Disk drive comprising current sense circuitry for a voice coil motor
View Patent ↗A disk drive is disclosed comprising a voice coil motor (VCM) for actuating a head radially over a disk. A VCM driver comprises first and second transistors forming a common collector amplifier for sensing a current flowing through a voice coil of the VCM. The common collector amplifier improves the accuracy of the current sense measurement when the VCM is driven in a pulse width modulated (PWM) mode.
1. A disk drive comprising:
(a) a disk;
(b) an actuator arm;
(c) a head connected to a distal end of the actuator arm;
(d) a voice coil motor (VCM) comprising a voice coil, the VCM for rotating the actuator arm about a pivot to actuate the head radially over the disk; and
(e) a VCM driver comprising:
an H-bridge driver comprising a plurality of driver switches for driving current from a supply voltage through the voice coil to ground;
a first sense resistor-connected in series between the supply voltage and at least one of the driver switches;
a second sense resistor connected in series between at least one of the driver switches and ground;
a first transistor having a first transistor terminal, a second transistor terminal, and a gate terminal;
a second transistor having a first transistor terminal, a second transistor terminal, and a gate terminal;
a third sense resistor having a first end connected to a node between the first sense resistor and the at least one of the driver switches and a second end connected to the first transistor terminal of the first transistor;
a fourth sense resistor having a first end connected to a node between the second sense resistor and the at least one of the driver switches and a second end connected to the first transistor terminal of the second transistor;
a first gain resistor having a first end and a second end, wherein:
the first end is connected to at least the second transistor terminal of the first transistor;
the second end connected to a reference voltage; and
when the gate terminals of the first and second transistors are forward biased, a voltage across the gain resistor represents the current flowing through the voice coil.
2. The disk drive as recited in claim 1 , wherein the VCM driver further comprises a current source for generating a current flowing through the first transistor.
3. The disk drive as recited in claim 2 , wherein the current source comprises a resistor having a first end connected to the supply voltage and a second end connected to the first transistor terminal of the first transistor.
4. The disk drive as recited in claim 1 , wherein the VCM driver further comprises a current source for generating a current flowing through the second transistor.
5. The disk drive as recited in claim 4 , wherein the current source comprises a resistor having a first end connected to the first transistor terminal of the second transistor and a second end connected to ground.
6. The disk drive as recited in claim 1 , wherein the first and second transistors are bipolar junction transistors.
7. The disk drive as recited in claim 1 , wherein the first and second transistors are field effect transistors.
8. The disk drive as recited in claim 1 , wherein the VCM driver further comprises a second gain resistor having a first end and a second end, wherein:
the first end is connected to the second transistor terminal of the second transistor;
the second end is connected to the reference voltage; and
when the gate terminals of the first and second transistors are forward biased a voltage across the second gain resistor represents the current flowing through the voice coil.