Vlcd Supply; Vcfl Supply; Preview Screen; Preview Display Power Supply Block Diagram - HP PageWriter XL M1705B Service Manual

VIN
SWITCHING
REGULATOR
OUTPUT SWITCH
VLCD
VLCDEN
VIN
VCFL SUPPLY
SWITCHING INVERTER
VCFLl
REGULATOR
VCLF2
u1700-10
Figure 5-14. Preview Display Power Supply Block Diagram.
VLCD Supply
The VLCD (contrast) supply is a buck-boost type DC-DC converter
that derives VLCD from VIN. The contrast potentiometer controls
VLCD voltage level.
The series transistor in the output is a switch that is controlled by
the VLCDEN signal. It prevents application of VLCD to the LCD
module before VCC is stable at power-on, and switches off VLCD
prior to VCC at power-off. This prevents damage to the LCD driver
ICS.
The VLCDEN signal may be controlled by the CPU software,
providing a software-controlled means of blanking the LCD.
VCFL Supply
The VCFL (brightness) supply consists of two stages. The first
stage is a switching regulator that converts VIN into a voltage that
is varied by the brightness potentiometer. The second stage is a
high voltage inverter module that generates the high voltage AC
signal necessary to operate the fluorescent lamps used for display
backlighting.
Preview Screen
The preview screen is a 640x400 dot LCD with cold cathode
fluorescent backlighting. The LCD driver circuitry receives control
and data signals that originate from the preview logic board. The
latch pulse (LP), f rame pulse (FP), and shift clock pulse (SCP)
control signals synchronize the LCD. The UD and LD lines provide
display data to the LCD.
Theory of Operation 5-27