Circuit and method for preventing reverse current flow into a voltage regulator from an output thereof

Abstract

A circuit and method are provided for interrupting current flow into a voltage regulator from an output thereof when a voltage source (Vpwr) drops below an output voltage (Vout). In one embodiment, the circuit comprises: (i) a comparator supplied by Vout including an output and inputs coupled to Vpwr and Vout; and (ii) transistors coupled to and controlled by the comparator, including a first transistor configured to interrupt a first current path extending between Vout and Vpwr through an output-leg of the regulator when Vpwr drops below Vout. Preferably, the regulator includes a reference-leg and a feedback-circuit coupling Vout thereto, and the first transistor also interrupts a second current path between Vout and Vpwr through the feedback-circuit and reference leg. More preferably, the reference-leg comprises resistors through which it is coupled to ground, and the transistors include a second transistor to interrupt a third current path between Vout and ground.

Description

TECHNICAL FIELD

The present invention relates generally to voltage regulators, and more particularly to a circuit and method to substantially prevent or interrupt reverse current flow into a voltage regulator from an output thereof.

BACKGROUND OF THE INVENTION

Voltage regulator circuits or voltage regulators are widely used in many applications to provide a nearly constant output voltage at a desired level that is substantially independent of a poorly specified and often fluctuating input voltage and output conditions (i.e., variation in a load current).

One type of voltage regulator is a replica voltage regulator. In a replica voltage regulator a voltage established in one portion or one leg of a circuit is replicated in another leg or portion of the circuit, typically by larger sized devices, to provide a desired load or output voltage. The output voltage is regulated by having it track the voltage in the first leg or portion as closely as possible.

An example of an output stage of a replica voltage regulator architecture for which a circuit and method of the present invention is particularly useful is shown in FIG. 1. Referring to FIG. 1A, the voltage regulator 100 includes a reference leg 102 coupled between a voltage source (V_pwr) and ground 104, and an output leg 106 coupled between V_pwr and an output node (V_out). The reference leg 102 includes a first transistor 108 connected as a source follower (SF) and including a gate node (V_gate) coupled to and controlled by for example an operational amplifier or a charge pump (not shown) in the voltage regulator 100, and an output node (V_source) coupled to ground 104 through a series resistor network 110. The output leg 106 includes a second larger transistor 112, also connected as a source follower and controlled by the gate node (V_gate) of the first transistor 108. The voltage regulator 100 further includes a small feedback resistor (R_f 114) coupling the output nodes of the first transistor 108 (V_source) and the second transistor 112 (V_out) to improve the accuracy and stability of the regulator. The first and the second transistors 108, 112 are selected so that the output voltage V_out is a replica of the V_source voltage. A ratio between resistors R₁ and R₂ in the series resistor network 110 is selected so that V_source is equal to the desired target voltage—that is it is the same as the desired V_out.

In normal operation V_pwr is greater than V_out and current flows through the reference leg 102, indicated by arrows 116, generating the desired target voltage at the output node of the first transistor 108 (V_source), which is then replicated at the output node of the second transistor 112 (V_out). Current, indicated by arrows 118 and 120, flows from the sources (V_source and V_out) of the first and the second transistors 108, 112 to the output node (V_out) of the voltage regulator 100.

Although the above described circuit provides a simple architecture that occupies a small area on a silicon die or substrate, it is not wholly satisfactory for a number of reasons. In particular, referring to FIG. 1B, when V_pwr goes lower than the output voltage (V_out) of the voltage regulator 100, the source potential (V_source and V_out) of the first and the second transistors 108, 112 becomes higher than the drain potential (V_pwr) causing reverse currents, indicated by arrows 122 and 124, to flow from the source to the drain of the source follower transistors. Yet another leakage path allows a reverse current, indicated by arrow 126, to flow from V_out through the feedback resistor (R_f) 114 and the resistor network 110. The sum of these reverse currents can be substantial, on the order of several milliamps (mA), and can induce a drop or droop in the output voltage (V_out) and will quickly discharge batteries in battery operated devices.

Accordingly, there is a need for a circuit and method that substantially prevents or interrupts a reverse current flow into a voltage regulator and the resultant droop in output voltage when a voltage of the voltage source (V_pwr) drops below a voltage at the output of the voltage regulator (V_out). It is further desirable that the circuit and method substantially not effect performance of the voltage regulator under normal operating conditions, i.e., when V_pwr is greater than V_out.

SUMMARY OF THE INVENTION

The present invention provides a solution to these and other problems, and offers further advantages over conventional voltage regulators and methods of operating the same.

In one aspect, the present invention is directed to a circuit for interrupting current flow into a voltage regulator from an output of the voltage regulator. The circuit comprises: (i) a comparator including an output, an input coupled to a voltage source, and an input coupled to the output of the voltage regulator; and (ii) a number of transistors coupled to the output of the comparator and controlled thereby. Generally, the number of transistors include a first transistor configured to interrupt a first current path extending between the output of the voltage regulator and the voltage source through an output leg of the voltage regulator when a voltage of the voltage source (V_pwr) drops below a voltage at the output of the voltage regulator (V_out). The comparator is powered by the output of the voltage regulator (V_out) rather than the voltage source (V_pwr) to avoid a varying or dropping V_pwr from adversely effecting operation of the comparator.

Preferably, the voltage regulator is a replica voltage regulator further including a reference leg and a feedback circuit coupling V_out to the reference leg, and the first transistor is also configured to interrupt a second current path extending between the output of the voltage regulator and the voltage source through the feedback circuit and at least partially through reference leg. More preferably, the reference leg further includes a resistor network through which the feedback circuit is coupled to a circuit ground, and the number of transistors include a second transistor configured to interrupt a third current path extending between the output of the voltage regulator and circuit ground through the feedback circuit and the resistor network when V_pwr drops below V_out.

In certain embodiments, the output leg includes a first source follower (SF) transistor in the first current path and the reference leg includes a second SF transistor in the second current path, and the first transistor is configured to pull gate nodes of the first and second SF transistors to a circuit ground when V_pwr drops below V_out.

In other embodiments, the comparator is also configured to signal a device comprising or coupled to the voltage regulator when V_pwr drops below V_out.

In another aspect, the present invention is directed to a method of operating a voltage regulator to interrupt current flow into the voltage regulator from an output thereof. Generally, the method including steps of: (i) a comparing V_pwr of a voltage source coupled to the voltage regulator to V_out at the output of the voltage regulator; and (ii) controlling a number of transistors to substantially prevent current from flowing from the output of the voltage regulator into the voltage regulator when V_pwr drops below V_out. Preferably, the voltage regulator is a replica voltage regulator comprising a reference leg and an output leg, and the method includes the step of interrupting a first current path extending between the output of the voltage regulator and the voltage source through an output leg of the voltage regulator when V_pwr drops below V_out. More preferably, the voltage regulator further comprises a feedback circuit coupling V_out to the reference leg, and wherein the method further includes the step of interrupting a second current path extending between the output of the voltage regulator and the voltage source through the feedback circuit and at least partially through reference leg when V_pwr drops below V_out.

In certain embodiments, the output leg comprises a first SF transistor in the first current path and the reference leg comprises a second SF transistor in the first current path, and the steps of interrupting the first and second current paths include the steps of pulling gate nodes of the first and second SF transistors to a circuit ground when V_pwr drops below V_out. Preferably, the reference leg further comprises a resistor network through which the feedback circuit is coupled to circuit ground, and the method further includes the step of interrupting a third current path extending between the output of the voltage regulator and circuit ground through the feedback circuit and the resistor network when V_pwr drops below V_out.

In other embodiments, the method can further include the step of signaling a device comprising or coupled to the voltage regulator when V_pwr drops below V_out.

BRIEF DESCRIPTION OF THE DRAWINGS

These and various other features and advantages of the present invention will be apparent upon reading of the following detailed description in conjunction with the accompanying drawings and the appended claims provided below, where:

FIG. 1A is a simplified schematic diagram illustrating current flow in an output stage of a voltage regulator when a power supply is greater than the output voltage for which a circuit and method of the present invention is particularly useful;

FIG. 1B is a simplified schematic diagram of the voltage regulator of FIG. 1A illustrating current flow into the voltage regulator when a power supply voltage drops below an output voltage;

FIG. 2 is a schematic diagram an output stage of a voltage regulator including a circuit to substantially prevent or interrupt reverse current flow into the voltage regulator from an output thereof according to an embodiment of the present invention;

FIG. 3 is a flowchart of a method according to an embodiment of the present invention for operating a voltage regulator to substantially prevent or interrupt reverse current flow into the voltage regulator from an output thereof; and

FIG. 4 are graphs illustrating the ability of a circuit according to the present invention to substantially prevent or interrupt reverse current flow into the voltage regulator from an output thereof.

DETAILED DESCRIPTION

The present invention is directed to a circuit and method for interrupting or substantially preventing reverse current flow into an output of a voltage regulator when a voltage of a voltage source of the voltage regulator drops below a voltage at the output of the voltage regulator.

The voltage regulator and method of the present invention are particularly useful in battery operated devices, such as a wireless computer mouse and other like devices, which include integrated voltage regulators.

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description.

Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. The term “to couple” as used herein may include both to directly connect and to indirectly connect through one or more intervening components.

Briefly, the circuit of the present invention includes a comparator including an output, an input coupled to a voltage source, and an input coupled to the output of the voltage regulator, and a number of transistors coupled to the output of the comparator to interrupt or substantially prevent current from flowing from the output of the voltage regulator into the voltage regulator when a voltage of a voltage source (V_pwr) of the voltage regulator drops below a voltage at the output of the voltage regulator (V_out).

The circuit and methods for operating the same according to various embodiments of the present invention will now be described in detail with reference to FIG. 2.

FIG. 2 shows a schematic view of an output stage of a voltage regulator 200 including an interrupt circuit or circuit 202 for interrupting or substantially preventing reverse current flow into an output 204 of the regulator when V_pwr drops below V_out according to an embodiment of the present invention. For purposes of clarity, many of the details of integrated circuit (IC) design in general and voltage regulators in particular that are widely known and are not relevant to the present invention have been omitted from the following description.

In the embodiment shown the voltage regulator 200 is a replica type voltage regulator and includes a reference leg 206 coupled between V_pwr and ground 208, and an output leg 210 coupled between V_pwr and the output node 204. The reference leg 206 includes a first transistor 212 connected as a source follower (SF) and including a gate node (V_gate) coupled to and controlled by an operational amplifier (OPAMP) or a charge pump and an output node (V_source) coupled to ground 208 through a series resistor network 214. The output leg 210 includes a second larger transistor 216, also connected as a source follower and controlled by the gate node (V_gate) of the first transistor 212. The voltage regulator 200 further includes a small a feedback resistor (R_f 218) coupling the output nodes of the first transistor 212 (V_source) and the second transistor 216 (V_out) to improve the accuracy and stability of the regulator. The first and the second transistors 212, 216 are selected so that the output voltage V_out is a replica of the V_source voltage. A ratio between resistors R₁ and R₂ in the series resistor network 214 is selected so that V_source is equal to the desired target voltage—that is it is the same as the desired V_out. In normal operation current, indicated by arrow 219, flows from the sources of the first and the second transistors 212, 216 to the output node 204 of the voltage regulator 200.

Referring to FIG. 2 in one embodiment the interrupt circuit 202 comprises a comparator 220 powered by the output node 204 (V_out) and including a non-inverting input 222 coupled to a filtered voltage from the voltage source (V_{pwr—filtered}) and an inverting input 224 coupled to a filtered voltage from the output of the voltage regulator (V_{out—filtered}). By filtered voltage it is meant the voltage is processed to attenuate or remove completely in unwanted variation or ripple in the voltage applied to the filter. Filtering can be accomplished by any known filter circuit (not shown) including, for example, an active or passive filter, such as a RC-filter. The comparator 220 further includes an output 226 coupled to a number of transistors configured or adapted to substantially prevent current from flowing from the output 204 into the voltage regulator 200 when V_pwr drops below V_out.

The number of transistors include a first transistor 228 configured to interrupt a first and second current paths extending between the output 204 of the voltage regulator 200 and the voltage source through the reference leg 206 and output leg 210 when V_pwr drops below V_out. Preferably, as in the embodiment shown, the first transistor is a leaker transistor configured to pull gate nodes of the first and second SF transistors 212, 216 to ground 208 when V_pwr drops below V_out. More preferably, the number of transistors include an inverter 232 and a second, normally closed switching transistor 230 configured to interrupt a third current path extending between the output 204 of the voltage regulator 200 and circuit ground through the feedback resistor 218 and the resistor network 214 when V_pwr drops below V_out.

A method or sequence of operating the circuit of FIG. 2 according to an embodiment of the present invention will now be described with reference to FIG. 3. FIG. 3 is a flowchart of a method according to an embodiment of the present invention for operating a voltage regulator to interrupt current flow into the voltage regulator from an output thereof. The method begins with comparing V_pwr of a voltage source coupled to the voltage regulator to V_out at the output of the voltage regulator (step 302). Next, a leaker transistor is controlled or operated to couple or pull a gate node of a first SF transistor in an output leg of the voltage regulator to a circuit ground when V_pwr drops below V_out (step 304). The leaker transistor is also operated to pull a gate node of a second SF transistor in a reference leg of the voltage regulator to circuit ground when V_pwr drops below V_out (step 306). A switching transistor is operated to open a current path coupling the reference leg to circuit ground when V_pwr drops below V_out (step 308). As indicated by the flowchart of FIG. 3 the steps of pulling gate nodes of the first and second SF transistors to ground, step 304 and 306 respectively, and operating the switching transistor, step 308, are performed at substantially the same time. In certain embodiments, as shown above, the switching transistor is in connected in series with a resistor network in the reference leg.

Optionally or preferably, the method can further include the step of signaling a device comprising or coupled to the voltage regulator when V_pwr drops below V_out (step 310). More preferably, the signaling step, step 310, is performed at substantially the same time as steps 304, 306 and 308.

The ability of a circuit and method according to the present invention to interrupt or substantially prevent reverse current into a voltage regulator from an output thereof when a power supply voltage drops below an output voltage will now be illustrated with reference to the graphs of FIG. 4. In particular, FIG. 4 includes four separate graphs illustrating exemplary inputs to and outputs from the circuits of FIG. 2. Line 402 in the top graph, labeled V_pwr and V_out (V), illustrates a voltage of the voltage source (V_pwr) and line 404 the voltage at the output of the voltage regulator (V_out). Line 406 in the second graph from the top, labeled Comparator output (V), illustrates a change in the output of the comparator (comparator 220 in FIG. 2) as V_pwr drops below V_out. Line 408 in the third graph, labeled Gate of SF (V), illustrates a voltage to the gate nodes of the first and second source followers (transistors 228 and 230 in FIG. 2). Line 410 in the fourth and final graph, labeled Current (A), illustrates the current flow through the output of the voltage regulator in milliamps (mA).

Referring to the graphs of FIG. 4 it seen that initially, at time 1.09 milliseconds (mS), V_pwr (line 402) is equal to 5.0 V, the comparator output (line 406) is 0V, the voltage applied to the gate nodes of the first and second source followers (line 408) is equal to about 4.5 V to provide a regulated output voltage V_out (line 404) of about 3.3 V and a current out of the voltage regulator of about +1.2 to about +1.4 mA. At about time T equal 1.10 mS V_pwr (line 402) begins dropping and current flow out of the voltage regulator (line 410) quickly drops to about 0 mA at time (T) equal 1.105 mS. V_pwr (line 402) continues to droop and at about T equal 1.132 mS drops below V_out (line 404). Immediately or soon thereafter at about T equal 1.138 mS the comparator output (line 406) goes high to about 3V operating the leaker transistors (transistor 228 in FIG. 2) to couple the gate nodes of the first and second source followers (line 408) to ground. As shown by line 410 in the bottom graph current flow out of the voltage regulator quickly settles at about 0 mA at T 1.14 mS after a brief dip (reverse current flow) indicated by dashed line 412 peaking at less than about −0.6 mA.

The advantages of the circuit and method of the present invention over previous or conventional systems and methods include: (i) interrupting or substantially preventing reverse current flowing into the voltage regulator when V_pwr drops below V_out; (ii) substantially preventing any voltage drop or droop in the output voltage when V_pwr drops below V_out; (iii) ability to signal a device comprising or coupled to the voltage regulator when V_pwr drops below V_out; (iv) increasing battery life time in battery operated devices, such as a wireless computer mouse and other like devices, by interrupting or substantially preventing reverse current flowing into the voltage regulator, which can quickly drain the battery; and (v) having substantially no impact on the performance of the voltage regulator in normal operating mode.

The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been described and illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications, improvements and variations within the scope of the invention are possible in light of the above teaching. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents. The scope of the present invention is defined by the claims, which includes known equivalents and unforeseeable equivalents at the time of filing of this application.

Claims

1. A circuit for interrupting current flow into a voltage regulator from an output of the voltage regulator, the circuit comprising: a comparator including an output, an input coupled to a voltage source, and an input coupled to the output of the voltage regulator; anda number of transistors coupled to the output of the comparator and controlled thereby, the number of transistors including a first transistor configured to interrupt a first current path extending between the output of the voltage regulator and the voltage source through an output leg of the voltage regulator when a voltage of the voltage source (Vpwr) drops below a voltage at the output of the voltage regulator (Vout).

2. A circuit according to claim 1, wherein the voltage regulator is a replica voltage regulator further comprising a reference leg and a feedback circuit coupling Vout to the reference leg, and wherein the first transistor is further configured to interrupt a second current path extending between the output of the voltage regulator and the voltage source through the feedback circuit and at least partially through reference leg when Vpwr drops below Vout.

3. A circuit according to claim 2, wherein the output leg comprises a first source follower (SF) transistor in the first current path, and wherein the first transistor is a leaker transistor configured to pull a gate node of the first SF transistor to a circuit ground when Vpwr drops below Vout.

4. A circuit according to claim 3, wherein the reference leg comprises a second SF transistor in the second current path, and wherein the first transistor is further configured to pull a gate node of the second SF transistor to circuit ground when Vpwr drops below Vout.

5. A circuit according to claim 4, wherein the reference leg further comprises a resistor network through which a source of the second SF transistor and the feedback circuit is coupled to circuit ground, and wherein the number of transistors include a second transistor configured to interrupt a third current path extending between the output of the voltage regulator and circuit ground through the feedback circuit and the resistor network when Vpwr drops below Vout.

6. A circuit according to claim 1, wherein the comparator is configured to signal a device comprising or coupled to the voltage regulator when Vpwr drops below Vout.

7. A circuit according to claim 1, wherein the comparator is powered by the output of the voltage regulator (Vout).

8. A method for interrupting current flow into a voltage regulator from an output thereof, the method comprising steps of: comparing a voltage (Vpwr) of a voltage source coupled to the voltage regulator to a voltage (Vout) at the output of the voltage regulator; andcontrolling a number of transistors to substantially prevent current flowing from the output of the voltage regulator into the voltage regulator when Vpwr drops below Vout.

9. A method according to claim 8, wherein the voltage regulator is a replica voltage regulator comprising a reference leg and an output leg, and wherein the method comprises the step of interrupting a first current path extending between the output of the voltage regulator and the voltage source through an output leg of the voltage regulator when Vpwr drops below Vout.

10. A method according to claim 9, wherein the voltage regulator further comprises a feedback circuit coupling Vout to the reference leg, and wherein the method further comprises the step of interrupting a second current path extending between the output of the voltage regulator and the voltage source through the feedback circuit and at least partially through reference leg when Vpwr drops below Vout.

11. A method according to claim 10, wherein the output leg comprises a first source follower (SF) transistor in the first current path and the reference leg comprises a second SF transistor in the first current path, and wherein the steps of interrupting the first and second current paths comprise the steps of pulling gate nodes of the first and second SF transistors to a circuit ground when Vpwr drops below Vout.

12. A method according to claim 11, wherein the reference leg further comprises a resistor network through which a source of the second SF transistor and the feedback circuit is coupled to circuit ground, and wherein the method further comprises the step of interrupting a third current path extending between the output of the voltage regulator and circuit ground through the feedback circuit and the resistor network when Vpwr drops below Vout.

13. A method according to claim 8, further including the step of signaling a device comprising or coupled to the voltage regulator when Vpwr drops below Vout.

14. A voltage regulator comprising: a comparator including an output, a non-inverting input coupled to a voltage source and an inverting input coupled to an output of the voltage regulator; anda number of transistors coupled to the output of the comparator and controlled thereby to substantially prevent current from flowing from the output of the voltage regulator into the voltage regulator when a voltage of a voltage source (Vpwr) of the voltage regulator drops below a voltage at the output of the voltage regulator (Vout).

15. A voltage regulator according to claim 14, wherein the voltage regulator is a replica voltage regulator comprising a reference leg and an output leg, and wherein the number of transistors include a first transistor configured to interrupt a first current path extending between the output of the voltage regulator and the voltage source through an output leg of the voltage regulator when Vpwr drops below Vout.

16. A voltage regulator according to claim 15, wherein the voltage regulator further comprises a feedback circuit coupling Vout to the reference leg, and wherein the first transistor is further configured to interrupt a second current path extending between the output of the voltage regulator and the voltage source through the feedback circuit and at least partially through reference leg when Vpwr drops below Vout.

17. A voltage regulator according to claim 16, wherein the output leg comprises a first source follower (SF) transistor in the first current path, and wherein the first transistor is a leaker transistor configured to pull a gate node of the first SF transistor to a circuit ground when Vpwr drops below Vout.

18. A voltage regulator according to claim 17, wherein the reference leg comprises a second SF transistor in the second current path, and wherein the first transistor is further configured to pull a gate node of the second SF transistor to circuit ground when Vpwr drops below Vout.

19. A voltage regulator according to claim 18, wherein the reference leg further comprises a resistor network through which the feedback circuit coupling Vout to the reference leg is coupled to electrical ground, and wherein the number of transistors include a second transistor configured to interrupt a third current path extending between the output of the voltage regulator and circuit ground through the feedback circuit and the resistor network when Vpwr drops below Vout.

20. A voltage regulator according to claim 13, wherein the comparator is configured to signal a device comprising or coupled to the voltage regulator when Vpwr drops below Vout.