Method of operating a voice coil motor and corresponding control circuit

A method of operating a voice coil motor in a hard disk drive via a transconductance loop having a supply voltage, wherein the transconductance loop comprises a power stage configured to apply a drive current to the voice coil motor, a differential sense amplifier having first and second inputs coupled to a current sensor of the voice coil motor, wherein the differential sense amplifier is configured to produce a sense signal indicative of the drive current applied to the voice coil motor, and the transconductance loop comprises an error amplifier configured to receive a loop control signal to produce a target drive current for the voice coil motor, wherein the error amplifier is coupled to the power stage to drive the power stage based on the loop control signal and the sense signal, wherein the method comprises:

• performing a calibration with a first calibration condition and a second calibration condition of the transconductance loop wherein, in the first calibration condition, the first and second inputs of the differential sense amplifier have applied thereto a first common input voltage and the error amplifier has applied thereto said loop control signal with a first offset value that produces a commutation in the sign of an error signal at an output of the error amplifier in the first calibration condition, and, in the second calibration condition, the first and second inputs of the differential sense amplifier have applied thereto a second common input voltage and the error amplifier has applied thereto said loop control signal with a second offset value that produces a commutation in the sign of the error signal at the output of the error amplifier in the second calibration condition, wherein the second common input voltage is different from the first common input voltage and at least one of the first and second common input voltages is a function of the supply voltage of the transconductance loop;
• establishing, based on the first offset value and the second offset value, a relationship between offset values and supply voltage of the transconductance loop;
• detecting an actual value of the supply voltage of the transconductance loop;
• producing an offset compensation signal of the transconductance loop as a function of the detected actual value of the supply voltage based on the relationship between the offset values and the supply voltage of the transconductance loop;
• and
• applying to the loop control signal of the transconductance loop the offset compensation signal.

A transconductance loop circuit comprising:

• a power stage configured to apply a drive current to a voice coil motor in a hard disk drive;
• a differential sense amplifier having first and second inputs configured to be coupled to a current sensor of the voice coil motor, wherein the differential sense amplifier is configured to produce a sense signal indicative of the drive current applied to the voice coil motor;
• an error amplifier configured to receive a loop control signal to produce a target drive current for the voice coil motor, wherein the error amplifier is coupled to the power stage to drive the power stage based on the loop control signal and the sense signal;
• offset calibration circuitry configured to be brought to a first calibration condition and said a second calibration condition and to establish a relationship between offset values and a supply voltage of the transconductance loop circuit, based on a first offset value and a second offset value;
• a detector configured to detect an actual value of the supply voltage;
• a processing circuit configured to produce an offset compensation signal of the transconductance loop circuit as a function of the detected actual value of the supply voltage based on the relationship between the offset values and the supply voltage of the transconductance loop circuit;
• and
• an offset compensation node configured to apply the offset compensation signal to the loop control signal of the transconductance loop circuit.