ViewSonic P90F Service Manual download pdf (Page 18)

8-1-2-8 Under-voltage protection

If Ac input voltage is too low, voltage across C844 will be too low that force Q814 to turn off

and voltage across C822 rise up. When pin2 of UC3842 rise up to 2.5V, pin6 stop to drive

output. Thus Q801 is protected.

8-1-2-9 Synchronization circuit

UC3842 working frequency is fixed at 33.3 Khz by adjusting R817A.

8-1-2-10 Power Saving control

At normal operating state, Q811 on, Q812 off, output voltage are normal. At stand-by or

suspend Mode, Q811 off, Q812 on, feedback voltage increase to reduce output voltages

down to 1/3 of normal approx.. If enter off Mode, Q811 and Q812 Off, feedback voltage

increase more to reduce output voltages down to

1/4 of normal. And the another great frequency control is to connect R821 to set power

supply working at low frequency about 10 Khz. By above two ways, it can meet DPMS and

NUTEK requirement.

When monitor enter power saving state, voltage across C817 controlled by I804, Q815

continue to supply pin 7 of UC3842 to operate and voltage across C811 controlled by

Q805,Q806 continue to supply 5V for IA01 to stand by.

8-2 DIGITAL CONTROL CIRCUIT( Signal control by Microprocessor unit; IA01)

Horizontal and vertical sync. signal coming from CRT drive Board(PWB—0237)are fed into

pin41(H. sync.) and pin42(V. sync.) of IA01.When the power cord plug in, pin4 of IA01 reset

and IA01 start to check if the sync. Signals are exist at pin 41 and pin 42. The control tables

are as follows:

Power state Normal Standby/suspend Off

Pin42,Pin41 V sync. H. sync. V or H. sync. exist Both sync. not exist

Pin23

Logic high Logic low Logic low

I804 ON OFF OFF

Pin24(G) High High Low

Pin22(A) Low High High

LED Green Yellow Amber

Standby/suspend Mode:

If only one of the sync signal exist, pin23 of IA01 change to low and turns off Q811 and pin21 of

IA01 change to high and turns on Q812. The feedback will be reduced through I808 and cut

down all the secondary output voltages. In order to maintain the supply voltage for I803 PWM,

I804 is then turned on, Q815 turns on too. The supply voltage from D812 is then connected to

Vcc of I803. The off signal from pin23 of IA01 also turns Q805, Q806 on, the supply voltage at

input of I802 is then taken from D804 to keep 5V for IA01 continue working. In order to keep

stand-by voltage for CRT heater, Q867 continue to conduct by the logic from pin21 of IA01.

Off mode:

When both sync. Signals are absent, IA01 will send a logic. Low at pin 23 and pin21 to turn

Q811, Q812 off. The secondary voltages will be reduced further, and Q867, Q866 are turned

off. The heater voltage become zero for further lower power consumption than stand-

by/suspend Mode.

Normal Power supply operation:

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ViewSonic P90F Service Manual Page 18

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