Feedback-controlled body-bias voltage source

Abstract

A body-bias voltage source having an output monitor, charge pump, and shunt. a shunt circuit having on/off control is coupled to the output monitor and to the output of the charge pump. Upon sensing that the output voltage of the charge pump is above a desired value, the output monitor may disable the charge pump circuit and may enable the shunt circuit to reduce the voltage at the output of the charge pump. When the voltage output of the charge pump is below the desired value, the output monitor may disable the shunt circuit and may enable the charge pump circuit. A shunt circuit having proportional control may be substituted for the shunt circuit with on/off control.

Description

FIELD OF THE INVENTION

Embodiments of the present invention relate to circuits for providing operational voltages in complementary metal-oxide semiconductor (CMOS) circuits. In particular, embodiments of the present invention relate to circuits for providing a body-bias voltage for CMOS transistors.

BACKGROUND ART

As the operating voltages for CMOS transistor circuits have decreased, variations in the threshold voltages for the transistors have become more significant. Although low operating voltages offer the potential for reduced power consumption, threshold voltage variations due to process and environmental variables often prevent optimum efficiency and performance from being achieved due to increased leakage currents.

Prior Art FIG. 1A shows a conventional CMOS inverter 100. A P-type substrate 105 supports an NFET 110 and a PFET 120. The NFET 110 comprises a gate 112, source 113, and drain 114. The PFET 120 resides in an n-well 115, and comprises a gate 122, drain 123, and a source 124. The substrate 105 and source 113 are coupled by a tie 130 that is connected to ground (GND), while source 124 and N-well 115 are coupled by a tie 135 that is connected to a supply voltage (V_DD). The input to the inverter is applied to the gates 112 and 122, with the output taken from the drain contact 125. In this conventional configuration, the transistors are often treated as three terminal devices.

Threshold voltage variations may be compensated for by body-biasing. Body-biasing introduces a reverse bias potential between the bulk and the source of the transistor that allows the threshold voltage of the transistor to be adjusted electrically. The purpose of body-biasing is to compensate for 1) process variations; 2) temperature variations; 3) supply voltage variations; 4) changes in frequency of operation; and 5) changing levels of switching activity.

Prior Art FIG. 1B shows an inverter having connections for body-biasing. Body-bias can provided to the PFET 120 through a direct bias contact 150a, or by a buried n-well 140 using contact 150b. Similarly, body-bias may be provided to the NFET 110 by a surface contact 155a, or by a backside contact 155b. An aperture 145 may be provided in the buried n-well 125 so that the bias potential reaches the NFET 110. In general, a PFET 120 or an NFET 110 may be biased by one of the alternative contacts shown.

Depending upon the environmental and operational conditions, a CMOS circuit may require different levels of bias for the transistors. For example, a microprocessor that is executing a computationally intensive routine for a real-time application will typically be biased for maximum speed, whereas during periods of low activity the bias will be adjusted to minimize leakage current.

For a CMOS integrated circuit, the load presented to a circuit providing a body-bias voltage and the bias circuit itself may vary with the environmental and operational conditions of integrated circuit. Thus, the variations in the required body-bias voltage and the load to which it is applied should be taken into account to achieve optimum performance.

SUMMARY OF INVENTION

Thus, a need exists for a system for providing a body-bias voltage for CMOS transistors that is capable of adapting to varying output voltage requirements and load conditions.

Accordingly, embodiments of the present invention provide a system that uses feedback controlled charge pump to establishing a desired output voltage. The system accepts an input reference voltage that is related to the desired output voltage in order to provide the desired output voltage.

In an embodiment of the present invention, a charge pump having a voltage output and an enable input for on/off control is coupled to an output monitor (e.g., a sense amplifier). The output monitor is coupled to the output of the charge pump and to the enable input of the charge pump. A shunt circuit having on/off control is coupled to the output monitor and to the output of the charge pump. Upon sensing that the output voltage of the charge pump is above a desired value, the output monitor may disable the charge pump circuit and may enable the shunt circuit to reduce the voltage at the output of the charge pump. When the voltage output of the charge pump is below the desired value, the output monitor may disable the shunt circuit and may enable the charge pump circuit.

In another embodiment similar to that described above, a shunt circuit having proportional control is substituted for the shunt circuit with on/off control. Upon sensing a deviation from a desired output value at the output of the charge pump, the output monitor provides a signal to the shunt circuit that is proportional to the deviation at the charge pump output. The effective resistance of the shunt is proportionally reduced in response to a positive deviation and proportionally increased in response to a negative deviation. Proportional control of the shunt circuit may be combined with on/off control of the charge pump circuit to regulate the output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention:

Prior Art FIG. 1A shows a conventional CMOS inverter without body-bias connections.

Prior Art FIG. 1B shows a conventional CMOS inverter with body-bias connections.

FIG. 2 shows a block diagram of a feedback controlled body-bias circuit in accordance with an embodiment of the present claimed invention.

FIG. 3 shows a circuit diagram of a body-bias supply with a servo loop for NFETs in accordance with an embodiment of the present claimed invention.

FIG. 4 shows a circuit diagram of a body-bias supply with a servo loop for PFETs in accordance with an embodiment of the present claimed invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the present invention, a feedback-controlled body-bias circuit, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one skilled in the art that the present invention may be practiced without these specific details. In other instances well known methods, procedures, components, and circuit elements have not been described in detail as not to unnecessarily obscure aspects of the present invention.

FIG. 2 shows a block diagram 200 of an embodiment of the present invention. A charge pump 210 has an output coupled to C_load that represents a substrate or well. Since body-bias is typically applied as a reverse bias to a p-n junction within a CMOS device, the load seen by the body-bias voltage source is generally a capacitive load; however, there is a certain amount of leakage current, represented by R_leak.

An output monitor 205 has a sense input coupled to the output of the charge pump 210. The output of the charge pump is compared to a reference voltage V_ref by the output monitor 205. Upon sensing a positive or negative deviation (overvoltage or undervoltage) that exceeds an allowed value, the output monitor provides a control signal to the charge pump circuit 210 and/or a shunt circuit 215.

For an overvoltage condition with loads having a large C_load and large R_leak (small leakage current), simply turning off the charge pump may not result in a sufficiently fast discharge of C_load to the desired value. Accordingly, the shunt 215 may be enabled to provide a discharge path that allows faster correction of the output voltage V_out.

Upon sensing an undervoltage condition, the output monitor 205 may enable the charge pump circuit 210 and/or disable the shunt circuit 215. In one embodiment, the charge pump is run continuously, with the shunt being cycled between enabled and disabled states to maintain the output voltage.

In determining the voltage deviation that is permitted in the system, a deadband having upper and lower control points may be used, or a single setpoint may be used (no allowable deviation).

In an alternative embodiment, the output monitor 205 provides a proportional signal to the shunt circuit 215 that is proportional instead of the on/off control described above. The effective resistance of the shunt is proportionally reduced in response to a positive deviation and proportionally increased in response to a negative deviation. Proportional control is preferably implemented using analog circuits, and thus is suitable for use in a mixed-signal integrated circuit.

FIG. 3 shows a circuit diagram 300 of a body-bias supply with a servo loop for NFETs in accordance with an embodiment of the present claimed invention. The current source 305 and variable resistor R combine to provide a reference voltage (e.g., V_ref of FIG. 2). The comparator 310, shunt 320, and charge pump 315 correspond to the output monitor 205, shunt 215, and charge pump 210 of FIG. 2. The output of charge pump 315 is a negative voltage that may be used to bias a P-type substrate or well to provide a body-bias for NFETs.

FIG. 4 shows a circuit diagram 400 of a body-bias supply with a servo loop for PFETs in accordance with an embodiment of the present claimed invention. The current sink 405 and variable resistor R combine to provide a reference voltage (e.g., V_ref of FIG. 2). The comparator 410, shunt 420, and charge pump 415 correspond to the output monitor 205, shunt 215, and charge pump 210 of FIG. 2. The output of charge pump 315 is a positive voltage that may be used to bias an N-type substrate or well to provide a body-bias for PFETs.

A description of the circuits shown in FIG. 3 and FIG. 4 is provided in the previously incorporated copending patent application entitled “Servo Loop for Well Bias Voltage Source” (U.S. Pat. No. 7,129,771). More specifically, descriptions of the variable resistor R and shunt (320, 420) shown in FIG. 3 and FIG. 4 are provided in the previously incorporated copending patent applications entitled “A Precise Control Component for a Substrate Potential Regulation Circuit” and “A Charge Stabilizing Component for a Substrate Potential Regulation Circuit” (U.S. Pat. No. 7,012,461).

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, an integrated circuit having a P-type substrate and an N-well disposed therein is described. More generally, the invention may be used with a semiconductor substrate of either N-type or P-type having a complementary well disposed therein. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims

1. A system comprising: first circuitry operable for generating an output voltage;second circuitry coupled to said first circuitry and operable for adjusting said output voltage in response to a control signal by providing a discharge path for said output voltage if enabled; andan output monitor coupled to said first circuitry and operable for comparing said output voltage and a reference voltage and operable for generating said control signal based on a result of said comparing, wherein said control signal has a state that is proportional to an amount of difference between said output voltage and said reference voltage, wherein said output monitor is further operable for enabling said first circuitry in response to sensing an undervoltage condition, and wherein further an amount of voltage discharged via said second circuitry is configured to be proportionally controlled according to said state of said control signal.

2. The system of claim 1 wherein said first circuitry comprises a charge pump.

3. The system of claim 1 wherein said second circuitry comprises a shunt circuit.

4. The system of claim 1 wherein an effective resistance of said second circuitry, if enabled, is configured to be proportionally controlled according to said state of said control signal.

5. The system of claim 1 wherein said output monitor is further operable for enabling said second circuitry to provide said discharge path for said output voltage in response to sensing an overvoltage condition.

6. The system of claim 1 wherein said output monitor is further operable for disabling said second circuitry in response to sensing said undervoltage condition.

7. The system of claim 1 wherein said output monitor is further operable for cycling said second circuitry between enabled and disabled states in response to sensing an undervoltage condition if said first circuitry is enabled.

8. The system of claim 1 wherein said first circuitry is coupled to a P-type well, and wherein said output voltage comprises a negative voltage usable as a body bias voltage to bias said P-type well.

9. The system of claim 1 wherein said first circuitry is coupled to an N-type well, and wherein said output voltage comprises a positive voltage usable as a body bias voltage to bias said N-type well.

10. The system of claim 1 further comprising a current source coupled to a variable resistor, wherein said current source and said variable resistor are operable for providing said reference voltage.

11. A device comprising: a well disposed in a substrate;first circuitry operable for generating a body bias voltage for biasing said well;an output monitor coupled to said first circuitry and operable for comparing said body bias voltage and a reference voltage and operable for generating a control signal based on a result of said comparing, wherein said control signal is proportional to an amount of difference between said output voltage and said reference voltage; andsecond circuitry coupled to said first circuitry and operable for adjusting said body bias voltage in response to said control signal, wherein said output monitor is further operable for enabling said first circuitry in response to sensing an undervoltage condition, and wherein further an amount of voltage discharged via said second circuitry is configured to be proportionally controlled according to said state of said control signal.

12. The device of claim 11 wherein said first circuitry comprises a charge pump and said second circuitry comprises a shunt circuit.

13. The device of claim 11 wherein said output monitor is further operable for enabling said second circuitry to provide a discharge path for said body bias voltage in response to sensing an overvoltage condition.

14. The device of claim 11 wherein said output monitor is further operable for disabling said second circuitry in response to sensing said undervoltage condition.

15. The device of claim 11 wherein said output monitor is further operable for cycling said second circuitry between an enabled state and a disabled state in response to sensing an undervoltage condition if said first circuitry is enabled, and wherein said second circuitry, if enabled, provides a discharge path for said body bias voltage.

16. A method comprising: sensing a first voltage at an output monitor coupled to an integrated circuit;sensing a reference second voltage at said output monitor;generating a control signal based on a comparison of said first voltage and said second voltage, wherein said control signal has a state that is proportional to an amount of difference between said first voltage and said second voltage; andadjusting said first voltage in response to said control signal via a discharge path that shunts said first voltage if enabled; andenabling a source of said first voltage in response to sensing an undervoltage condition, and wherein further an amount of voltage discharged via said second circuitry is configured to be proportionally controlled according to said state of said control signal.

17. The method of claim 16 further comprising: enabling said discharge path for said first voltage in response to sensing an overvoltage condition; andvarying an effective resistance of said discharge path according to said state of said control signal.

18. The method of claim 16 further comprising disabling said discharge path for said first voltage in response to sensing an undervoltage condition.

19. The method of claim 16 further comprising cycling said discharge path for said first voltage between an enabled state and a disabled state in response to sensing an undervoltage condition if a source of said first voltage is enabled, and wherein, in said enabled state, an effective resistance of said discharge path is varied according to said state of said control signal.

20. The method of claim 16 wherein said first voltage comprises a negative voltage usable as a body bias voltage to bias a P-type well of said integrated circuit.

21. The method of claim 16 wherein said first voltage comprises a positive voltage usable as a body bias voltage to bias an N-type well of said integrated circuit.