Panasonic TC-L42D2 Technical Manual page 36

LED Backlight Control
The inverter receives 174V from the SMPS. The 174V source is provided to
the panel to power the LED. The Backlight_On command from pin 13 of
connector P2 starts the inverter drive signals. The inverter control signal,
PWM, is delivered by the TCON board at pin 15 of connector P2 to control
the brightness of the LEDs. The inverter is responsible for controlling the
amount of current passing through the LEDs.
To avoid catastrophic failures, when the inverter circuit is defective, the
BACKLIGHT_SOS command is output to IC8001 to trigger the shutdown of
the TV. The power LED blinks one time.
Slide # 36