KX-TDA6178X, KX-TDA6178XJ - Panasonic PBX Service Manual (repair manual). Page 16

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KX-TDA6178X/KX-TDA6178XJ
5.2.15.  Power Supply Voltage Generator 
• Block Diagram
The input from the main power supply is in 2 voltage levels: +15V, +40V.
Since the voltage levels: +15V, +3.3V, +80V, -30V and -55V are required onboard, voltage levels are generated onboard as written
below.
• Protection of 3.3V output
The protection circuit consists of an output short protection and an overvoltage protection.
When an output of power supply is short, IC1 works to stop the output as the output short protection.
When +3.3V output become overvoltage caused by Q4 short, it is detected by D7. Q3 is cut by Q2 to shut down the input power
supply as the overvoltage protection.
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