A hard disk drive generally includes a stack of rotating disks or platters, a spindle motor which causes the disk to rotate, read/write heads which fly above the surface of the disks, an actuator motor (known as a "voice coil motor" or VCM) which controls the positioning of the read/write heads, power circuitry to provide electrical power to the spindle and voice coil motors, and control circuitry to control the operation of the spindle and voice coil motors.
A read/write head reads data from the disk by sensing flux changes on the magnetic surface of the disk as it passes beneath the read/write head. To synchronize the data being read from the disk with the operation of the data processing circuitry, it is necessary to carefully control the speed of the rotation of the disks. This is usually accomplished by controlling the current delivered to the spindle motor.
The switching characteristics of the VCM power devices are very important in achieving good performance from the motor and other favorable characteristics. However, a problem has been developed in the control of the voice coil motors in that a "glitch" occurs in the high-side and low-side drivers. This glitch couples a noise spike into the channel chip and reduces the bit error rate, making it difficult to obtain a high performance read channel. This glitch is caused by the input signal saturating the driver circuits. More particularly, the output stage of the driver circuits are driven to saturation. As the input stage comes out of saturation, the output stage does not follow the input signal as a result of the saturation. Once the output stage comes out of saturation and starts to respond, a glitch or rapid decrease in output signal results.