Canon LBP-1000 Service Manual page 54

CHAPTER 2
2. Automatic power control of laser diode
The laser driver IC conducts the automatic power control (APC) of the laser diode so that the
laser diode emits a beam of constant intensity. The APC is conducted prior to the image for-
mation (Initial APC) and between lines (between line APC).
When the CNTRL0 signal and CNTRL1 signal input to the laser driver IC from the CPU are
"L" and "H" respectively, the sample hold circuit in the laser driver IC enters the sample mode
(mode in which the APC is performed). At the same time, the laser diode (LD) emits the laser
beam forcibly.
The intensity of the laser beam emitted from the laser diode is detected by the photo diode
(PD) and fed back to the sample hold circuit via the current/voltage converting circuit and dif-
ferential amplifier.
The laser driver IC controls the output current of the drive circuit to make the fed back laser
intensity become equal to the target laser intensity set in the IC.
When the CNTRL0 signal and CNTRL1 signal become "H", the laser diode is turned OFF
forcibly and the sample hold circuit enters the hold mode. The laser driver IC converts the con-
trolled laser intensity to voltage and stores it in the C804.
3. Horizontal sync control
The CPU generates the UNBLANKING signal based on the BD INPUT signal (/BDI) sent from the
BD PCB in the laser/scanner unit.
During unblanking period, the CPU controls the LASER CONTROL signal to emit laser
forcibly.
There is a small fixed mirror (BD mirror) on the scanning start position in the optical path.
The laser beam is reflected by the BD mirror and sent to the BD PCB in the laser/scanner unit.
The BD PCB generates the BD INPUT signal (/BDI) based on the detected laser beam and
sends the signal to the CPU.
The CPU generates HORIZONTAL SYNC signal (/BD) based on the /BDI and starts sending
the signal to the video controller when it detects that the picked up paper has reached the spec-
ified position.
4. Image masking control
The CPU performs the image masking control in order to prevent laser beam emission in the
non-image area other than the unblanking period.
The laser beam emission enable period of the laser diode is controlled by masking the /VDO
and VDO with the LASER CONTROL signals (CNTRL0, CNTRL1) in the logic circuit in the laser
driver IC.
The CPU sets the CNTRL0 and CNTRL1 to "H" in the image mask area other than the
unblanking period so that the laser diode will not be turned ON/OFF when the /VDO and VDO
are sent to the laser driver IC from the video controller.
In image unmasking area, the laser driver IC turns ON/OFF the laser diode based on the
/VDO and VDO by setting CNTRL0 to "H" and CNTRL1 to "L."
2 - 20