POWER SUPPLY MONITOR

This circuit monitors the logic supply (5V) and the

power supply (12V) and activates the power on

reset output (POR) and the VCM PARK circuit.

After both logic and power supply reach their

nominal value a timing capacitor (T_CAP) has to

be charge before the POR output change from

low to high level.

POR delay

=

C

⋅ V

I

where:

C is the capacitor value connected at pin

T_CAP

V is delta voltage that capacitor have to be

charged (2.3V)

I is the costant current charging the capacitor

(4

µA typ.)

At the two input pins, +12 FILTER CAP and + 5

FILTER CAP, can be connected two capacitors

for filtering the noise on the power supply, avoid-

ing in this case undesired commutations of the

POR signal because of some fast negative spikes

on the line.

BRAKE AND PARKING CIRCUITS

The voice coil driver is switched into the parking

condition through the VCM PARK input or when

the POR signal is low. In such condition immedi-

ately the output stage turns on the two lower

DMOS of the power bridge to activate the BRAKE

of the voice coil motor.

After a delay generated by the capacitor at the

BRAKE DELAY pin, only one of the two lower

DMOS stays on while the opposite half bridge is

tristated.

BRAKE delay

=

C

⋅ V

I

where:

C is the capacitor value connected at pin

BRAKE DELAY

V is delta voltage that capacitor have to be

charged (3V)

I is the costant current charging the capacitor

(5

µA typ.)

The parking voltage is then supplied by the

PARKING circuit connected to the output that has

been tristated.

The value of such a voltage is set by connecting

an external resistor between the RPARK pin and

ground.

=

Vbandgap

Rpark

where:

Vr is the retract voltage for parking the heads

Vbandgap is the internal bandgap reference

voltage of 1.4V

Rpark is value of the resistor connected at

RPARK pin

The parking circuit takes the power supply from

the spindle driver through the VBEMF pin, so that

in case of power fail the retract of the heads is

possible using the rectified BEMF voltage coming

from the spindle motor.

CHARGE PUMP

The charge pump circuit is used as a means of al-

most doubling the power supply voltage (12V) in

order to drive the upper DMOS of the power

bridge.

The energy stored in the in the capacitor con-

nected at VCP pin is also used to drive the gate

of the external N-MOSFET.

GATE DRIVER

This circuit provide the voltage driving the gate of

the external isolation N-MOSFET, and it is con-

trolled by the POR signal.

THERMAL

The thermal protection circuit has two threshold,

the first if the pre shut down alarm that activates

the THERMAL SD signal and the second is the

shut down temperature that tristates the output

stage when the junction temperature increases

over this level.

APPLICATION INFORMATION

Example of calculation of the error amplifier com-

pansation for the stability of the current control

loop. As can be seen from the draw of the current

control loop circuit of the next page, the voltage

across the load is:

#1

ror, Sense and Input Amplifier.

Changing in the #1 the transfer function between

#2

=

L6246

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