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参数资料
| 型号： | 8936 |
| 厂商： | Allegro MicroSystems, Inc. |
| 元件分类： | 基准电压源/电流源 |
| 英文描述： | VOICE COIL MOTOR DRIVER |
| 中文描述： | 音圈电机驱动 |
| 文件页数： | 6/10页 |
| 文件大小： | 102K |
| 代理商： | 8936 |
8936
VOICE COIL MOTOR DRIVER
TM
McroSystems, Inc.
A
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
Current Amplifier.
The A8936CJT and A8936CLW
voice coil motor drivers feature a wide transconductance
bandwidth and no measurable 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. Two 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 auxiliary amplifier 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.
Under- & Over-Velocity Fault.
For a constant load,
motor current (I
) and therefore V
are proportional
to motor velocity. V
is amplified by the uncommitted
amplifier and compared against the internal 2 V refer-
ence and used to indicate a velocity fault if the voltage is
greater than a nominal
±
0.75 V from the 2 V reference.
EN VEL FLT may be tied to the FAULT terminal to reset
the velocity fault after a tripout.
Retract and Brake.
A retract-brake sequence is initiated
on receiving a fault indication from the internal thermal
shutdown (TSD), under-voltage lockout (UVLO), the
under- or over-velocity fault, or an externally applied logic
High at the RETRACT input.
If the velocity control scheme is implemented, the
head can be retracted under the full control of INPUT in
conjunction with OUT
back-EMF voltage if no fault
condition exists. If a fault condition were to occur how-
ever, the retract velocity would be controlled by applying a
constant user-defined voltage across the load:
(Equation 3)
where R
7
+ R
8
>> 1000
.
When the sequence is operated, the output voltage is
forced to approximately V
to retract the heads, and
then a fault command (
“
brake
”
) is sent to the spindle-
motor driver. The user determines the total time for the
retract sequence, before the spindle brake is enabled, by
the choice of an external resistor and capacitor at the
FAULT output.
Power for the retract function is provided by the
rectified back EMF of the spindle motor by way of the
V
terminal. The A8936CJT/CLW will perform the
retract function under low supply conditions (nominally
down to 2 V). Operation down to almost 0 V requires an
energy-storage capacitor at the V
FLT
terminal.
Protective Features.
The A8936CJT/CLW has a number
of protective features incorporated into the design.
Under-voltage lockout provides system protection in the
event of reduced primary supply voltages. The under-
voltage trip point is internally set at approximately 4.3 V.
It can be user-defined with an external resistor voltage
divider:
(Equation 4)
where R
5
+ R
6
<< 200 k
.
Thermal shutdown circuitry is included to protect the
device from excessive junction temperature. It is only
intended to protect the chip from catastrophic failures due
to excessive junction temperature.
DEVICE DESCRIPTION
A
i
R
gm
g
m
=
R
S
I
LOAD
A
iL
V
SENS
=
V
RET-SET
=
2 R
8
1000 + R
7
+ R
8
2 (R
5
+ R
6
)
R
6
UV
TRIP
=
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