LG CED-8081B Service Manual page 40

1-2. ALPC(Automatic Laser Power Control) Circuit Operation
This circuit consists of Feedback Loop to maintain light output of the Laser Diode (FPDO).
Feedback signal, output voltage from PD (Photo Detector) of P/U, is used monitor the light power of Laser
Diode.
RREF (Read Reference Voltage) of IC401 (OTI-9071) Pin 10, which is from DAC (IC409) Pin 18, is the
reference level of the Read Loop part of this ALPC Circuit.
WREF (Write Reference Voltage) of IC401 (OTI-9071) Pin 5, which is from DAC (IC409) Pin 3&4, is the
reference level of the Write Loop part of this ALPC Circuit.
* Read Loop
• When playback
VRDC (Pin 12) signal of OTI-9701 is output to P/U through Gain Control S/W and drives Laser Diode
during play back.
This S/W Circuit is designed to reduce transition time from CD-RW writing mode to playback mode.
• When writing mode
- CD-R
Three Laser Power Levels, Read, Write, and Overwrite, are used to write on CD-R disc, and Read
Level is used to monitor the output laser power.
For stabilizing read loop, the S/H signal (FRPDSH), which sample and hold the Read Level of laser
power in the CD-R writing mode, is input through Pin 50 of OTI-9071.
- CD-RW
Three Laser Power Levels, Read, Erase and Write, are used to write on CD-RW disc, and Erase Level
is used, during CD-RW writing, to monitor output laser power.
It is not VRDC but VWDC that is the output signal of the control loop performed by Erase level.
ERGCNT makes ALPC Loop stable when it is changed to playback mode from writing mode.
* Write Loop
For stabilizing Write Loop, the S/H signal (WFPDSH), which sample and hold the Erase Level of laser
power in the CD-RW writing mode, is input through Pin 51 of OTI-9071.
Output voltage of Write Loop, VWDC (Pin 6 of OTI-9071), is protected by the high limit diode applied
to P/U.
In the writing mode, the reference signal of Write Loop is WREF (Pin 3&4 of IC409) and it is input to
Pin 5 of IC401 (OTI-9071).
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