Actuator driver circuit

Details Actuator driver circuit Actuator driver circuit

1999-07-23

2001-05-08

Ip, Paul (Department: 2709)

Electricity: motive power systems

Switched reluctance motor commutation control

C318S132000, C318S560000, C318S434000, C318S293000, C360S075000, C360S078040

Reexamination Certificate

active

06229273

ABSTRACT:

BACKGROUND OF THE INVENTION
The actuator arm in a hard disk drive is operated by a voice coil motor (VCM) to move the read and write heads across the disk. In most disk drive systems, a motor controller integrated circuit (IC) contains the necessary circuitry to regulate the current through the VCM. In higher performance systems, the VCM driver stage is connected directly to the VCM and is not part of the motor controller IC. Typically, the driver stage includes power field-effect transistors, such as N-channel metal oxide field-effect transistors (NMOS FETs), arranged in an H-bridge configuration. To achieve proper class AB operation of the driver stage, all of the FETs in the driver stage must be closely matched. This typically means that all of the FETs are implemented on a single IC, resulting in higher resistance and higher cost. Therefore, a cheaper, more efficient method of achieving class AB operation of unmatched FETs in a driver stage for a voice coil motor is needed.
BRIEF SUMMARY OF THE INVENTION
The present invention is an actuator driver circuit that includes an H-bridge configured driver stage where the upper half of the bridge consists of NMOS sense FETs and the lower half consists of standard NMOS power FETs. The driver circuit includes a differential amplifier configuration connected to the control electrodes of the sense FETs of the H-bridge. A bias circuit is connected to the H-bridge for controlling bias currents through sense and power FETs on both sides of the H-bridge. A common mode feedback circuit provides feedback to the differential amplifier configuration.
In preferred embodiments, the bias circuit comprises a first bias amplifier connected to the H-bridge and a current source to control bias current through one sense FET and power FET to maintain the sense FET in an active region. A second bias amplifier is connected to the H-bridge and a current source to control bias current through the other sense FET and power FET to maintain the other sense FET in an active region.
In preferred embodiments, the power FETs are operated to establish the bias current through the sense FETs. The output common mode voltage of the differential amplifier configuration is adjusted so that a common mode voltage on the voice coil motor is equal to a predetermined common mode reference voltage. Preferably, the H-bridge is operated to class AB operation.
In one embodiment, the differential amplifier configuration comprises a differential amplifier having respective inputs for receiving command inputs and respective outputs for operating the sense FETs to operate a VCM. The common mode feedback circuit comprises a transconductance amplifier connected to the H-bridge for sensing the common mode voltage of the VCM to control the differential amplifier and adjust an output common mode voltage of the differential amplifier. In another embodiment, the differential amplifier configuration comprises a pair of amplifiers each connected to the command inputs and providing an output to a respective sense FET to operate the VCM. The common mode feedback circuit includes a pair of dividers each having at least two divider impedances connected across a voltage supply. One input of each amplifier is connected to the junction between the divider impedances of a respective one of the pair of dividers, and a feedback circuit is connected between the respective node and the other input of the respective amplifier.


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