Laser Control Circuit - Canon LBP 2460 - B/W Laser Printer Service Manual

B. Laser Control Circuit
1. Outline
DC controller PCB
IC201
J201-C6 /PRNT
CPU
J201-C1 VDO
J201-C16 /BD
This circuit switches the laser diode (LD) ON and OFF at a constant intensity in response to the
VDO signal sent from the video controller.
The VDO signal sent from the video controller to the DC controller is sent as is, to the
switching circuit in the laser driver circuit.
The microprocessor (CPU: IC201) outputs the INTENSITY CONTROL signals (APC1,
APC2) from DA output ports to the analog adder circuit according to the internal data. The
output from analog adder circuit is sent to the laser current setting circuit in the laser driver as the
LASER OUTPUT signal (LLCONT).
This analog adder circuit performs weighted addition for the APC1 signal and APC2 signal at
a ratio of approximately 8:1. The CPU adjusts the value of the LLCONT signal by changing the
APC1 signal for rough adjustment and APC2 signal for fine adjustment.
The output current of the laser current setting circuit is sent to the switching circuit and the
laser diode is switched ON and OFF in response to the VDO signal when the VIDEO DATA
ENABLE signal (/LENBL) (described later) is "L".
2. Automatic laser emisison control
The CPU on the DC controller controls the intensity of the laser beam according to the APC1
signal (laser beam intensity rough adjustment signal) and APC2 signal (laser beam intensity fine
adjustment signal). The analog adder circuit adds the APC1 and APC2 signals, generates a
LASER OUTPUT signal (LLCONT), and outputs it to the laser current setting circuit in the laser
driver.
The CPU performs the automatic laser power control (APC) to let the laser diode emit
constant light, then adjusts the LLCONT signal.
This control is divided into initial APC and between-page APC.
APC1
Analog
J202-2 J1001-8
adder
APC2
circuit
IC205
J202-5 J1001-5
/RD
/WR
Gate
J202-4 J1001-6
ADBUS
array
DATA
J202-7 J1001-3
J202-3 J1001-7
J206-2
Figure 2-3-2
Laser driver PCB
LLCONT
Laser current
setting circuit
/LON
/LENBL
Switching
VDO
circuit
PD
AMP
PD
(Photo diode)
/BDI
J6-2
BD PCB
CHAPTER 2
(Laser diode)
Vcc
LD
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