LG CED-8083B 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.
Feedback signal, output current from PD (Photo Detector) of P/U, is used to monitor the light power of
Laser Diode.
RREF (Read Reference Voltage) of IC401 (CXA2551R) Pin 12, which is from DAC (IC409) Pin 18, is the
reference level of this Loop Circuit.
* Read Loop
• When playback
VRDC (Pin 14) signal of CXA2551R is output to P/U through Gain Control S/W and drives Laser Diode
during playback.
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 (RFPDSH), which sample and hold the Read Level of laser
power in the CD-R writing mode, is input through Pin 39 of CXA2551R.
Hence, S/H circuit makes feedback laser power level constant.
- 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 38 of CXA2551R.
Output voltage of Write Loop, VWDC (Pin 10 of CXA2551R), 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 of IC409) and it is input to Pin 5
of IC401 (CXA2551R).
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