PWM/linear driver for disk drive voice coil actuator

A disk drive comprising a storage disk rotated by a disk spindle motor and a voice coil actuator operated by a voice coil actuator motor (VCM) having a single coil, and a motor chip, the motor chip including a control interface for receiving motor control values from drive electronics and for thereupon generating driving signals for operating the spindle motor with a motor driver and current command signals for operating the VCM with a VCM driver, the VCM driver comprising:

• four field effect transistors connected to the coil as an H-bridge,
• a sense resistor in series with the coil and the H-bridge for sensing coil current,
• a feedback amplifier for amplifying voltage developed across the sense resistor to provide a feedback signal,
• a VCM driver summing junction for summing current command signals with the feedback signal,
• a transconductance loop compensation amplifier having an inverting input connected to the VCM driver summing junction, having a non-inverting input connected to a reference level, and having an output control signal node, for providing loop compensation to a control level at the output control signal node,
• the control level being connected to control linear transconductance differential driver amplifiers including the four field effect transistors during a linear drive mode,
• pulse width modulation (PWM) generation circuitry connected to the output control signal node for generating variable duty cycle pulses related to the control level during a PWM drive mode and for applying the pulses to control elements of a first one of the four field effect transistors, and
• polarity control means responsive to a driving current direction signal for applying a control level to control elements of other ones of the four field effect transistors during PWM drive mode.

A method for controlling a voice coil actuator motor (VCM) having a single coil with a VCM motor driver chip within a disk drive, the motor chip including a control interface and an H-bridge driver for generating driving signals for operating the VCM, current sensing means for sensing current flow through a voice coil of the VCM and a transconductance control loop, comprising the steps of:

• receiving and decoding VCM control values from disk drive electronics via the control interface,
• operating the H-bridge driver circuit in a linear mode during track following operations and during an acceleration phase of track seeking operations,
• switching the H-bridge driver circuit from linear mode to pulse width modulation (PWM) mode at a commanded time to begin a deceleration phase of track seeking operations and without generating a step disturbance in the transconductance control loop,
• operating the H-bridge driver circuit in pulse width modulation (PWM) mode during a deceleration phase of track seeking operations, and
• switching the H-bridge driver circuit from PWM mode to linear mode at an end of the deceleration phase without generating a step disturbance in the transconductance control loop.