Preview Display Power Supply Board - HP PageWriter XL M1705B Service Manual

The SRAM is a CMOS 32Kx8 static RAM that contains a complete
graphics bit map of the LCD. Display data is written to or read from
SRAM.
The LCD controller reads the display data from SRAM and outputs
it on the upper and lower screen data lines (UD and LD). The
controller also generates synchronization signals that send the data to
the LCD.
The LCD enable circuitry generates the VLCDEN signal that allows
the system to turn the LCD (and backlight) on and off. Functionally,
this circuit is an AND gate that generates VLCDEN when +5 volts
is stable (+5READY) and the system has turned on the LCD
(LCDON). The latch pulse signal (LP) must also be present before
the LCD can be turned on, thus preventing damage to the LCD.
The upper screen and lower screen data lines (UD and LD), sync
signals (FP, LP, and SCP), VLCDEN, VIN, +5 volts (VCC), and
GROUND leave the preview logic board and connect to the preview
display power supply board.
Preview Display Power
The preview display power supply board, shown in Figure 5-14,
Supply Board
provides three sources of power to the LCD module:
m VCC (+5 V DC)
n
VLCD (6 to -23.5 V DC)
w VCFL (about 400 V RMS @ 30 kHz)
Note .I
w
The upper and lower screen data signals (UD and LD), and the sync
signals (LP, FP, and SCP) pass unaffected through this board to the
LCD module.
VCC, which is obtained from the main system +5V power supply,
provides basic power to the LCD and its associated driver ICs.
VLCD controls the LCD contrast (the optical characteristic of the
LCD), and VCFL controls the LCD brightness (the intensity of the
cold cathode fluorescent backlighting).
5-26 Theory of Operation