Over Current Protection Circuit; Latch Circuit; Standby Operation - Panasonic GP 31 Technical Manual

8.5

Over Current Protection Circuit

1. This circuit is performed by detecting the peak drain current of the internal MOSFET
transistor of IC801, which is used to reverse the output of the internal oscillator of
IC801.
2. The MOSFET drain current is detected by inputting the voltage drop developed
across R820, R821 into pin 7 of IC801 via R830.
3. When this input voltage exceeds the internally generated reference signal of 0.94V,
the drive output is pulled LOW, resulting in the internal MOSFET of IC801
switching OFF and the power supply stopping.
8.6

Latch Circuit

1. This latch circuit is used to keep the output from the oscillator low, stopping the
power supply operating when the overvoltage and overload circuits are in operation.
2. In this condition, the Vin terminal (pin 4) voltage decreases until the Vin terminal
reaches the shut-down voltage of 9.7V. At this point pin 4 begins to rise again but
when it reaches the start-up level 18.2V, the latch circuit continues to stop the drive.
3. When latch is ON, Vin terminal pin 4 voltage increases and decreases within the 9.7V
to 18.2V range.
4. Cancellation of the latch is achieved by switching OFF the TV.
8.7 Stand-by Operation
1. The stand-by power supply circuit keeps the IR, UOC IC601 and EEPROM IC1103
operational during stand-by mode.
2. These circuits require 5V supply during standby so as a switch-ON command from
the remote controller can be processed.
3. During standby, pin 70 of IC601 pulls LOW. When base of Q852 goes LOW, Q857
turns ON. Photocoupler IC860 starts conducting, pulling 140V line to about 7V,
which is determined by R867 and D862.
4. When 140V line is reduced to 7V, all outputs from secondary winding are reduced to
approximately 0V. This 7V is then fed to stand-by IC IC880 to provide 5V to IR,
UOC and EEPROM during stand-by.
5. When the secondary load is lighter, feedback signal is feed to pin 6 of IC801 and
IC801 controls the ON time of MOSFET with a fixed OFF time, during stand-by
mode.
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