Converting a pulse-width modulation signal to an analog voltage

A disc drive, comprising:

• a motor;
• and
• circuitry which controls rotational velocity of the motor, comprising:
• a rotational velocity demand circuit which outputs a pulse-width modulation (PWM) signal having a series of cycles each with a first and a second level and a duty cycle expressed as a percentage of time the PWM signal is at the first level over each cycle, the duty cycle indicative of a desired velocity of the motor;
• and
• a PWM to analog voltage converter which converts the PWM signal to a corresponding analog voltage having a magnitude indicative of the duty cycle of the PWM signal, comprising:
• a current source which supplies a current at a selected magnitude;
• first and second sampling capacitors;
• and
• a ramp capacitor which, during successively occurring cycles of the PWM signal:
• (a) stores charge in response to application of the current to the ramp capacitor during substantially the entire duration of a first cycle;
• (b) transfers charge to the first sampling capacitor during a second cycle, the first sampling capacitor outputting a feedback voltage to adjust the magnitude of the current from the current source, the feedback voltage having a magnitude in relation to a magnitude of the charge transferred to the first sampling capacitor;
• (c) stores charge in response to application of the current to the ramp capacitor during a portion of a third cycle corresponding to the first level of the PWM signal;
• and
• (d) transfers charge to the second sampling capacitor during a fourth cycle, the second sampling capacitor outputting the analog voltage in relation to a magnitude of the charge transferred to the second sampling capacitor.

A method for converting a pulse-width modulation (PWM) signal to an analog voltage, the PWM signal comprising a series of cycles each having a first and a second level and a duty cycle expressed as a percentage of time the PWM signal is at the first level over each cycle, the analog voltage having a magnitude indicative of the duty cycle, the method comprising steps of:

• (a) providing a current at a selected magnitude;
• (b) applying the current to a ramp capacitor over each of a plurality of successively occurring cycles to repetitively store charge on the ramp capacitor at a rate determined by the magnitude of the current;
• (c) transferring charge stored during a selected cycle from the ramp capacitor to a first sampling capacitor to output a feedback voltage used to adjust the magnitude of the current, the charge transferred to the first sampling capacitor determined in relation to the charge stored on the ramp capacitor over the entire cycle;
• and
• (d) transferring charge stored during a selected cycle from the ramp capacitor to a second sampling capacitor to output the analog voltage, the charge transferred to the second sampling capacitor determined in relation to the charge stored on the ramp capacitor while the PWM signal remains at the first level during said selected cycle.