Disk drive controlling ripple current of a voice coil motor when driven by a PWM driver

1. A disk drive comprising:

(a) a disk;

(b) a head;

(c) a voice coil motor (VCM) for actuating the head radially over the disk, the VCM comprising a voice coil;

(d) a plurality of driver switches for controlling a voltage applied to the voice coil; and

(e) a pulse width modulated (PWM) signal generator for generating PWM control signals applied to the driver switches, the PWM control signals comprising:

a PWM cycle time;

a Tforward time interval of the PWM cycle time wherein a positive control voltage is applied to the voice coil;

a Treverse time interval of the PWM cycle time wherein a negative control voltage is applied to the voice coil; and

a Tdead time interval of the PWM cycle time wherein a substantially zero control voltage is applied to the voice coil, wherein the Tdead time interval is adjusted to control a magnitude of an actual ripple current flowing through the voice coil.

2. The disk drive as recited in claim 1 , wherein a first and second end of the voice coil are shorted to ground during the Tdead time interval.

3. The disk drive as recited in claim 1 , wherein the Tforward time interval is computed in response to a target ripple current flowing through the voice coil.

4. The disk drive as recited in claim 3 , wherein the Treverse time interval is computed in response to the Tforward time interval and the target ripple current.

5. The disk drive as recited in claim 4 , wherein the Tdead time interval is computed in response to the Tforward and Treverse time intervals.

6. The disk drive as recited in claim 5 , wherein the Tforward and Treverse time intervals are adjusted proportionally in response to a current command.

7. The disk drive as recited in claim 1 , wherein the Tdead time interval is computed in response to a target ripple current and a measured ripple current.

8. The disk drive as recited in claim 7 , wherein the measured ripple current is computed by:

(a) detecting the current flowing through the voice coil to generate a detected current;

(b) integrating the detected current over a first time interval of the PWM cycle time to generate a negative current measurement;

(c) integrating the detected current over a second time interval of the PWM cycle time to generate a positive current measurement; and

(d) computing a difference between the negative current measurement and the positive current measurement.

9. The disk drive as recited in claim 8 , wherein the Tforward and Treverse time intervals are computed in response to the Tdead time interval.

10. The disk drive as recited in claim 9 , wherein the Tforward and Treverse time intervals are adjusted proportionally in response to a current command.

11. The disk drive as recited in claim 1 , wherein:

(a) the voice coil comprises a resistance R and an effective inductance L;

(b) the effective inductance L is a function of the actual ripple current flowing through the voice coil;

(c) the resistance R changes with temperature drift; and

(d) the Tdead time is adjusted to maintain a substantially constant ratio L/R.

12. A method of controlling ripple current in a voice coil motor (VCM) of a disk drive when driven in a PWM mode, the disk drive comprising a disk, a head, and the VCM for actuating the head radially over the disk, wherein the VCM comprise a voice coil, the method comprises the steps of:

(a) computing a Tforward time interval of a PWM cycle time;

(b) applying a positive control voltage to the voice coil during the Tforward time interval;

(c) computing a Treverse time interval of the PWM cycle time;

(d) applying a negative control voltage to the voice coil during the Treverse time interval;

(e) computing a Tdead time interval of the PWM cycle time;

(f) applying a substantially zero control voltage to the voice coil during the Tdead time interval; and

(g) adjusting the Tdead time interval to control a magnitude of an actual ripple current flowing through the voice coil.

13. The method as recited in claim 12 , further comprising the step of shorting a first and second end of the voice coil to ground during the Tdead time interval.

14. The method as recited in claim 12 , wherein the Tforward time interval is computed in response to a target ripple current flowing through the voice coil.

15. The method as recited in claim 14 , wherein the Treverse time interval is computed in response to the Tforward time interval and the target ripple current.

16. The method as recited in claim 15 , wherein the Tdead time interval is computed in response to the Tforward and Treverse time intervals.

17. The method as recited in claim 16 , further comprising the step of adjusting proportionally the Tforward and Treverse time intervals in response to a current command.

18. The method as recited in claim 12 , wherein the Tdead time interval is computed in response to a target ripple current and a measured ripple current.

19. The method as recited in claim 18 , wherein the measured ripple current is computed by:

(e) detecting the current flowing through the voice coil to generate a detected current;

(f) integrating the detected current over a first time interval of the PWM cycle time to generate a negative current measurement;

(g) integrating the detected current over a second time interval of the PWM cycle time to generate a positive current measurement; and

(h) computing a difference between the negative current measurement and the positive current measurement.

20. The method as recited in claim 19 , wherein the Tforward and Treverse time intervals are computed in response to the Tdead time interval.

21. The method as recited in claim 20 , further comprising the step of adjusting proportionally the Tforward and Treverse time intervals in response to a current command.

22. The method as recited in claim 12 , wherein:

(a) the voice coil comprises a resistance R and an effective inductance L;

(b) the effective inductance L is a function of the actual ripple current flowing through the voice coil;

(c) the resistance R changes with temperature drift; and

(d) the Tdead time is adjusted to maintain a substantially constant ratio L/R.