AN1139 STMicroelectronics, AN1139 Datasheet - Page 33

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AN1139

Manufacturer Part Number
AN1139
Description
L6254 - L6268 - L6269 12V DISK DRIVE POWER COMBO IC
Manufacturer
STMicroelectronics
Datasheet
The second input of the two comparators is one of the two anti-phase triangle waves whose 50% voltage is VCC/2.
As shown in Figure #15, when the Vx signal is equal to VCC/2, the outputs voltage VCM+ and VCM- are in phase
(both high or both low) resulting zero current in the coil. When the Vx signal is grater or less than VCC/2, the duty
cycle of VCM+ and VCM- changes (if one increase the other decrease) resulting a positive or negative current
into the coil.
Figure 15. Voice Coil Psm Output Voltages & Current Signals
The amplitude of the anti-phase triangle waves (T1,T2) has been chosen to keep the effective gain from Vx to
the output the same as in linear mode. This minimize transient DC shift in the control loop voltages when switch-
ing between PSM and Linear modes. The frequency of the triangle waves is generated by an internal oscillator
and its value is about 50KHz +/- 30%. As can be noticed in Figure #15, the effective chopping frequency is twice
the frequency of the oscillator.
5.2.3 D/A converter
A 14 bit DAC is available through the serial port in the Reg#0.0:5 (Vcm Dac High) and Reg#1.0:7 (Vcm DAC
Low). It is required to write on register #1 to make effective changes on register #0. This means that if a change
is done to a bit in a low VCM_DAC register (Bit 0 through Bit 7), only the Register #1 needs to be written. If a
change is done to a bit in a high VCM_DAC register (Bit 8 through Bit 13), both register #0 (first) and register
#1 need to be written.
The ladder of the DAC and the DAC circuit itself, are connected internally to a floating power supply in which a
feedback system is keeping the supply centered to VCC/2. This is done mainly to compensate the variation of
the power supply.
The output of the DAC swings from VCC/2 + 1V (DAC set as Hex.4000 or Dec.16384) and VCC/2 -1V (DAC set
OUTPUT
CURRENT
OUTPUT
CURRENT
OUTPUT
CURRENT
VCM+
VCM-
VCM+
VCM-
VCM+
VCM-
Vx = Vcc/2
Vx > Vcc/2
Vx < Vcc/2
AN1139 APPLICATION NOTE
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