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Número de pieza

componentes Descripción

Fabricante

8936

VOICE COIL MOTOR DRIVER

Allegro MicroSystems 

8936

VOICE COIL MOTOR DRIVER

DEVICE DESCRIPTION

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:

gm =

Ai

Rgm

(Equation 1)

where A is either 2000 (H GAIN = Low) or 8000 (H GAIN

i

= 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 and C for

Z

Z

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:

VSENS

=

RS ILOAD

A

iL

(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

LOAD

SENS

to motor velocity. V is amplified by the uncommitted

SENS

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 OUTSW 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:

VRET-SET

=

2 R8

1000 + R7

+

R8

where

R

7

+

R

8

>>

1000

Ω.

(Equation 3)

When the sequence is operated, the output voltage is

forced to approximately V

to retract the heads, and

RET-SET

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

BEMF

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 terminal.

FLT

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:

2 (R + R )

UV =

5

6

TRIP

R

6

(Equation 4)

where

R

5

+

R

6

<<

200

kΩ.

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.

A

TM

MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

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