FIELD OF THE INVENTION
[0001] This invention relates to battery operated circuits and,
more particularly, to a method and system for detecting low battery
voltage utilizing a stable reference voltage and a
switched-capacitor circuit.
BACKGROUND OF THE INVENTION
[0002] In many battery operated applications, some means for
detecting a low battery voltage is needed. In such applications, a
warning signal might be generated or, alternatively, vulnerable
circuits within a system might be selectively disabled by an
appropriate control signal in order to avoid damage to or
malfunction of such circuits. A straightforward method of
implementing a low voltage detection function is to compare a
scaled battery voltage and a stable reference voltage such as a
voltage from a bandgap circuit. It is known, as will be discussed
in greater detail later, that a bandgap circuit can generate a
stable, substantially temperature independent reference voltage.
Normal time-continuous bandgap references typically need
well-matched resistors and transistors and some sort of trimming to
get good accuracy.
[0003] U.S. Pat. No. 5,196,833, which issued Mar. 23, 1993, to Kemp
describes a low voltage detection circuit which includes a bandgap
reference and a differential comparator circuit for generating a
signal proportional to the level of the supply voltage. Since many
battery operated circuits are subject to ambient temperature
variations, such as an automobile application as disclosed in the
U.S. Pat. No. 5,196,833, it is important that some form of
temperature compensation be used in deriving the reference voltage.
The bandgap cell, as disclosed in the aforementioned patent,
includes bipolar transistors in which the base to emitter voltage
is used as a reference source. Typically, two transistors are used
having different current densities through each. In the U.S. Pat.
No. 5,196,833, the different current densities are achieved by
using four parallel transistors in one current path and a single
transistor in the second. It has been shown that the base to
emitter voltage (V.sub.BE) of a bipolar transistor exhibits a
negative temperature coefficient with respect to temperature. On
the other hand, it has also been shown that the difference of base
to emitter voltages .DELTA.V.sub.BE of the two bipolar transistors
operating at different current densities exhibit a positive
temperature coefficient with respect to temperature. Thus, the sum
of the base to emitter voltage V.sub.BE of a bipolar transistor and
a differential voltage .DELTA.V.sub.BE will be relatively
independent of temperature when the sum of the voltages equals the
energy gap of silicon. Such temperature stable references have been
created by generating a V.sub.BE and summing a .DELTA.V.sub.BE of
such value that the sum substantially equals the bandgap voltage of
1.205V.
[0004] U.S. Pat. No. 5,814,995, which issued Sep. 29, 1998, to
Tasdighi, discloses a voltage detector for a battery operated
device employing two bipolar transistors and an operational
amplifier as a comparator. The system of this reference also
employs resistors which, as noted hereinbefore, must be
well-matched and usually require some sort of trimming to obtain
good accuracy.
[0005] U.S. Pat. No. 4,375,595, which issued Mar. 1, 1983, to Ulmer
et al., relates to a temperature independent bandgap reference
which employs switched capacitors to input the V.sub.BE and
.DELTA.V.sub.BE of the bipolar transistors. A proper selection of
the ratio of the switched capacitors is used to get around the need
for matched and/or trimmed resistors. The switched capacitor
implementation employs clock signals in order to establish a
precharge phase and an output reference stage. In the U.S. Pat. No.
4,375,959, three separate clock signals are required.
[0006] U.S. Pat. No. 5,563,504, which issued Oct. 8, 1996, to
Gilbert et al., also relates to a switching bandgap voltage
reference in which one bipolar transistor is used with two
different current sources providing the current densities needed to
obtain the .DELTA.V.sub.BE value.
[0007] Thus, in an application where a battery voltage detector is
required in order to detect a low voltage value, and providing that
a suitable clock exists within the application, a switched
capacitor architecture can provide an effective solution.
SUMMARY OF THE INVENTION
[0008] The present invention provides lower untrimmed errors by
utilizing offset cancellation techniques. This, in combination with
a switched capacitor network having well-matched capacitors provide
suitable weighting factors.
[0009] The invention also provides for a reduced component count by
combining a voltage reference circuit and a voltage comparator into
a single circuit.
[0010] In a preferred embodiment, the comparator implementation
eliminates the need to actually generate a 1.2V bandgap reference
voltage.
[0011] The invention also provides for lower power requirements by
eliminating part of the static current by replacing resistors in
the bandgap circuit with a switched capacitor circuit.
[0012] Therefore, in accordance with a first aspect of the present
invention there is provided a detector for providing a low battery
voltage indication comprising: a stable voltage reference and
comparator circuit for comparing the battery voltage and the
voltage reference; switch means to switch between a first
operational state and a second operational state; a switched
capacitor circuit to store charges related to the battery voltage
and the reference voltage in each operational state; and a clock to
initiate switching between the first and second states, wherein an
output from the comparator during the second state indicates
whether the battery voltage is below a preset threshold.
[0013] In accordance with a second aspect of the present invention
there is provided a method of detecting a low battery voltage
supplied to an integrated circuit comprising: providing a stable
voltage reference and comparator circuit for comparing battery
voltage against the reference voltage; providing a capacitor
circuit for storing charges associated with the battery voltage and
the reference voltage; providing switching means to switch between
a first phase wherein the capacitors are charged to a first voltage
level, and a second phase wherein the capacitors are charged to a
second level; and comparing the first and second levels to
determine whether the battery voltage is above or below a preset
threshold.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The invention will now be described in greater detail with
reference to the attached drawings wherein:
[0015] FIG. 1 illustrates a prior art voltage detector utilizing
resistors;
[0016] FIG. 2 illustrates a switched capacitor,
temperature-independent, bandgap reference utilizing three input
clocks;
[0017] FIG. 3 is a simplified schematic of the low battery voltage
detector according to the present invention;
[0018] FIG. 4 illustrates a second embodiment of the bandgap
reference circuit; and
[0019] FIG. 5 is a circuit diagram of the CMOS low voltage battery
detector according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] FIG. 1 shows a basic, low battery voltage detector 2
according to the prior art. The scaled battery voltage taken across
the voltage divider circuit comprising resistors 4 and 5 is
supplied to the positive input of an operational amplifier 6 and
the voltage reference 7 which in a preferred embodiment is a
bandgap reference is supplied to the negative input of the
operational amplifier 8. The output of the operational amplifier is
0 if the battery voltage is low and is a 1 otherwise.
[0021] A discussed previously, the voltage divider circuit
comprising the resistors 4,5 increases the power requirement to the
detector circuit and can lead to measurement inaccuracies.
[0022] To overcome the inaccuracies created by the resistor
combination, a switched capacitor circuit 10 as shown in FIG. 2 has
also been disclosed in the prior art. In this circuit bipolar
transistors 12 and 14 are used to obtain the stable bandgap
reference voltage. Comparator 42 evaluates the stored voltage
across switched capacitors 28 and 34. Clock 16 provides clock
signals A, B and C to switches 30, 32, 36, 38 and 48.
[0023] The present invention provides a low voltage monitoring
circuit 100 utilizing the switched capacitor arrangement which is
illustrated at a high level in FIG. 3. The circuit 100 includes
switches 110, 112, 114, 116, 118 and 120, operational amplifier 122
bipolar transistors 124 and 126, current source 128, capacitors 130
(C1), 132 (C2), 134 (C3) and battery voltage input 136.
[0024] As mentioned previously, the object of the invention is to
compare an appropriately scaled battery voltage to a stable
reference voltage. The reference voltage from the bandgap reference
is the sum of a V.sub.BE with a negative temperature coefficient
and a K.sub.1.DELTA.V.sub.BE with a positive temperature
coefficient, were K.sub.1 is a scaling factor chosen to balance the
negative and positive coefficient and .DELTA.V.sub.BE is the
difference in V.sub.BE between the two transistors with different
current densities. The sum of V.sub.BE and K.sub.1.DELTA.V.sub.BE
is equal to the silicon bandgap voltage or approximately 1.2
volts.
[0025] In essence, the object of the invention is carried out by
determining the sign of the expression
V.sub.BE+K.sub.1.DELTA.V.sub.BE-K.- sub.2V.sub.BAT. In evaluating
this expression it is possible to see whether V.sub.BAT is above or
below a minimum voltage V min. The value of the Vmin depends on
K.sub.2 and is approximately 1.2/K.sub.2 volts
[0026] In a switched capacitor circuit, the above mentioned three
voltages can be sampled on capacitors C1, C2 and C3 and then summed
to get the answer. The size of each capacitor controls the scaling
of each voltage and the accuracy of the scaling is normally a
magnitude better then if resistors are used. Another inherent
advantage of the present invention is that the typical 1.2 volt
bandgap reference voltage is actually never created, so in
principle the circuit will work for supply voltages below 1.2
volts. Further, there is not a problem in setting Vmin to be less
than 1.2 volts.
[0027] As discussed previously, a clock signal (not shown in FIG.
3) is required in the implementation of the detector circuit. The
clock is required to generate two phases or stages which are shown
in FIG. 3 as phase 1 (.PHI.1) and phase 2 (.PHI.2). During phase 1
or the pre-charge phase switches 110, 112, 114 and 116 are closed
while switches 118 and 120 are open. In phase 2 switches 110, 112,
114 and 116 are open while switches 118 and 120 are closed. All of
the above switches are shown in FIG. 1 as generic switches for
simplicity while it is known that these switches in a preferred
embodiment are actually MOS devices as shown in greater detail in
FIG. 5.
[0028] In pre-charge phase 1 switch 114 is closed so that current
from current source 128 flows through both transistors 124 and 126.
Hence both transistors are active and the positive input of
operational amplifier 122 will be biased at V.sub.BE also known as
V.sub.BElow. Since switch 116 is also closed the negative input of
operational amplifier will also be raised to V.sub.BElow. Since
switch 110 and 112 are closed and switch 120 open, capacitor 130
and capacitor 132 will be charged to V.sub.BElow+V.sub.offset where
V.sub.offset is the off set voltage in the operational amplifier
122. Capacitor 134 will be charged to V.sub.offset.
[0029] During the second stage or phase 2, switches 110, 112, 114
and 116 are opened while switches 118 and 120 are closed. Now all
of the current from source 128 flows through the transistor 126 and
this higher current density results in a higher base to emitter
voltage across transistor 126. The positive input of operational
amplifier 122 is now raised to V.sub.BE2 or V.sub.BEhigh. As
discussed previously the difference between V.sub.BEhigh and
V.sub.BElow is the .DELTA.V.sub.BE value. It has a value kT/qln
(N+1) when N is the relative emitter area of transistor 124.
[0030] When the battery voltage V.sub.BAT at input 136 is at the
threshold or trip point, V.sub.min the negative input of
operational amplifier 122 will be at a voltage equal to
V.sub.BEhigh plus V.sub.offset. Capacitor 130 is charged to
V.sub.BEhigh+V.sub.offset-V.sub.BAT while capacitor 132 and
capacitor 134 are charged to V.sub.BEhigh+V.sub.offset. The
transfer of charges between the capacitors when switching between
phase 1 and phase 2 can be written has:
C3V.sub.BEhigh+(C1+C2).DELTA.V.sub.BE-C1V.sub- .BAT. It will be
recognized that this is similar to the expression previously given
as V.sub.BE+K.sub.1.DELTA.V.sub.BE-K.sub.2V.sub.BAT. In this
expression the weighting factor K.sub.1 will be determined by
(C1+C2)/C3 and the factor K.sub.2 is given by C1/C3. It is
significant that the off set voltage in the operational amplifier
122 will not affect the above result as long as it does not change
when the reference voltage at the positive input changes from
V.sub.BElow to V.sub.BEhigh so a small and simple amplifier can be
used to save current and area.
[0031] At relatively low operating voltages MOS switches can have a
sufficient on resistance especially if they are biased at a voltage
between the supply voltages. This is not a problem in a switched
capacitor circuit as long as the time constants are much shorter
then the clock. But the switch in series with the transistor 124
will have a DC current and this will result in a voltage drop and
consequently reduced accuracy. A different topography may therefor
be used to switch transistor 124 on and off and this alternate
topography is shown in FIG. 4. In this implementation, two current
sources are provided and switch 114 is replaced with switches 142
and 144. In phase 1 switch 144 is closed and the switch 142 is open
and in phase 2 switch 142 is closed while switch 144 is open.
Therefore in phase 1 the base of transistor 124 is shorted to
ground and the transistor is active. In phase 2 the base of
transistor 124 is biased at 1V.sub.BE above ground. If the
transistors 126 and 140 are matched the value V.sub.BE is the same
voltage as the emitter voltage at transistor 124 so that V.sub.BE
is 0 volts and the transistor will be turned off. There is still
current flowing through switch 144 to ground during phase 1 but
only the base current. With the switch connected to ground the on
resistance will also be lower.
[0032] FIG. 5 is a detailed circuit diagram of the CMOS low voltage
detector circuit including the circuit topography of FIG. 4.
[0033] Although specific embodiments of the invention have been
described and illustrated it will be apparent to one skilled in the
art that numerous variations can be made without departing from the
basic concept. It is to be understood, however that such variations
will fall within the true scope of the invention as defined in the
appended claims.
* * * * *