Method of operating a voice coil motor and corresponding control circuit
Embodiments provide a method of operating a voice coil motor via a transconductance loop. The method includes detecting an actual value of a supply voltage of the transconductance loop. An offset compensation signal of the transconductance loop is produced as a function of the detected actual value of the supply voltage based on a relationship between offset values and the supply voltage of the transconductance loop. The offset compensation signal is applied to a loop control signal of the transconductance loop. A drive current is applied to the voice coil motor. The drive current is related to a target drive current that is based on the loop control signal.
1. A method of operating a voice coil motor via a transconductance loop, the method comprising:
detecting an actual value of a 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 a relationship between offset values and the supply voltage of the transconductance loop;
applying the offset compensation signal to a loop control signal of the transconductance loop, wherein a target drive current for the voice coil motor is based on the loop control signal; and
applying a drive current to the voice coil motor, the drive current related to the target drive current.
2. The method of claim 1 , wherein the offset values comprise a first offset value that produces a commutation in a sign of an error signal at an output of an error amplifier of the transconductance loop in a first calibration condition, and a second offset value that produces a commutation in the sign of the error signal at the output of the error amplifier in a second calibration condition.
3. The method of claim 2 , wherein the drive current is applied by a power stage of the transconductance loop, the method further comprising at least partially de-activating the power stage to discontinue supplying drive current to the voice coil motor during the first calibration condition and the second calibration condition.
4. The method of claim 2 , wherein the relationship between the offset values and the supply voltage of the transconductance loop is a straight line passing through the first offset value and the second offset value.
5. The method of claim 2 , wherein producing the offset compensation signal of the transconductance loop comprises reading the offset compensation signal in a look-up table having stored therein the relationship between the offset values and the supply voltage of the transconductance loop and having entries for the detected actual value of the supply voltage.
6. The method of claim 2 , wherein producing the offset compensation signal of the transconductance loop comprises calculating a relationship between the first offset value and the second offset value and first and second common input voltages applied to first and second inputs of a sense amplifier of the transconductance loop.
7. A transconductance loop circuit configured to operate with the method of claim 1 , the transconductance loop comprising:
a power stage configured to apply a drive current to the voice coil motor;
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; and
an error amplifier configured to receive the loop control signal to produce the 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.
8. 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 a 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.
9. The method of claim 8 , wherein both of the first and second common input voltages applied to the first and second inputs of the differential sense amplifier are a function of the supply voltage of the transconductance loop.
10. The method of claim 8 , comprising at least partially de-activating the power stage to discontinue supplying drive current to the voice coil motor during the first calibration condition and the second calibration condition.
11. The method of claim 8 , wherein the relationship between offset values and the supply voltage of the transconductance loop is a straight line passing through the first offset value and the second offset value.
12. The method of claim 8 , further comprising producing the offset compensation signal of the transconductance loop by reading it in a look-up table having stored therein said relationship between offset values and the supply voltage of the transconductance loop and having entries for the detected actual value of the supply voltage.
13. The method of claim 8 , further comprising producing the offset compensation signal of the transconductance loop by calculating a relationship between the first offset value and the second offset value and the first and second common input voltages applied to the first and second inputs of the differential sense amplifier.
14. 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 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.
15. The transconductance loop circuit of claim 14 , wherein the power stage is configured to be at least partially de-activated to discontinue supplying drive current to the voice coil motor during the first calibration condition and the second calibration condition.
16. The transconductance loop circuit of claim 14 , wherein the processing circuit is configured to produce the offset compensation signal of the transconductance loop circuit by reading it in a look-up table having stored therein said relationship between the offset values and the supply voltage of the transconductance loop circuit and having entries for the detected actual value of the supply voltage.
17. The transconductance loop circuit of claim 14 , wherein the processing circuit is configured to produce the offset compensation signal of the transconductance loop circuit by calculating a relationship between the first offset value and the second offset value and first and second common input voltages applied to the first and second inputs of the differential sense amplifier.
18. A power combo integrated circuit for use with a hard disk drive that includes a hard disk, a spindle motor, and a voice coil motor, the power combo integrated circuit comprising:
the transconductance loop circuit according to claim 14 ; and
a spindle motor driver;
wherein the power combo integrated circuit is configured to operate with the hard disk drive such that the spindle motor driver drives the spindle motor to cause the hard disk to spin and the transconductance loop circuit drives the voice coil motor to position a read/write head at a surface of the hard disk.
19. A hard disk drive, comprising:
a hard disk;
a voice coil motor functionally coupled to the hard disk; and
the transconductance loop circuit according to claim 14 , wherein the power stage is coupled to the voice coil motor to apply the drive current to the voice coil motor.
20. The hard disk drive of claim 19 , further comprising:
a spindle motor functionally coupled to the hard disk to cause the hard disk to spin, wherein the voice coil motor is functionally coupled to the hard disk to position a read/write head at a surface of the hard disk; and
a spindle motor driver coupled to the spindle motor to apply a spindle motor drive current to the spindle motor.