L6256 datasheet(7/28 Pages) STMICROELECTRONICS

7 / 28 page

SATURATED SEEK BIT

The processor can command the VCM amplifiers

to hard saturate, in the polarity determined by the

sign bit of the DAC. The saturation detector bit is

not the echo of this bit, but is a separate compa-

rator bit representing the true state of the ampli-

fier. Thus, it can be used for loopback testing of

the DOLPHIN.

VCM CURRENT SENSE AMPLIFIER

The input differential voltage of the amplifier can

be limited to low voltage, but common mode re-

jection is very high. The amplifier is capable of

operating smoothly when the VCM amplifiers are

saturated, providing no input charge buildup or

other anomalies.

Charge does not build up on

the inputs even when VCM inductance forces the

inputs substantially above the supply or below

ground.

SATURATION DETECTOR

This detector notifies the processor when the

commanded VCM current does not match the ac-

tual VCM current.

The threshold is set by

VC_Sat.

Fault Detection

UV DETECTION

The power supply undervoltage protection is set

up for the appropriate tolerances, and causes a

low signal on POR\.

A small hysteresis is in-

cluded on the voltage comparators, and band-

width limiting techniques are used. Current limit

from the 3.3V regulator has been added to the

POR error inputs.

POWER ON RESET (see appendix B)

The power on reset circuit provides the following

functions:

● A retriggerable one shot of several millisec-

onds.

● An interlock circuit which provides for dis-

charge of the one shot, and a clamp to hold

the POR\ line low during the timing interval.

● Circuitry to pull POR\ high quickly after the 1

shot has timed out.

● A current source or weak pullup to pull the

POR\ line high against external leakage cur-

rents.

Undervoltage conditions override external inputs

and force POR\ low. External inputs do not cause

pulse stretching; all internal inputs do.

PCBA in-circuit testing can arbitrarily pull this line

low as necessary to restart the system.

Alter-

nately, a 1 milliamp current can be introduced to

the timing capacitor to speed up the POR timeout.

THERMAL LIMIT

The thermal limit of the chip is set for THlimit with

THhyst degrees of hysteresis. Thermal limit is a

relative voltage, above Thwarn for tolerance rea-

sons, and must protect the part; it indicates that

thermal limit is taking place by disabling the serial

port (see serial port section). A park and a spin-

dle driver tristate is performed when thermal limit

occurs.

THERMAL WARNING

Thermal warning is made available to the proces-

sor as a status bit in every register, to allow a

modified control algorithm strategy that reduces

power dissipation and drops chip temperature.

PARK CIRCUIT

The park circuit provides smooth head retraction.

In Park, the VCM is switched to voltage mode.

Bout is grounded. The A amplifier’s positive in-

put is switched from the normal half supply refer-

ence down to Vpark, and Aout applies the volt-

age determined by Vpark and Rp1 and Rp2. This

damps any motion that may been in progress and

causes the head to retract into the latch.

L6256

7/28

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