
8932-A
VOICE-COIL MOTOR DRIVER
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
0.4 1 2
Ai
VBEMF
Ai
KT
1/S
KB
1/RVgm
.
..
+
–
K1
1/Rgm
VIN
Dwg. OP-003B
∑
Current Amplifier.
The A8932CLWA voice-coil motor
driver features a wide transcon-ductance bandwidth and
minimal crossover distortion. The transconductance gain
is user selectable:
(Equation 1)
where A
i
is either 2000 (H GAIN = Low) or 8000 (H GAIN =
High).
The error amplifier
’
s bandwidth and load compensa-
tion zero are set utilizing external resistor and capacitor
feedback components around the amplifier.
The actuator main loop compensation can be set by
applying a square wave and adjusting R
Z
and C
Z
for
optimum response.
Current and Voltage Sensing.
The load current is
sensed internally. Three auxiliary amplifiers are also
included to allow various control functions to be imple-
mented. The first of these amplifiers provides a voltage
output that is proportional to the load current:
(Equation 2)
The second and third auxiliary amplifiers may be used
in conjunction with the first to provide a closed-loop
velocity control system for the actuator arm during a
controlled retract for head parking. This is achieved by
determining the back-EMF voltage generated by the voice
coil and feeding back this information to the main actuator
control input. The back-EMF feedback voltage can be
switched in as required by means of the SWITCH logic
input.
DEVICE DESCRIPTION
The back-EMF voltage represents the velocity of the
actuator. By subtracting the I
R
voltage component
from the voltage across the load, the back-EMF term can
be isolated and fed back to close a velocity control loop.
The amplifier output voltage V
is proportional to the
voltage across the load (A
VD
[V
- V
OUTP
]). R
S
is selected
so that V
represents I
while R
R
LOAD
as shown in the following equations:
V
LOAD
= -A
VD
([I
LOAD
R
LOAD
] + V
BEMF
)
where A
i
= 2000 (H GAIN = logic Low)
V
SENSE
=
R
S
I
LOAD
A
i
where J is the moment of inertia, K
is the back-EMF
motor constant, and K
T
is the torque constant.
Velocity loop compensation =
L
LOAD
/R
LOAD
= R
1
C
1
= R
3
C
2
.
V
SENSE
= R
S
I
LOAD
/A
iL
g
m
=
A
i
R
gm
OUT
SW
= 0.4 (V
BEMF
R
1
/R
2
)
R
2
R
S
0.4 A
i
R
LOAD
R
3
=
=
V
BEMF
V
IN
R
1
K
B
K
T
A
i
2
π
R
VGM
R
2
J
BW = 0.4
R
2
R
Vgm
0.4 R
gm
R
1