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ViewSonic VG181 - Power System

ViewSonic VG181
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ViewSonic
Service
Manual
July
2000
-
Version
1.0
VG181
THEORY
OF
CIRCUIT
OPERATION
Power
System
The
VG181
can
reduce
power
consumption
effectively
when
the
display
enters
the
power
saving
mode.
With
the
smart
power
system
implementation,
the
system
consumes
less
than
3
Watts.
It
also
supports
auto
source
detection
for
the
digital
and
analog
port,
while
in
the
power
saving
mode.
The
auto-recover
function
provides
users
with
a
convenient
feature
if
they
try
to
use
two
PCs
on
one
display.
As
shown
on
the
schematic,
“sheet
8
of
8,“
the
VG181
power
system
contains
an
electronic
switch
for
main
power,
a
switching
regulator
for
3.3V,
and
other
power
MOS
switches
for
different
parts
of
the
system.
The
electronic
switch
is
based
on
a
D
type
flip-flop
(U21)
and
a
power
MOS
(U20).
There
is
also
a
over
voltage
protector
which
is
composed
of
Q5,
Q6,
and
ZD2
to
prevent
high
voltage
from
getting
into
the
system.
When
the
input
exceeds
approximately
15
volts,
Q5
and
Q6
will
be
turned
on.
So,
the
D
flip-flop
will
be
preset
to
a
high
state
and
turn
off
the
power
MOS
(U20).
The
PW1
and
PW2
are
connected
to
the
keypad
of
“
POWER’
directly.
Once
the
user
presses
the
POWER
keypad,
there
will
be
a
pulse
on
pin3
of
U21
for
triggering
the
flip-flop.
Then
pin1
of
U21
will
cycle
it’s
state
to
high
or
low
level
for
controlling
the
power
on
or
off.
:
The
3.3V
regulator
is
composed
of
a
comparator
(U26),
a
power
MOS
(U25),
and
L25,
D15,
C157,
C158
etc.
The
reference
voltage
is
made
from
the
ZD03,
R120,
R121,
and
VR1.So
the
regulator
output
voltage
will
follow
the
voltage
on
pind
of
U26.
There
is
also
a
over
voltage
protector
within
the
regulator.
If
the
voltage
on
pin11
(which
feeds
back
from
regulator
output)
is
higher
than
the
voltage
on
pind,
then
U27
will
be
turned
off
so
it
can
prevent
an
over
voltage
from
damaging
most
of
the
devices
on
the
main
board.
The
LCDPWON
is
the
control
signal
from
CPU.
It
will
turn
off
the
SW-12V
which
is
the
main
power
source
of
LCD
module
when
the
system
enters
the
power
saving
mode.
The
AUXPWON
is
the
enable
signal
for
inverter
by
the
same
condition.
Page
27
Confidential
-
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Not
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