Philips EM7U LCOS, Service Manual

Page: 19 / 104

3.3 VOLT REGULATOR (Figure 25)
IC 7307 and transistor 7308 make up the 3.3-volt regulator located on the Main Power Supply
board. When the Oscillator in 7307 goes Low, the flip-flop is reset causing Q to go Low and
the output to go High. When the Feedback voltage on Pin 5 is below 1.25 volts, the
comparator outputs a High. When the Oscillator goes High, the flip-flop is Set, causing Q to
go High. This causes Pin 1 to go Low, turning Transistor 7308 On. 7308 will stay on until the
Oscillator goes Low, resetting the flip-flop. If the output voltage monitored on Pin 5 goes
below 3.3 volts, the voltage on Pin 5 will drop below the 1.25 volt reference. The flip-flop will
then Set when the Oscillator goes High. If the output voltage goes above 3.3 volts, the
reference voltage on Pin 5 will be above the 1.25 volt internal reference. The flip-flop will not
Set when the Oscillator goes High until the feedback reference voltage on Pin 5 drops below
the 1.25 volt internal reference. This method controls the On time of 7308 to regulate the
3.3-volt supply. Pin 7 monitors the current by reading the voltage drop across resistors 3349
and 3351. If the output current is excessive, the Oscillator will go Low to shorten the On time
of 7308.
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POWER INPUT BOARD IN SERVICE POSITION MAIN POWER BOARD