55
DC Control
IC851
4
1
Backlight Block
The LCD panel needs a backlight to view a picture on the rear panel. A
backlight On/Off switch on the rear panel permits the user to shut off the
backlight, saving battery power when the display is not used. The cam-
era can then remain on for those spontaneous pictures.
The backlight circuitry is on the rear PK board. It is powered from the
battery or AC adapter via fuse F003 and switched on when Panel 4.9V is
applied to the circuitry.
On/Off Control
The backlight On/Off switch at the rear panel is input to HI Control IC404
on the FC-72 main board (not shown). IC404 sends data and clock sig-
nals to EVR IC902 to switch Panel 4.9V to the backlight circuit.
IC404 instructs EVR IC902 using a common serial data bus (HI SO input
IC902/pin 20) and an individual chip select line (XCS Panel D/A input
IC902/pin 2). When the XCS chip select line goes LOW, the data on the
serial bus line (HI SO) is for IC902.
One of the pieces of data on the bus instructs IC902 to set a voltage at pin
10. The voltage at pin 10 controls the backlight, turning on or off DC-DC
control IC851. IC851 powers the backlight.
B acklight C ontrol
B acklight
IC 902/pin 10
IC 851/pin 2
O FF
0V dc
0V dc
O N
2V dc
4.9V dc
Regulated Power Source
The backlight stage has its own regulated power supply necessary to
keep the light intensity stable. The PWM regulator stage consists of
four main parts that output a stable 5.2Vdc to the oscillator.
Backlight Regulator
Name
Input / Output
Purpose
Q856
BL Unreg Voltage / Regulated
5.2Vdc
Switching regulator
L852 &
C861
Pulsating DC / DC
LC filters pulses to DC
R860-
R862
5.2Vdc input R860 / 1Vdc output
to IC851/pin 4
Returns a sample of the
output voltage for
regulation.
IC851
DC error voltage input pin 4 /
6.8Vp-p 480kHz-pulse width
modulation output pin 1.
Changes the Pulse
Width to maintain a
constant voltage at pin 4
When the backlight is switched ON, IC851/pin 2 receives 4.9V to operate.
IC851 generates a pulse from pin 1 that turns switching transistor Q856
on and off. These pulses are output Q856/collector into L/C filter L852
and C861. This L/C filter smoothes the pulses into an average DC volt-
age. This output voltage is applied to the oscillator, but a sample of this
voltage is returned to IC851 for regulation.
Regulation
The 5.2 V output is reduced by voltage divider R860-R862 so a represen-
tative sample of the output (about 1 volt) is returned to IC851/pin 4 for
regulation. IC851 monitors the input voltage at pin 4 and adjusts the
pulse width output pin 1 to control the average time Q856 is turned on.
The longer Q856 is turned on (wider low going pulse width), the higher
the output voltage.
