LG GCC-4520B, Service Manual

Page: 27 / 66

DESCRIPTION OF CIRCUIT
1. ALPC (Automatic Laser Power Control) Circuit
1-1. Block Diagram
30
MT1628E
Write
Strategy
Write
S/H Signal
Micro
Processor
IC 100
CN 101
IC200 ( MT1616E)
Optical
Pick-up
LPC-711A
LD
Drive
33 114
113
FPDO
FPDOLP
105 23
VWDC1O
VWDC1
106 24
VWDC2O
VWDC2
61 177
WLDON
59
179
RLDON
104 21
VRDCO
RREF
VRDC
58
181
RFPDSH
S/H &
VRDCG &
Level Shift
60
S/H &
VWDC1G &
Level Shift
VWDC1
DAC
& Gain
WFPDSH
178
WDAC2
&
Amplifier
WDAC1
& x1/x2
RDAC
VRDC
DAC
& Gain
32 112
FVREF
+
-
-
+
1-2. APC (Automatic Power Control) Block
APC (Automatic Power Control) function in CD-R/RW analog front-end is used for constant power level control
purpose. Based on the accurate power sensor (FMD) in PUH, APC feedback loop maintains constant power level against
laser diode’s temperature variation.
There are three power control loops in MT1616, two for CD and one for DVD, which are used with different combination
for different applications. Generally, the first APC loop is used for read-power control. The second APC loop is used for write
(erase) power control for CD-R (CD-RW) disc. And the third APC loop is used for DVD read power control.
Owing to the small signal level in read-power control mode, the first APC loop amplifies (x5/x10/x15/x20) the FMD
signal (FPDO) to enhance the accuracy of read power control. The register VRDCG[1:0] is used to adjust the gain of FMD
signal. The registers RDAC[7:0] and VBDAC[7:0] will set the read power level at the built-in 8-bit DAC (RDAC). The second
APC loop is used for high power control. The built-in 10-bits DAC(WDAC1) is used to set the wanted power level. And the
register VWDC1G is employed to adjust the gain of FMD signal.
The following potentiometers (VRDCDAC, VWDC1DAC, and VWDC2DAC) and amplifiers (VRDCDAG, VWDC1DAG,
and WDAC2G) are used to set the wanted levels of VRDCO, VWDC1O, and VWDC2O. Moreover, the input source of the
VWDC2O path can be fed from the output of VWDC1O or directly programmed by the built-in 10-bit DAC (VWDC2DAC).
Moreover, the input signal FPDO (form CD FMD) after low-pass filtering $ amplification (x1 or x2) will send the
MPXOUT2 for monitoring.
Furthermore, the third loop is used for DVD read power control loop. Read power level is adjusted by the RREF voltage
set in the first APC loop.