Calibration of driver output current

1. A method for driving an output node with a constant current, the method comprising:

driving a control node of an output device coupled to the output node according to an input signal and using a fixed regulated voltage and a variable regulated voltage;

generating the fixed regulated voltage based on a first power supply voltage, a second power supply voltage, and a first reference voltage;

generating the variable regulated voltage based on the first power supply voltage, the second power supply voltage, and a second reference voltage; and

generating the second reference voltage based on the first power supply voltage, the second power supply voltage, a reference current, and a predetermined target voltage level of the control node of the output device.

2. The method as recited in claim 1 wherein generating the second reference voltage comprises periodically calibrating the second reference voltage.

3. The method as recited in claim 2 wherein periodically calibrating the second reference voltage comprises:

after the first power supply voltage stabilizes, providing a replica current to a replica device by downscaling the reference current by a predetermined amount, the replica device being a version of the output device downscaled by the predetermined amount; and

after a control voltage of the replica device stabilizes, sampling the control voltage of the replica device using the replica current.

4. The method as recited in claim 2 wherein generating the second reference voltage further comprises enabling periodic calibration after a delay sufficient to damp ringing of the first power supply voltage in response to a change in state of the input signal from a first level to a second level, thereby synchronizing the calibrating to the input signal.

5. The method as recited in claim 1 wherein generating the variable regulated voltage further comprises:

maintaining a control voltage of a replica device at a level that corresponds to the predetermined target voltage level of the control node of the output device, the replica device being a version of the output device downscaled by a predetermined amount; and

boosting the control voltage by the predetermined amount to generate the second reference voltage.

6. The method as recited in claim 1 further comprising:

driving a second control node of a second output device coupled to the output node according to the input signal and using a second fixed regulated voltage and a second variable regulated voltage, the second output device being complementary to the output device;

generating the second fixed regulated voltage based on the first power supply voltage, the second power supply voltage, and a third reference voltage;

generating the second variable regulated voltage based on the first power supply voltage, the second power supply voltage, and a fourth reference voltage; and

generating the fourth reference voltage based on the first power supply voltage, the second power supply voltage, the reference current, and a second predetermined target voltage level of the control node of the output device.

7. The method as recited in claim 6 wherein the output node is coupled between a drain terminal of the output device and a second drain terminal of the second output device.

8. The method as recited in claim 1 wherein driving the control node comprises:

driving an intermediate signal according to the input signal and using the fixed regulated voltage; and

driving the control node according to the intermediate signal and using the variable regulated voltage.

9. An integrated circuit comprising:

an output node;

an output device of a first type coupled between the output node and a first power supply node;

a pre-driver circuit coupled to the first power supply node and configured to drive a control node of the output device according to an input signal and using a fixed regulated voltage and a variable regulated voltage;

a fixed voltage regulator configured to generate the fixed regulated voltage based on a first power supply voltage, a second power supply voltage, and a first reference voltage level;

a variable voltage regulator configured to generate the variable regulated voltage based on the first power supply voltage, the second power supply voltage, and a second reference voltage; and

a reference voltage generator configured to generate the second reference voltage based on the first power supply voltage, the second power supply voltage, a reference current, and a predetermined target voltage level of the control node of the output device.

10. The integrated circuit as recited in claim 9 wherein the reference voltage generator is configured to periodically calibrate the second reference voltage.

11. The integrated circuit as recited in claim 9 wherein the reference voltage generator comprises a replica device, the replica device being a version of the output device downscaled by a predetermined amount.

12. The integrated circuit as recited in claim 11 wherein the reference voltage generator further comprises a replica current generator selectively coupled to the replica device and configured to generate a replica current based on the reference current and the predetermined amount.

13. The integrated circuit as recited in claim 12 wherein the reference voltage generator further comprises a buffer configured to boost a control voltage on a second control node of the replica device by the predetermined amount to generate the second reference voltage.

14. The integrated circuit as recited in claim 13 wherein the reference voltage generator further comprises a sampling capacitor coupled to the buffer and selectively coupled to the second control node of the replica device.

15. The integrated circuit as recited in claim 14 wherein the reference voltage generator further comprises a first switch coupled between the sampling capacitor and the control node of the replica device and the integrated circuit further comprises a control circuit configured to provide a control signal that periodically causes the first switch to couple the control node of the replica device to the sampling capacitor.

16. The integrated circuit as recited in claim 15 wherein the reference voltage generator further comprises a second switch coupled between a current source and the replica device, the control circuit being further configured to provide a second control signal that periodically causes the second switch to couple the replica device to the replica current generator.

17. The integrated circuit as recited in claim 13 wherein the reference voltage generator further comprises a feedback circuit configured to maintain the control voltage of the replica device at a level that corresponds to a predetermined output voltage of the output device.

18. The integrated circuit as recited in claim 9 wherein the pre-driver circuit comprises:

a first driver configured to drive an intermediate node according to the input signal using the fixed regulated voltage; and

a second driver configured to drive the control node according to an intermediate signal on the intermediate node using the variable regulated voltage.

19. The integrated circuit as recited in claim 9 further comprising:

a second output device coupled between the output node and a second power supply node, the second output device having a second type, the second type being complementary to the first type;

a second pre-driver circuit configured to drive a second control node of the second output device responsive to the input signal and using a second fixed regulated voltage and a second variable regulated voltage;

a second fixed voltage regulator configured to generate the second fixed regulated voltage based on the first power supply voltage, the second power supply voltage, and a third reference voltage;

a second variable voltage regulator configured to generate the second variable regulated voltage based on the first power supply voltage, the second power supply voltage, and a fourth reference voltage; and

a second reference voltage generator configured to generate the fourth reference voltage based on the first power supply voltage, the second power supply voltage, a second reference current, and a second predetermined target voltage level of the second control node of the second output device.

20. An apparatus comprising:

means for driving a control node of an output device coupled to an output node according to an input signal and using a fixed regulated voltage and a variable regulated voltage, the variable regulated voltage being based on a first power supply voltage, a second power supply voltage, and a reference voltage; and

means for generating and periodically calibrating the reference voltage based on the first power supply voltage, the second power supply voltage, a reference current, and a predetermined target voltage level of the control node of the output device.