Canon LBP5000 Series, Service Manual

Page: 39 / 162

Chapter 2
2-5
2.3.2.3 Automatic power control (APC)
0011-1371
LBP5000 / LBP5100
The machine makes sure that the intensity of the laser diode (LD) remains at a specific level at all times.
The APC mechanism may be either initial APC or between-line APC, and both are controlled by the laser driver circuit as follows:
1) When the DC controller PCB switches the operating mode of the laser driver circuit to LD forced activation mode, the laser driver circuit forces the LD to go on.
2) A photodiode (PD) checks the intensity of the LD, and converts it into a voltage; it is then compared against the reference voltage (equivalent of the target laser
intensity) within the laser driver IC.
3) The laser driver circuit controls the laser current until the voltage of the laser is identical to the reference voltage.
4) Thereafter, the DC controller PCB switches the laser drive circuit to LD forced deactivation mode; in response, the LD is forced to go off. The laser driver circuit
serves to retain the adjusted laser intensity using a capacitor (C107, C115).
Initial APC
The initial APC is executed when the scanner motor starts up, and is used to adjust the laser intensity.
Between-Line APC
The between-line APC is executed while printing is under way. The laser intensity is adjusted for each single line before drawing any line.
2.3.2.4 Horizontal synchronous control
0011-1382
LBP5000 / LBP5100
The machine controls the laser write start print in horizontal direction as follows:
1) The DC controller PCB causes the laser driver circuit to execute LD forced activation mode in the course of an unblanking period, thereby forcing the laser diode
(LD) to go on.
2) The point in the laser path at which the laser beam starts to scan is equipped with a small fixed mirror (BD mirror) used to reflect the laser beam, guiding it to
the BD circuit inside the laser/scanner assembly.
3) The BD circuit detects the laser beam to generate the BD input signal (/BDI12) and sends it to the DC controller PCB.
4) The DC controller PCB generates the horizontal sync signal (M: /BD1; C: /BD2) based on /BDI12, and sends the result to the video controller PCB.
5) In response to the /BD1 and /BD2 signal, the video controller PCB sends video signals (VDO1, /VDO1, VDO2, /VDO2) to the DC controller PCB so that the
image write position will be correct in horizontal direction.
1. The term "unblanking period" is used to refer to the period of time in which the laser diode is turned on in the non-image area.
2. The /BD1 signal is a timing signal like the /BDI12 signal. The /BD2 signal, however, is a signal that is newly generated based on the /BDI 2 signal.
2.3.2.5 Image Mask Control
0010-6047
LBP5000 / LBP5100
The machine makes sure that the laser beam will not be emitted in a non-image area unless BD detection is under way.
While the laser beam scans a non-image area (except at the time of BD detection), the DC controller PCB causes the laser driver circuit to assume LD forced de-
activation mode, thus forcing the laser diode to go off. The resulting state is referred to as an "image mask state," and the LD is not turned on in the presence of a
video signal. The timing at which image mask control is executed is determined based on the paper size information coming from the video controller PCB. (The
machine is not equipped with a mechanism used to identify the size of paper in the cassette.)
F-2-6
1. The shaded area indicates that an image may be drawn using the laser beam.
2. The time intervals T1 through T4 vary depending on the selected paper size.
3. The time interval T5 varies according to individual colors.
4. If the paper size command from the video controller PCB does not specify the size of print paper in manual feed mode, the machine does not have a means to
find out the width of paper; as such, the T1, T2, and T3 values will be based on LTR size, and the T4 value will be determined based on the length of paper
detected by the registration paper sensor.
/BD
Left/Right
margin masking
T4
T2
2 mm
2 mm
2 mm 2 mm
/T
OP
T1
T5
Top/Bottom margin masking
/BD
Left/Right
margin masking
T2
2 mm
2 mm
2 mm 2 mm
T3
<C, Bk image masking> <M, Y image masking>