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Fabricante

A8932CLWA

VOICE-COIL MOTOR DRIVER

Allegro MicroSystems 

8932-A

VOICE-COIL MOTOR DRIVER

Retract and Brake. A retract-brake sequence is initiated

on receiving a fault indication from the internal thermal

shutdown (TSD), or under-voltage lockout (UVLO), 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 VIN 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:

2R

VRET-SET

=

1000

+

8

R7

+

R8

(Equation 3)

where

R

7

+

R

8

>>

1000

Ω.

When the sequence is operated, the output voltage is

forced to approximately VRET-SET 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 VBEMF

terminal. The A8932CLWA will perform the retract func-

tion under low supply conditions (nominally down to 2 V).

Operation down to almost 0 V requires an energy-storage

capacitor at the VFLT terminal.

Protective Features. The A8932CLWA 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 4.2 V. It can be

user-defined with an external resistor voltage divider:

2 (R + R )

UVTRIP =

5

6

R6

where R5 + R6 << 200 kΩ.

(Equation 4)

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.

TEST CIRCUIT AND TYPICAL APPLICATION

RZ

V IN

Rgm

V REF = 2 V

CZ

X

H GAIN

PWR OFF

R FAUL≥T 200 kΩ

ERROR

– AMP

+

+5 V

V CC

VDD

I SENSE +

POWER

AMP

+

VOLTAGE-SENSE

AMP

–

+

V REF

VOICE COIL

MOTOR

2V –

POWER

CONTROL

LOGIC

TSD

CURRENT-SENSE

AMP

I SENSE –

–

V REF

+

R2

RS

R3

C2

PROTECTION

RETRACT

CIRCUITRY

CONTROL

–

VREF

+

R5

V CC

R7

R6

R8

C1

R1

R Vgm

X

SWITCH

BACK EMF FROM

SPINDLE MOTOR

VDD

DIODE FOR RETRACT FUNCTION

OTHERWISE SHORT TO SUPPLY

Dwg. EP-041D

www.allegromicro.com

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