LG CED-8042B 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 IC601 (AK8563) Pin 77, which is from DAC (IC503) Pin 19, is the
reference level of this Loop Circuit.
PD output current is converted to voltage through external R, C and AK8563 internal I/V Amp.
* Read Loop
• When playback
VRDC (Pin 79) signal of AK8563 is output to P/U through Gain Control S/W and drive 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 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 38 of AK8563.
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.
As the Control Loop is performed by Erase Level and resulting output of the ALPC is VWDC instead of
VRDC. Therefore, WREN is active so that Read Loop can not operate.
WRHOLD 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 39 of AK8563.
Output voltage of Write Loop, VWDC (Pin 6 of AK8563), is divided by external resisters and applied to
P/U.
In the writing mode, the reference signal of Write Loop is WREF (Pin 5 of AK8563) from DAC port
(IC503 Pin 7, 8).
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