Maybe this will help. A while back we needed a low battery warning for a circuit that was already in production so it had to be done in reverse. The PIC was powered by a 6V battery with a diode to drop it to 5.4V. A jumper from an ADC pin was attached to a 3.3V regulator already on another section of the circuit board. Now as the battery voltage drops, the ADC result will rise, since the 255 reference will always be the battery voltage. The lower the battery voltage the higher the 3.3V ADC reading becomes.
That is what I was suggested, just to to use diode instead of regulator...
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