Apparatus and method for providing selective hysteresis for zero crossings of a disk drive spindle motor

A hard disk drive comprising:

• a rigid disk for storing digital data;
• a transducer for writing the digital data onto the disk;
• a spindle motor for spinning the rigid disk, wherein the spindle motor is comprised of a plurality of windings coupled to a center-tap;
• a comparator having one input coupled to one of the windings and another input coupled to the center-tap, wherein an output signal from the comparator an occurrence of a zero-crossing corresponding to when a back emf signal corresponding to the winding equals a center-tap voltage;
• a control circuit coupled to the comparator for generating a control signal that indicate a slope of the voltage corresponding to one of the windings, wherein the determination is based on commutation signals that provide sequencing for the spindle motor;
• a hysteresis circuit coupled to the comparator for providing hysteresis to an input to the comparator only after a zero-crossing has been detected, wherein the hysteresis circuit is comprised of a first transistor coupled to an output of the comparator for determining when the hysteresis circuit is to provide a positive hysteresis voltage to the comparator according to the control signal and a second transistor coupled to the output of the comparator for enabling the hysteresis circuit to provide a negative hysteresis voltage to the comparator according to the control signal.

In a hard disk drive having a disk for storing digital data, a transducer for writing the digital data onto the disk, and a spindle motor for rotating the disk about a spindle, a method for providing hysteresis voltage to a comparator that detects zero-crossings, the method comprising the steps of:

• detecting a zero crossing by comparing a winding voltage to a center-tap voltage, wherein if the zero crossing occurs when the winding voltage equals the center-tap voltage;
• switching a state of an output signal generated by the comparator when the zero crossing is detected;
• applying the hysteresis voltage to an input of the comparator when the output signal generated by the comparator switches states, wherein the hysteresis voltage is applied only after the zero crossing has been detected;
• generating a control signal as a function of commutation signals used to drive the spindle motor, wherein the control signal indicates a slope of a signal associated with a winding of the motor;
• applying a positive hysteresis voltage to the input of the comparator when the slope is negative;
• and
• applying a negative hysteresis voltage to the input of the comparator when the slope is positive.