Decreasing spin up time in a disk drive by adjusting a duty cycle of a spindle motor PWM signal to maintain constant average input current

1. A disk drive connectable to a host computer, the disk drive comprising:

(a) a disk;

(b) a head;

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

(d) a spindle motor for rotating the disk, the spindle motor comprising a plurality of windings;

(e) an interface for receiving a supply voltage from the host computer;

(f) a spindle driver for applying a current from the supply voltage to the windings of the spindle motor in response to a pulse width modulated (PWM) signal;

(g) a current sensor for generating a current sense signal representing a current flowing from the supply voltage through at least the VCM and the windings of the spindle motor;

(h) an integrator for integrating the current sense signal to generate an integration signal; and

(i) a comparator for comparing the integration signal to a predetermined threshold, wherein an output of the comparator adjusts a duty cycle of the PWM signal.

2. The disk drive as recited in claim 1 , wherein the integrator is reset at the end of the on-period of the PWM signal.

3. The disk drive as recited in claim 1 , further comprising circuitry for limiting the duty cycle of the PWM signal to a predetermined maximum.

4. The disk drive as recited in claim 1 , wherein the integrator comprises a capacitor for integrating the current sense signal.

5. The disk drive as recited in claim 1 , wherein the current sense signal is indicative of substantially all current flowing from the supply voltage.

6. The disk drive as recited in claim 1 , further comprising a comparator for comparing the current sense signal to a predetermined threshold, wherein the duty cycle of the PWM signal is adjusted in response to an output of the comparator.

7. The disk drive as recited in claim 1 , wherein the integrator comprises:

(a) a current controlled oscillator for generating a periodic signal proportional to a magnitude of the current sense signal; and

(b) a counter responsive to the periodic signal, wherein an output of the counter adjusts the duty cycle of the PWM signal.

8. The disk drive as recited in claim 1 , further comprising a voltage sensor for generating a voltage sense signal representing a voltage level of the supply voltage, wherein:

(a) the spindle driver is responsive to the current sense signal to control the amount of current drawn by the windings from the supply voltage during a spin up mode of the disk drive; and

(b) the spindle driver is responsive to the voltage sense signal to control the amount of current drawn by the windings from the supply voltage during a steady state mode of the disk drive.

9. The disk drive as recited in claim 8 , wherein a duty cycle of the PWM signal is adjusted in response to the voltage sense signal.

10. A method of operating a disk drive connected to a host computer, the disk drive comprising a disk, a head, a voice coil motor (VCM) for actuating the head over the disk, and a spindle motor for rotating the disk, the spindle motor comprising a plurality of windings, the method comprising the steps of:

(a) receiving a supply voltage from the host computer;

(b) generating a pulse width modulated (PWM) signal;

(c) applying a current from the supply voltage to the windings of the spindle motor in response to the PWM signal;

(d) sensing a current flowing from the supply voltage through at least the VCM and the windings of the spindle motor to generate a current sense signal;

(e) integrating the current sense signal to generate an integration signal;

(f) comparing the integration signal to a predetermined threshold; and

(g) adjusting a duty cycle of the PWM signal in response to the comparison.

11. The method as recited in claim 10 , further comprising the step of resetting the integration at the end of the on-period of the PWM signal.

12. The method as recited in claim 10 , further comprising the step of limiting the duty cycle of the PWM signal to a predetermined maximum.

13. The method as recited in claim 10 , wherein the step of integrating uses a capacitor for integrating the current sense signal.

14. The method as recited in claim 10 , wherein the current sense signal is indicative of substantially all current flowing from the supply voltage.

15. The method as recited in claim 10 , further the steps of:

(a) comparing the current sense signal to a predetermined threshold; and

(b) adjusting the duty cycle of the PWM signal in response to the comparison.

16. The method as recited in claim 10 , wherein the step of integrating comprises the steps of:

(a) generating a periodic signal proportional to a magnitude of the current sense signal;

(b) counting cycles of the periodic signal; and

(c) adjusting the duty cycle of the PWM signal in response to the step of counting cycles of the periodic signal.

17. The method as recited in claim 10 , further comprising the steps of:

(a) controlling the amount of current drawn by the windings from the supply voltage during a spin up mode of the disk drive in response to the current sense signal;

(b) sensing a voltage level of the supply voltage to generate a voltage sense signal during a steady state mode of the disk drive; and

(c) controlling the amount of current drawn by the windings from the supply voltage during the steady state mode of the disk drive in response to the voltage sense signal.

18. The method as recited in claim 17 , wherein the step of controlling the amount of current drawn by the windings from the supply voltage during the steady state mode comprises the step of adjusting a duty cycle of the PWM signal in response to the voltage sense signal.