Power Supply Failure

For no apparent reason the main voltage regulator burnt up while operating from a 3-cell Lipo battery (about 12V).

It may be that if the supply voltage drops low enough, the switch-mode regulator can’t cope and self destructs.

It looks like the failed switch-mode regulator drops the supply voltage and feeds a small 3.3V linear regulator.  There is a unused circuit on-board that looks like a back-light driver, probably for a different model or older screen.

Supplying 3.3V directly to the linear regulator, it draws almost 800mA; a lot of this is probably back-light power.  The switch-mode regulator is an efficient and usually reliable way to drop the 12V.  The supply current is about 350mA with the 12V dropped to about 4V, after repair.

Repaired

The first thing is to find out if the power supply failure damaged anything else.  This involved removing the damaged components then using a current limited power supply to test it.

Having decided it was repairable, a small off-board switch-mode regulator was pre-set to 3.3V then connected to the LCD.  This seemed to work OK but a couple of voltages on-board were down around 3.15V, so I increased the regulator voltage to bring them up to 3.3V.  This compensates for the volt-drop through the on-board 3.3V regulator.

Tested OK

Still working after running over night; 14+ hour.