Voltage monitoring system for a negative supply voltage

Abstract

A voltage monitoring circuit portion is arranged to monitor a negative supply voltage (V.sub.neg) and comprises a negative voltage generator arranged to generate the negative supply voltage (V.sub.neg) and to output the negative supply voltage (V.sub.neg) at an output terminal. A capacitor is arranged so that a first capacitor plate is connected to the output terminal of the generator and to a reference node via a potential divider. The potential divider is arranged to produce a monitor voltage (V.sub.monitor) between the resistors, where the reference node is supplied with a positive predetermined reference voltage (V.sub.ref). A comparator compares the monitor voltage (V.sub.monitor) to a threshold voltage (V.sub.ref_low) and to produce an output signal having a first value when the monitor voltage (V.sub.monitor) is below the threshold voltage (V.sub.ref_low) and having a second value otherwise. The negative voltage generator is enabled only when the output signal has its second value.

Claims

1. A voltage monitoring circuit portion arranged to monitor a negative supply voltage, said voltage monitoring circuit portion comprising: a negative voltage generator arranged to generate the negative supply voltage and to output said negative supply voltage at an output terminal of the negative voltage generator; a capacitor having a first capacitor plate and a second capacitor plate, said first capacitor plate being connected to the output terminal of the negative voltage generator, said first capacitor plate being further connected to a reference node via a potential divider comprising first and second resistors connected in series, wherein the potential divider is arranged to produce a monitor voltage at a monitor node between said first and second resistors and wherein the reference node is supplied with a positive predetermined reference voltage; and a comparator arranged to compare the monitor voltage to a threshold voltage and to produce an output signal having a first value when the monitor voltage is below the threshold voltage and having a second value when the monitor voltage is not below the threshold voltage; wherein the negative voltage generator is enabled only when the output signal has its second value and is not enabled only when the output signal has its first value; and wherein the second capacitor plate is connected to a first input of a sense comparator and to a second reference voltage, said sense comparator having a second input connected to a third reference voltage, said sense comparator being arranged to produce a sense signal having a first value when the voltage at its first input is above the third reference voltage and having a second value when the voltage at its first input is not above the third reference voltage.

2. The voltage monitoring circuit portion as claimed in claim 1, wherein the predetermined reference voltage provided at the reference node to which the potential divider is connected is a positive supply voltage.

3. The voltage monitoring circuit portion as claimed in claim 1, wherein the second reference voltage is the threshold voltage to which the monitor voltage is compared by the comparator.

4. The voltage monitoring circuit portion as claimed in claim 1, wherein the sense comparator has a power terminal thereof connected to a constant current source, wherein the sense comparator is always on.

5. The voltage monitoring circuit portion as claimed in claim 1, wherein the sense comparator consumes less current than the comparator arranged to compare to the monitor voltage to the threshold voltage.

6. The voltage monitoring circuit portion as claimed in claim 1, further comprising: a controller arranged to receive the output signal from the comparator and to receive the sense signal from the sense comparator; wherein the controller is arranged to produce a control signal having a first value when the sense signal has its respective first value and having a second value when the output signal has its respective first value; wherein the controller is further arranged to supply the control signal to the negative voltage generator to enable the negative voltage generator when the control signal has its first value and to disable the negative voltage generator when the control signal has its second value.

7. The voltage monitoring circuit portion as claimed in claim 6, wherein the comparator is arranged such that said comparator is enabled when the control signal produced by the controller has its respective first value and such that said comparator is disabled when the control signal produced by the controller has its respective second value.

8. The voltage monitoring circuit portion as claimed in claim 7, wherein the potential divider is disconnected from the comparator when the comparator is disabled.

9. The voltage monitoring circuit portion as claimed in claim 7, wherein the potential divider is disconnected from the reference node when the comparator is disabled.

10. The voltage monitoring circuit portion as claimed in claim 7, wherein the control signal comprises a start-up control signal and a shut-down control signal, wherein the controller is arranged to produce the start-up control signal when the sense signal has its respective first value and to produce the shut-down control signal when the output signal has its respective first value.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Certain embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings in which:

(2) FIG. 1 is a block diagram of a voltage monitoring circuit portion in accordance with an embodiment of the present invention;

(3) FIG. 2 is a state diagram illustrating the operation of the voltage monitoring circuit portion of FIG. 1; and

(4) FIG. 3 is a timing diagram illustrating the operation of the voltage monitoring circuit portion of FIG. 1.

DETAILED DESCRIPTION

(5) FIG. 1 is a block diagram of a voltage monitoring circuit portion 2 in accordance with an embodiment of the present invention. The voltage monitoring circuit portion 2 comprises: a negative voltage generator 4; a comparator 6; a sense comparator 8; a controller 10; first and second capacitors 12, 14; first and second resistors 16, 18; first, second, and third current sources 20, 22, 24; and eight switches 26, 28, 29, 30, 31, 32, 34, 36.

(6) The negative voltage generator 4 is arranged to produce a negative supply voltage V.sub.neg, which is used as a negative supply rail by other parts of the circuit (not shown), e.g. as the negative back bias voltage of an FDSOI device. This negative supply voltage V.sub.neg is supplied to a first capacitor plate of the first capacitor 12, which is also connected to one side of a potential divider constructed from the two resistors 16, 18. The other side of the potential divider formed from these resistors 16, 18 is connected to a reference voltage V.sub.ref via a switch 29, where the reference voltage V.sub.ref is known a priori, i.e. it is predetermined.

(7) At the midpoint of these resistors 16, 18 is a ‘monitor node’, which supplies a monitor voltage V.sub.monitor to one of the inputs of the comparator 6, via one of the switches 30. In this arrangement, the monitor voltage V.sub.monitor is connected to the inverting input of the comparator 6. The other input of the comparator 6, i.e. the non-inverting input, is connected to a low reference voltage V.sub.ref_low via a switch 31, where the low reference voltage V.sub.ref_low acts as a threshold voltage to which the comparator 6 compares the monitor voltage V.sub.monitor.

(8) The second capacitor plate of the first capacitor 12 is connected to a first input of the sense comparator 8, in this case the non-inverting input of the sense comparator 8. This second capacitor plate of the first capacitor 12 and the first input of the sense comparator 8 are also connected to the low reference voltage V.sub.ref_low via one of the switches 28. The other, i.e. inverting, input of the sense comparator 8 is connected to a first capacitor plate of the second capacitor 14, which is also connected to a high reference voltage V.sub.ref_high via one of the switches 26. The second capacitor plate of the second capacitor 14 is connected to ground GND.

(9) The comparator 6 is arranged to produce an output signal 38 which depends on the relative values of the monitor voltage V.sub.monitor and the low reference voltage V.sub.ref_low, i.e. the threshold voltage, where this output signal 38 is a ‘flag’ that may either be set to a first value (e.g. a binary ‘1’) or to a second value (e.g. a binary ‘0’), depending on whether the monitor voltage V.sub.monitor is below (i.e. is more negative than) the low reference voltage V.sub.ref_low or not respectively.

(10) Similarly, the sense comparator 8 is arranged to produce a sense signal 40 which depends on the relative values of the high reference voltage V.sub.ref_high (or a sampled version of the high reference voltage V.sub.ref_high present on the capacitor 14 when the corresponding switch 26 is open) and a voltage V.sub.pos which, as is explained in further detail below, depends on the low reference voltage V.sub.ref_low and the negative supply voltage V.sub.neg. It can be seen, however, that this voltage V.sub.pos is in the positive voltage domain. The sense signal 40 is a further ‘flag’ that may either be set to a first value (e.g. a binary ‘1’) or to a second value (e.g. a binary ‘0’), depending on whether the voltage V.sub.pos is above the sampled version of the high reference voltage V.sub.ref_high or not respectively.

(11) The output signal 38 and the sense signal 40 are both input to the controller 10, which produces a number of control signals 42, 44, 46. The first of these control signals 42 is supplied to the negative voltage generator 4 and is used to selectively enable and disable (i.e. switch on and off) the negative voltage generator 4. Switching on and off the negative voltage generator 4 switches on and off a charge pump, which produces the negative supply voltage V.sub.neg, and an oscillator within the negative voltage generator 4. The negative voltage generator 4 uses the high reference voltage V.sub.ref_high to produce the negative supply voltage V.sub.neg using the negative charge pump in a manner known in the art per se.

(12) The second control signal 44 is used to operate the switches 26, 28, 29, 30, 32, 34, 36. While all of these switches 26, 28, 29, 30, 32, 34, 36 are operated using the same control signal 44, it will be appreciated that typically the relative timings of enabling and disabling these components is typically important for system stability. As such, there is generally some degree of timing differences between when each component controlled by the controller 10 using the second control signal 44 is switched on and/or off in order to avoid undesirable race conditions.

(13) The third control signal 46 is used to enable and disable the various reference voltages used by the voltage monitoring circuit portion 2.

(14) The sense comparator 8 is powered by a relatively low power constant current source 22. The current produced by this current source 22 is generally on the scale of nanoamps. Conversely, the current source 20 that supplies current to the comparator 6, and the current source 24 that supplies current to the negative voltage generator 4 supply relatively large currents (when compared to the current supplied by the low power constant current source 22) but are not operated constantly, instead only supplying current to the comparator 6 and negative voltage generator 4 when the respective switches 36, 34 are closed.

(15) Operation of the voltage monitoring circuit portion 2 of FIG. 1 will now be described with reference to the accompanying state diagram of FIG. 2 and timing diagram of FIG. 3.

(16) Firstly, the voltage monitoring circuit portion 2 enters a hold/monitoring phase 102, during which the control signal 44 is set such that the switches 26, 28, 29, 30, 31, 32, 34, 36 are opened (i.e. not conducting). During this hold/monitoring phase 102, the inverting input of the sense comparator 8 is provided with the sampled version of the high reference voltage V.sub.ref_high held by the second capacitor 14. The value of V.sub.pos, which is provided to the non-inverting input of the sense comparator 8, is equal to the low reference voltage V.sub.ref_low plus the voltage change in the negative supply voltage V.sub.neg, where this change is caused by leakage current at the output node of the negative voltage supply 4, i.e. unwanted leakage current being drained from the negative supply rail due to e.g. temperature, process, and/or voltage variations. Ideally, the leakage of the second capacitor 14 is less than the leakage of V.sub.neg and V.sub.pos for correct operation.

(17) Subsequent to the hold/monitoring phase 102, the voltage monitoring circuit portion 2 enters a start-up and settling phase 104. As the leakage current of V.sub.neg node rises, the value of V.sub.pos rises above V.sub.ref_high, which sets the sense output 40 of the sense comparator 8 high, i.e. to a binary ‘1’. The sense output 40 being set causes the controller 10 to set the control signals 42, 44, 46 high, thus starting the references; closing the switches 26, 28, 29, 30, 31, 32, 34, 36; and starting the negative voltage generator 4 (including the charge pump and oscillator). Notably, the closing of the switches 29, 30, 31, 36 associated with the comparator 6 causes the comparator 6 to start monitoring the monitor voltage V.sub.monitor, as outlined below.

(18) During a subsequent sample and refresh phase 106, the negative charge pump within the negative voltage generator 4 acts to regulate Vneg to its set point, where a closed control loop is formed by the output V.sub.neg of the negative voltage generator 4 varying the monitored voltage V.sub.monitor, which in turn is used to determine when the charge pump is on, which drives the negative supply voltage V.sub.neg. The monitored voltage V.sub.monitor is set by the ratio of the resistors 16, 18 that form the potential divider and the difference between the predetermined reference voltage V.sub.ref and the negative supply voltage V.sub.neg, and provides a measure of the absolute value of the negative supply voltage V.sub.neg.

(19) Finally, a shut down and hold phase 108 is started once the negative supply voltage V.sub.neg returns to its desired value, as determined by the comparisons made by the comparator 6, i.e. because V.sub.monitor has dropped below V.sub.ref_low. The output signal 38 produced by the comparator 6 is set high (i.e. binary ‘1’), which causes the controller 10 to set the control signals 42, 44, 46 low, thus the references are turned off; the negative voltage generator 4 is turned off; and the switches 26, 28, 29, 30, 31, 32, 34, 36 are opened.

(20) Following the shut down and hold phase 108, operation returns to the hold/monitoring phase 102 and operation loops through the phases 102, 104, 106, 108 as outlined above.

(21) Thus the voltage monitoring circuit portion 2 may advantageously provide power savings by powering on the negative voltage generator 4 only when the sense comparator 8 determines that there has been a sufficiently significant change in the negative supply voltage V.sub.neg. The negative voltage generator 4 is then powered off when the comparator 6 determines that negative supply voltage V.sub.neg has reached its desired value (i.e. it is sufficiently negative). In other words, the output of the sense comparator 8 controls the turning on the negative voltage generator 4 and the comparator 6, while the output of the comparator 6 controls the turning off the negative voltage generator 4 and the comparator 6.

(22) Thus it will be appreciated by those skilled in the art that embodiments of the present invention provide an improved voltage monitoring circuit that ‘passively’ senses the state of the negative supply voltage without drawing any current from the output terminal of the generator as a result of the monitoring process, which may provide significant power savings compared to conventional voltage monitoring techniques. The monitoring of the negative supply voltage is advantageously carried out entirely within the positive supply voltage domain. Those skilled in the art will appreciate that the specific embodiments described herein are merely exemplary and that many variants within the scope of the invention are envisaged.