Title: Disk drive comprising current sense circuitry for a voice coil motor
Abstract: 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.
Patent Number: 6,850,383 Issued on 02/01/2005 to Bennett
| Inventors:
|
Bennett; George J. (Murrieta, CA)
|
| Assignee:
|
Western Digital Technologies, Inc. (Lake Forest, CA)
|
| Appl. No.:
|
376819 |
| Filed:
|
February 28, 2003 |
| Current U.S. Class: |
360/75; 360/78.01; 360/78.04 |
| Intern'l Class: |
G11B 021/02; G11B005/596 |
| Field of Search: |
360/46,67,78.01,78.04,75,77.02
318/293,256,280,560,565,568.16
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Tran; Sinh
Assistant Examiner: Olson; Jason
Attorney, Agent or Firm: Shara, Esq.; Milad G., Kim, Esq.; Won Tae C., Sheerin, Esq.; Howard H.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS AND PATENTS
This application is related to co-pending U.S. Patent Application Ser. No.
10/376,821 entitled "DISK DRIVE COMPRISING OSCILLATORS AND COUNTERS FOR
SENSING CURRENT IN A VOICE COIL MOTOR" filed on Feb. 28, 2003, the
disclosure of which is incorporated herein by reference.
Claims
I claim:
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.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to disk drives for computer systems. More
particularly, the present invention relates to a disk drive comprising
current sense circuitry for a voice coil motor (VCM).
2. Description of the Prior Art
FIG. 1 shows a prior art disk drive comprising a disk 2 rotated about a
center axis by a spindle motor (not shown). A head 4 attached to a distal
end of an actuator arm 6 is actuated radially over the disk 2 by a voice
coil motor (VCM) 8. The VCM 8 comprises a voice coil 10 which interacts
with permanent magnets of a VCM yoke in order to rotate the actuator arm 6
about a pivot. The VCM 8 is typically driven in either a linear mode or in
a pulse width modulated (PWM) mode. In addition, the motion of the VCM 8
may be controlled using a current feedback loop by sensing the amount of
current flowing through the voice coil 10 which is proportional to the
amount of torque applied to the actuator arm 6.
FIG. 1 also shows a VCM driver 12 comprising a conventional H-bridge driver
for driving the voice coil 10 shown as a resistance Rvcm 14 and an
inductance Lvcm 16. The H-bridge driver comprises a plurality of driver
switches 18A-18D for selectively connecting the ends of the voice coil 10
to a supply voltage 20 or to ground 22 depending on the desired direction
of rotation. A plurality of diodes D1-D4 protect the driver switches
18A-18D from flyback currents generated from driving an inductive load.
In order to control the motion of the VCM 8 accurately using a current
feedback loop it is important to measure the total integral of the current
flowing through the voice coil 10. Several problems arise when attempting
to use the conventional techniques for sensing the current flowing through
the voice coil 10 when driven in a PWM mode. Referring again to FIG. 1, if
a single sense resistor Rsense 24 in series with the voice coil 10 is used
to sense current, the PWM voltage appears on both sides of the resistor
Rsense 24 at several volts at very high slew rates. This chop voltage (a
square wave) must be rejected by sense amplifier 31 so that the very small
voltage across Rsense 24 can be accurately measured. This high frequency
AC voltage capacitively couples into the sense amplifier 31, and creates
offsets and nonlinearities which distort the current sense measurement.
This problem exacerbates as the frequency of the PWM increases.
Another prior art current sensing technique uses a sense resistor Rsensep
26 in series with the supply voltage 20 and an amplifier 28, or a sense
resistor Rsensem 30 in series with ground 22 and an amplifier 32. This
technique avoids the common mode voltage problem associated with sense
resistor Rseries 24, however, it also leads to other problems related to
inductive flyback currents. Assume, for example, that current is flowing
to the right through the voice coil 10. Initially, driver switches 18A and
18D are on, allowing Vpwr 20 to source the current through the sense
resistors Rsensep 26 or Rsensem 30. Driver switch 18A is driven by a PWM
signal, while driver switch 18D is left on continually. When the PWM
signal turns driver switch 18A off, the inductive load keeps current
flowing to the right in the coil regardless of the voltage applied because
of the magnetic flux stored in the coil and its magnetic structure. This
inductive current can cause diode D2 or driver switch 18B to conduct
current, depending on the ratio of impedances. Since current is also
flowing through switch 18D, the flyback current momentarily cancels the
current through sense resistor Rsensem 30, which can distort the current
sense measurement by creating a blank spot in the voltage waveform.
Additionally, if the two halves of the H-bridge are switched alternately,
flyback current from the inductive current can drive the voltage at the
top of sense resistor Rsensem 30 below ground. When this happens,
substrate parasitic transistors (shown as parasitic transistor 31 in FIG.
1) are activated, drawing current from elsewhere in the driver circuitry
in a random manner, both distorting the current measurement with this
additional current and disrupting operation of the driver circuitry.
Regardless of how the H-bridge PWM switching is timed, shootthrough
currents (caused by a brief simultaneous conduction between driver
switches 18A and 18B or driver switches 18C and 18D) or gate charge
injections can also create false values for current that distort the true
measurement. These problems are generally avoided using sample/hold
circuits 34 and 36, which sample the voltage across the resistors 26 and
30 at a point in the PWM chop cycle where distortions due to flyback,
shootthrough, switching, or diode conduction, do not occur. However, the
sampling process adds delay to the loop. Additionally, an abrupt change
from a large current to a small current leaves a time related sample
distortion in the waveform that can be larger than the actual voltage
value corresponding to the small current. The control system spends time
responding to these spurious distortions which cause unwanted motion in
the VCM. Still further, the sense amplifiers 28 and 32 must be designed
such that their inputs can be driven below ground, or above the power
supply, respectively, in order to sense current of all polarities. Sensing
current above or below the power supply rails significantly increases the
circuit complexity of a monolithic IC sense amplifier due to substrate
current injection, which also removes current from the sense resistor in a
nonlinear manner.
There is, therefore, a need to accurately sense the current flowing through
the voice coil of disk drive VCM in order to implement a current feedback
loop while driving the VCM in a PWM mode.
SUMMARY OF THE INVENTION
The present invention may be regarded as a 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, the VCM for rotating the
actuator arm about a pivot to actuate the head radially over the disk, and
a VCM driver. The VCM driver comprises 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 is connected in
series between the supply voltage and at least one of the driver switches,
and a second sense resistor is connected in series between at least one of
the driver switches and ground. The VCM driver further comprises a first
transistor having a first transistor terminal, a second transistor
terminal, and a gate terminal, and a second transistor having a first
transistor terminal, a second transistor terminal, and a gate terminal. A
third sense resistor has 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 has 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 has a first end connected to at least
the second transistor terminal of the first transistor, and a second end
connected to a reference voltage. 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.
In one embodiment, the VCM driver further comprises a current source for
generating a current flowing through the first transistor, for example by
connecting a resistor between the supply voltage and the first transistor
terminal of the first transistor. In another embodiment, the VCM driver
further comprises a current source for generating a current flowing
through the second transistor, for example by connecting a resistor
between the first transistor terminal of the second transistor and ground.
In one embodiment, the first and second transistors are bipolar junction
transistors, and in an alternative embodiment, the first and second
transistors are field effect transistors.
In yet another embodiment, the VCM driver further comprises a second gain
resistor having a first end connected to the second transistor terminal of
the second transistor, and a second end connected to the reference
voltage. 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.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows prior art techniques for sensing the current flowing through
the voice coil of a disk drive VCM.
FIG. 2A shows a disk drive according to an embodiment of the present
invention comprising a VCM driver employing first and second transistors
forming a common collector amplifier for sensing the current flowing
through the voice coil of the VCM.
FIG. 2B shows an embodiment for generating the bias voltages for the first
and second transistors of FIG. 2A.
FIG. 3A shows a disk drive according to an embodiment of the present
invention wherein the VCM driver further comprises first and second
current sources for optimizing the operating mode of the first and second
transistors.
FIG. 3B shows an embodiment of the present invention wherein the first and
second current sources of FIG. 3A are implemented using resistors.
FIG. 4 shows a disk drive according to an embodiment of the present
invention wherein the VCM driver further comprises auto-zero circuitry to
compensate for drift.
FIG. 5 shows voltage waveforms representing the current sense signals
relative to the PWM signal controlling the driver switches.
FIG. 6 shows a disk drive according to an embodiment of the present
invention wherein the VCM driver comprises auto-zero circuitry and a first
and second current source for optimizing the operating mode of the first
and second transistors.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 2A shows a disk drive according to an embodiment of the present
invention comprising a disk 40, an actuator arm 42, a head 44 connected to
a distal end of the actuator arm 42, a voice coil motor (VCM) 46
comprising a voice coil 48, the VCM 46 for rotating the actuator arm 42
about a pivot to actuate the head 44 radially over the disk 40. A VCM
driver 50 comprises an H-bridge driver having a plurality of driver
switches 52A-52D for driving current from a supply voltage 54 through the
voice coil 48 to ground 56. A first sense resistor Rsensep 58 is connected
in series between the supply voltage 54 and at least one of the driver
switches 52A-52D, and a second sense resistor Rsensem 60 is connected in
series between at least one of the driver switches 52A-52D and ground 56.
The VCM driver 50 further comprises a first transistor 62 having a first
transistor terminal, a second transistor terminal, and a gate terminal,
and a second transistor 64 having a first transistor terminal, a second
transistor terminal, and a gate terminal. A third sense resistor Rsp 66
has a first end connected to a node between the first sense resistor
Rsensep 58 and the at least one of the driver switches 52A-52D and a
second end connected to the first transistor terminal of the first
transistor 62. A fourth sense resistor Rsm 68 has a first end connected to
a node between the second sense resistor Rsensem 60 and the at least one
of the driver switches 52A-52D and a second end connected to the first
transistor terminal of the second transistor 64. A gain resistor Rg 70 has
a first end connected to at least the second transistor terminal of the
first transistor 62, and a second end connected to a reference voltage
X*Vref 72. When the gate terminals of the first and second transistors 62
and 64 are forward biased (by bias voltage Vbias 74 and 76), a voltage
across the gain resistor Rg 70 represents the current flowing through the
voice coil 48.
The bias voltage Vbias 74 and 76 sets the amount of bias current flowing
through sense resistor Rsp 66 and sense resistor Rsm 68. Since the
currents through sense resistors Rsp 66 and Rsm 68 are substantially
matched through the biasing arrangement, this bias current generates
insignificant voltage on Rg 70.
The supply voltage 54 may be supplied by any suitable source, such as from
a host computer or generated internally during a power failure using the
back EMF voltage of the spindle motor (not shown).
When driving the VCM 46 in a particular direction (e.g., moving the head 44
from the inner diameter toward the outer diameter of disk 40), a PWM
signal (not shown) turns on driver switches 52A and 52D. Current flowing
from the supply voltage 54 through the voice coil 48 to ground 56
generates a voltage drop across sense resistor Rsensep 58 which reduces
the voltage across sense resistor Rsp 66 and therefore reduces the amount
of current flowing through transistor 62. Since the current flowing
through transistor 64 does not change, the gain resistor Rg 70 sources the
difference in current from the reference voltage X*Vref 72 and the voltage
developed across the gain resistor Rg 70 represents the current flowing
through the voice coil 48. When the PWM signal turns off driver switches
52A and 52D and turns on driver switch 52B and 52C, current is forced by
the inductance to flow from ground, through sense resistor Rsensem 60,
through driver switch 52B, through the voice coil 48, through driver
switch 52C to the supply voltage 54. This flow generates a voltage drop
below ground across sense resistor Rsensem 60 which increases the voltage
across sense resistor Rsm 68 and therefore increases the amount of current
flowing through transistor 64. Since the current flowing through
transistor 62 does not change, the gain resistor Rg 70 sources the
difference in current from the reference voltage X*Vref 72 and the voltage
developed across the gain resistor Rg 70 again represents the current
flowing through the voice coil 48.
When driving the VCM 46 in the opposite direction (e.g., moving the head 44
from the outer diameter toward the inner diameter of disk 40), the PWM
signal turns on driver switches 52C and 52B. Current flowing from the
supply voltage 54 through the voice coil 48 to ground 56 generates a
voltage across sense resistor Rsensem 60 which reduces the voltage across
sense resistor Rsm 68 and therefore reduces the amount of current flowing
through transistor 64. Since the current flowing through transistor 62
does not change, the gain resistor Rg 70 sinks the difference in current
and the voltage developed across the gain resistor Rg 70 represents the
current flowing through the voice coil 48. When the PWM signal turns off
driver switches 52C and 52B and turns on driver switch 52A and 52D,
current is forced by the inductance to flow from ground, through driver
switch 52D, through the voice coil 48, through driver switch 52A, through
sense resistor Rsensep 58 to the supply voltage 54. This flow generates a
voltage rise above the supply voltage 54 across sense resistor Rsensep 58
which increases the voltage across sense resistor Rsp 66 and therefore
increases the amount of current flowing through transistor 62. Since the
current flowing through transistor 64 does not change, the gain resistor
Rg 70 sinks the difference in current and the voltage developed across the
gain resistor Rg 70 again represents the current flowing through the voice
coil 48.
If driver switches 52A and 52B momentarily cross conduct, a current spike
flows through sense resistor Rsensep 58 and sense resistor Rsensem 60.
Since the current is identical in both sense resistors, and the resulting
voltage spike generated across sense resistor Rsensep 58 and sense
resistor Rsensem 60 is opposite in sign, sense resistor Rsp 66 and sense
resistor Rsm 68 cause an identical change in current through transistors
62 and 64. The result is a net zero change in the voltage across the gain
resistor Rg 70, and consequently the circuit rejects common mode currents,
bias currents, and any current flowing through both sense resistors
identically. The extent that the circuitry rejects common mode depends on
the match between the various parts of the circuitry. This embodiment
therefore generates an accurate voltage representation of the current
regardless of the state of the driver switches 52A-52D or the direction
the current is flowing through the voice coil 48, and suppresses secondary
currents generated in the circuitry that do not actually flow through the
voice coil 48.
FIG. 2B shows an embodiment of the present invention for generating the
bias voltage Vbias 74 and 76 (FIG. 2A) applied to the gates of transistors
62 and 64. A reference voltage vRef 78 is applied as the bias voltage 76
for transistor 64. The reference voltage vRef 78 is also applied to
transistor 80 to generate the bias voltage across resistors 82 and 84. The
voltage across resistor 84 is the bias voltage Vbias 74 for transistor 62.
A field effect transistor (MOSFET) 86 is used to buffer the voltage across
the gain resistor Rg 70 representing the current flowing through the voice
coil 48. This buffer arrangement is very simple, fast, and very high
impedance at its input.
FIG. 3A shows an embodiment of the present invention wherein the VCM driver
50 further comprises a first current source 88 for optimizing the
operating mode of the first transistor 62, and a second current source 90
for optimizing the operating mode of the second transistor 64. In this
embodiment, the current sources 88 and 90 are configured so that the
transistors 62 and 64 operate in a more linear region which improves the
accuracy and speed of the current sense measurement (the voltage across
the gain resistor Rg 70). The first and second current sources 88 and 90
may be implemented using any suitable circuitry, such as conventional
current mirror circuits, or as shown in FIG. 3B, by adding resistors 92
and 94. Resistors 92 and 94 can be somewhat mismatched, adding an offset
to the voltage across Rg 70, but the AC portion of the current sense
signal remains intact.
FIG. 4 shows an embodiment of the present invention wherein the VCM driver
50 comprises auto-zero circuitry to compensate for drift in the operating
characteristics of the first and second transistors 62 and 64 (due, for
example, to temperature drift or component mismatches). This embodiment
employs first and second gain resistor Rgp 96 and Rgm 98 for generating a
voltage with respect to X*Vref 72 representing the current flowing through
the voice coil 48. Transistors 100 and 102 have been added to provide bias
currents to transistors 64 and 62, respectively. Resistor Rb is
approximately equal to sense resistors Rsp 66 and Rsm 68, nominally
zeroing the voltage across gain resistors Rgp 96 and Rgm 98 when zero
current flows in the H-bridge.
When driving the VCM 46 in a particular direction (e.g., moving the head 44
from the inner diameter toward the outer diameter of disk 40), the PWM
signal turns on driver switches 52A and 52D. Current flowing from the
supply voltage 54 through the voice coil 48 to ground 56 generates a
voltage drop across sense resistor Rsensep 58 which reduces the voltage
across sense resistor Rsp 66 and therefore reduces the amount of current
flowing through transistor 62. Since the current flowing through
transistor 102 does not change, the gain resistor Rgp 96 sources the
difference in current from the reference voltage X*Vref 72 and the voltage
developed across the gain resistor Rgp 96 represents the current flowing
through the voice coil 48. When the PWM signal turns off driver switches
52A and 52D and turns on driver switch 52B and 52C, current is forced by
the inductance to flow from ground, through sense resistor Rsensem 60,
through driver switch 52B, through the voice coil 48, through driver
switch 52C to the supply voltage 54. This flow generates a voltage drop
below ground across sense resistor Rsensem 60 which increases the voltage
across sense resistor Rsm 68 and therefore increases the amount of current
flowing through transistor 64. Since the current flowing through
transistor 100 does not change, the gain resistor Rgm 98 sources the
difference in current from the reference voltage X*Vref 72 and the voltage
developed across the gain resistor Rgm 98 represents the current flowing
through the voice coil 48.
When driving the VCM 46 in the opposite direction (e.g., moving the head 44
from the outer diameter toward the inner diameter of disk 40), the PWM
signal turns on driver switches 52C and 52B. Current flowing from the
supply voltage 54 through the voice coil 48 to ground 56 generates a
voltage across sense resistor Rsensem 60 which reduces the voltage across
sense resistor Rsm 68 and therefore reduces the amount of current flowing
through transistor 64. Since the current flowing through transistor 100
does not change, the gain resistor Rgm 98 sinks the difference in current
and the voltage developed across the gain resistor Rgm 98 represents the
current flowing through the voice coil 48. When the PWM signal turns off
driver switches 52C and 52B and turns on driver switch 52A and 52D,
current is forced by the inductance to flow from ground, through driver
switch 52D, through the voice coil 48, through driver switch 52A, through
sense resistor Rsensep 58 to the supply voltage 54. This flow generates a
voltage rise above the supply voltage 54 across sense resistor Rsensep 58
which increases the voltage across sense resistor Rsp 66 and therefore
increases the amount of current flowing through transistor 62. Since the
current flowing through transistor 102 does not change, the gain resistor
Rgp 96 sinks the difference in current and the voltage developed across
the gain resistor Rgp 96 represents the current flowing through the voice
coil 48.
A first and second MOSFETs 104 and 106 are used to buffer the respective
voltages across the gain resistors Rgp 96 and Rgm 98 representing the
current flowing through the voice coil 48.
Since the driver switches 52A-52D in the H-bridge driver are driven with a
PWM signal, there is a known period of time during the PWM cycle when zero
current is flowing through sense resistor Rsensep 58 and a known period of
time when zero current is flowing through sense resistor Rsensem 60. The
voltage across the gain resistors Rgp 96 and Rgm 98 during these time
intervals, which represents zero current, is used to adjust the voltage
measurements when current is flowing through the gain resistors Rgp 96 and
Rgm 98. This auto-zero cycle compensates for drift in the operating
characteristics of the entire sense circuit. It also allows the use of
crude depletion mode MOSFETs 104 and 106 that need not be matched since
the auto-zero cycle calibrates out the voltage difference.
FIG. 5 shows the voltage waveforms 108 and 110 across gain resistors Rgp 96
and Rgm 98 relative the PWM signal 112 controlling the driver switches
52A-52D. The waveforms illustrate that the operating characteristics of
transistors 62 and 64 may drift creating an offset in the voltage
measurement across the gain resistors Rgp 96 and Rgm 98. In this example
when the PWM signal 112 is high, driver switches 52B and 52C are turned on
and driver switches 52A and 52D are turned off. Zero current flows through
sense resistor Rsensep 58 such that the voltage 108 across gain resistor
Rgp 96 represents the zero-level offset voltage, while the voltage 110
across gain resistor Rgm 98 represents the zero-level offset voltage plus
the current flowing through the voice coil 48. When the PWM signal 112 is
low, driver switches 52A and 52D are turned on and driver switches 52B and
52C are turned off. Zero current flows through sense resistor Rsensem 60
such that the voltage 110 across gain resistor Rgm 98 represents the
zero-level offset voltage, while the voltage 108 across gain resistor Rgp
96 represents the zero-level offset voltage plus the current flowing
through the voice coil 48. The voltage 108 across gain resistor Rgp 96
while the PWM signal 112 is high is subtracted from the voltage 108 across
gain resistor Rgp 96 while the PWM signal 112 is low. Similarly, the
voltage 110 across the gain resistor Rgm 98 while the PWM signal 112 is
low is subtracted from the voltage 110 across the gain resistor Rgm 98
while the PWM signal 112 is high. The resulting composite signal 114
represents the current following through the voice coil 48 with the offset
voltage canceled. Any suitable circuitry may be employed to subtract the
offset from voltages 108 and 110, including sample/hold circuitry
operating relative to the PWM cycle. In an alternative embodiment,
oscillators and counters are used to subtract the offset from voltages 108
and 110 as disclosed in the above-referenced U.S. patent application
entitled "DISK DRIVE COMPRISING OSCILLATORS AND COUNTERS FOR SENSING
CURRENT IN A VOICE COIL MOTOR".
FIG. 6 shows an embodiment wherein separate bias current adjustments are
included by adding resistors 116 and 118 in order to optimize the
operating mode (improve linearity and speed) of the first and second
transistors 62 and 64 by increasing the idle current and thus reducing the
transistor's internal impedances.
Any suitable transistor technology may be employed to implement transistors
62, 64, 100, 85, 102 and 80. In the embodiments described above, the first
and second transistors are bipolar junction transistors (BJT) wherein the
first transistor 62 is a pnp BJT and the second transistor 64 is a npn
BJT. In an alternative embodiment, the first and second transistors 62 and
64 comprise field effect transistors (FETs). Since the first and second
transistors 62 and 64 are emitter (or source) driven, the current sense
circuitry operates extremely fast (high bandwidth) with high fidelity.
*
