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 first amplifier for amplifying a voltage across the first sense resistor to generate a first sense signal;
a second sense resistor connected in series between at least one of the driver switches and ground;
a second amplifier for amplifying a voltage across the second sense resistor to generate a second sense signal;
a first oscillator for generating a first frequency signal in response to the first sense signal;
a second oscillator for generating a second frequency signal in response to the second sense signal; and
a first counter responsive to a reference frequency, the first frequency signal, and the second frequency signal, wherein an output of the first counter represents a current flowing through the voice coil.
2. The disk drive as recited in claim 1, wherein the output of the first counter represents a difference between the reference frequency and the first and second frequency signals.
3. The disk drive as recited in claim 1, wherein the VCM driver further comprises pulse width modulated (PWM) circuitry responsive to the output of the first counter for generating a PWM signal for controlling the driver switches.
4. The disk drive as recited in claim 3, wherein:
(a) current flows through the first sense resistor and the voice coil while the PWM signal is in a first state;
(b) current flows through the second sense resistor and the voice coil while the PWM signal is in a second state;
(c) the first counter is responsive to the first frequency signal while the PWM signal is in the first state and to the second frequency signal while the PWM signal is in the second state;
(d) the VCM driver further comprises a second counter responsive to at least the first frequency signal while the PWM signal is in the second state; and
(e) an output of the second counter is used to cancel an offset in the measured current flowing through the voice coil.
5. The disk drive as recited in claim 4, wherein the output of the second counter is used to adjust the first and second sense signals.
6. The disk drive as recited in claim 4, wherein the first counter comprises:
(a) a first up counter responsive to the first frequency signal while the PWM signal is in the second state, the first up counter for counting the number of periods in the first frequency signal over a period of the PWM signal;
(b) a first down counter responsive to the first frequency signal, wherein an output of the first up counter is loaded into the first down counter;
(c) a second up counter responsive to the second frequency signal while the PWM signal is in the first state, the second up counter for counting the number of periods in the second frequency signal over a period of the PWM signal;
(d) a second down counter responsive to the second frequency signal, wherein an output of the second up counter is loaded into the second down counter; and
wherein an output of the first down counter and an output of the second down counter are combined to generate the output of the first counter in order to compensate for a difference between the operating characteristics of the first and second oscillators.
7. A method of operating a disk drive, the disk drive comprising a disk, an actuator arm, a head connected to a distal end of the actuator arm, a voice coil motor (VCM) comprising a voice coil for rotating the actuator arm about a pivot to actuate the head radially over the disk, and an H-bridge driver comprising a plurality of driver switches for driving current from a supply voltage through the voice coil to ground, the method comprising the steps of:
amplifying a voltage across a first sense resistor to generate a first sense signal, the first sense resistor being connected in series between the supply voltage and at least one of the driver switches;
amplifying a voltage across a second sense resistor to generate a second sense signal, the second sense resistor being connected in series between at least one of the driver switches and ground;
generating a first frequency signal in response to the first sense signal;
generating a second frequency signal in response to the second sense signal;
counting periods in the reference frequency, the first frequency signal, and the second frequency signal, to generate a first count representing a current flowing through the voice coil.
8. The method as recited in claim 7, wherein the first count represents a difference between the reference frequency and the first and second frequency signals.
9. The method as recited in claim 7, further comprising the step of generating a PWM signal for controlling the driver switches in response to the first count.
10. The method as recited in claim 9, wherein:
current flows through the first sense resistor and the voice coil while the PWM signal is in a first state;
current flows through the second sense resistor and the voice coil while the PWM signal is in a second state;
the first count is generated in response to the first frequency signal while the PWM signal is in the first state and to the second frequency signal while the PWM signal is in the second state;
the method further comprises the steps of:
(a) counting periods in at least the first frequency signal while the PWM signal is in the second state to generate a second count; and
(b) canceling an offset in the measured current flowing through the voice coil in response to the second count.
11. The method as recited in claim 10, further comprising the step of adjusting the first and second sense signals in response to the second count.
12. The method as recited in claim 10, wherein the step of generating the first count comprises the steps of:
(a) counting up periods in the first frequency signal while the PWM signal is in the second state to generate a third count, wherein the counting up occurs over a period of the PWM signal;
(b) loading the third count into a first down counter for counting down periods in the first frequency signal to generate a fourth count;
(c) counting up periods in the second frequency signal while the PWM signal is in the first state to generate a fifth count, wherein the counting up occurs over a period of the PWM signal;
(d) loading the fifth count into a second down counter for counting down periods in the second frequency signal to generate a sixth count; and
(e) combining the fourth and sixth count to generate the first count.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A compound semiconductor device containing a hetero junction bi-polar transistor comprising a sub-collector layer, a collector layer, a base layer and an emitter layer formed as thin film crystal layers in this turn on a compound semiconductor substrate by vapor growth wherein the ratio of current gainbase sheet resistance of the hetero junction bi-polar transistor is not less than 0.60.