L7250 datasheet(29/46 Pages) STMICROELECTRONICS

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L7250

3.6.1 Voltage Mode

In Voltage Mode, the VCM power outputs will apply a voltage to the VCM motor commanded by the VCM DAC.

This is implemented by tristating the sense amp and error amp outputs, and connecting DAC_OUT to

ERR_OUT with an internal switch (switch S2). Skipping the err_out amplifier the DAC command will enter the

power section without any inversion, then the DAC codification must be considered in opposite direction respect

to the current mode operation.

3.7 VCM Loop Offset Calibration Mode

The VCM Loop Calibration mode can be implemented following two different approach:

1) The VCM loop is enabled (sense amp, error amp, DAC), but the VCM power stage is tri-stated. Thus,

the sense amp is guaranteed to be monitoring a zero current condition.

To implement offset calibration, the current command is swept through zero by the controller ASIC.

Since the Gm loop is open, the error amp output will be saturated in one direction or the other depending

on the current command (to configurate the error opamp as a comparator the external compensation

network will be disconnected opening the switch S1). As the command sweeps through the zero current

command point, the error amp output will swing to the other extreme. The comparator senses the out-

put swing of the error amp, and through the serial port (Reg. 00H -> b6) interrupts the ASIC. The ap-

propriate DAC value corresponding to the trip point interrupt is the loop zero current offset.

Figure 9. VCM Current Loop Offset Calibration 1

Set VCM Offset Calibration

Write Reg.03H

VCMState[2:0] = 101

START

VCM Current Loop

Offset Calibration Routine

UPDATE 15 Bit DAC

Write Reg.09H

dac[14:0]= DAC_VAL

EXIT

VCM Current Loop

Offset Calibration Routine

Select

Sense Amplifier

Gain

4.5 V/V

16 V/V

DAC_VAL = 0

Flag1 = 0 , Flag2 = 0

Set SenseAmpl.Gain

Write Reg.09H

GainSW bit = 0

Set SenseAmpl.Gain

Write Reg.09H

GainSW bit = 1

Read Error Ampl Output

Read Reg.00H

VCMcalOut bit value

VCMcalOut = 0

YES

DAC_VAL = DAC_VAL +1

Flag2 =1

Flag1 = 1

DAC_VAL = DAC_VAL -1

Flag2 = 1

YES

Store the DAC_VAL

as the zero loop offset

* DAC_VAL is in 2 complement format

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