Brother HL-3450CN Service Manual page 92

(2) Laser Drive Control Circuit
Laser Drive Control Circuit (LDC) consists of the video signal input circuit, laser drive
circuit, laser diode, output sensing circuit and output control circuit, as shown in Fig.4-
34.
<Operation>
(1) When the video signal is received, the laser drive control circuit switches the laser
diode according to the video signal data.
(2) The radiated laser beam is sensed by the photo detector (PD) and the signal is fed
back to the output control circuit.
(3) The output control circuit controls the laser output to make the level constant by
comparing the laser output with the feed-back value transmitted from the output
sensing circuit.
(4) The scanning laser beam is sensed by the beam detector (PD), then the beam
detecting timing (BDT) signal is output.
PRINT Signal
VIDEO Signal
MCTL PWB
Frequency Control
Circuit
Beam Scanning
Synchronizie
Control Circuit
Control Circuit
Fig. 4-34
4-39
HL-3450CN SERVICE MANUAL
PD
Laser
BDT
LDC PCB
OPC Belt
Scanner Lens
Scanner Motor