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LG GSA-4040B Service Manual page 39

Super multi dvd drive
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1-2. ALPC (Automatic Laser Power Control) Circuit
ALPC function DVD/CD anlaog front-end IC(IC101 HD153721TF) is for constant power control purpose. Based on the
accurate power sensor(FMON) in P/U, ALPC feedback loop maintains constant power level against laser diode's
temperature variation.
The ALPC loop amplifies(10x) the FMON signal to enhance the accuracy of read power level control. Swithching of
amplification is made by combination of a logical WRGATE2 signal and a logical RWAPC signal.
There are two set of APCDAC, which are used at different term to monitor different power level. Generally, the one is used
at reading and the other one is used at writing. The logical WRGATE2 signal which is switched between reading and writing
changes the two APCDAC.
The ALPC loop supplies same currency (IAPC) to Laser Diode Driver(LDD) during reading and writing.
There is another ALPC which controls Full-Scale-DAC (FsDAC). FsDAC determines scale of WDAC of Laser Diode Driver.
MPU monitors write-power level of FMON signal and changes FsDAC directly to maintain constant write-power level. There
are two write power signal. The one is based on S/H signal and the other one is based on B/H(Bottom-Hold) signal. The
write power levels are monitored as digital levlels which are converted by built-in Anlog-Digital Converter in DSP. The MPU
monitors each write power level and calculates proper setting of FsDAC, and changes value through serial interface
between LDD and DSP.



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