Canon ImageCLASS MF4690, Service Manual

Page: 62 / 182

Chapter 5
5-2
5.2 Controlling the Laser Activation Timing
5.2.1 Laser ON / OFF Control
0016-1834
This is the control where the laser driver turns on / off the laser diode (LD) based on the laser control signal sent from the DCNT board.
The following is the circuit diagram for laser control.
F-5-2
The DCNT board sends the video signal (VDO, /VDO) for image formation and the laser control signal (CNT0, CNT1) for switching the laser operation mode to
the logic circuit in the laser driver IC.
The laser driver IC performs laser control based on the combination of CNT0 and CNT1 signals.
The following chart shows the combination of the laser control signals (CNT0, CNT1).
T-5-1
5.3 Controlling the Intensity of Laser Light
5.3.1 Auto Photoelectric Current Control (APC)
0016-1835
This is the control to emit the definite level of laser diode.
There are initial APC during initial rotation (note 1) and line space APC during printing (note 2), which are performed in the same procedure. The following is its
procedure.
1) When the laser control signal (CNT0, CNT1) turned into LD APC mode, the laser driver emits LD forcedly.
2) After being detected with the photodiode (PD), the LD emission level as current is converted to the one as voltage and compared with the reference voltage (the
voltage equivalent to the target laser level).
3) The laser driver controls the laser current until it reaches the voltage of LD target level.
4) When the laser control signal turns into LD forced OFF mode, LD is forcedly off and the laser driver converts the adjusted laser level to the voltage of the con-
denser and keeps it.
Note 1. Initial APC
APC during initial rotation. Laser level is adjusted by APC.
Note 2. Line space APC
APC during printing. Adjustment of laser level for one line is performed before writing one line.
Operation mode CNT0 CNT1 Remarks
Discharge mode
L L
Discharge of C803
Print mode
H L
Use at normal printing
LD APC mode
L H
Use at LD APC
LD forced OFF mode
H H
Use at masking image
J931-8
-9
-5
-6
-3
-2
/BDI
CNT1
CNT0
VDO
/VDO
IC902
CPU
Laser driver circuit
+5V
SCNT board
J902-2
-1
-5
-4
-8
-7
/BD
VDO
/VDO
J901-6
-3
-2
Comparator
PD LD
C803
DCNT board
BD sensor
Photo diode) Laser diode)
Laser driver IC
Logic
circuit
Drive
circuit
Sample
hold
circuit