First, some background. I have two boards, Board A and Board B. Two separate, but extremely similar designs. Let's start with the similarities:
- Based on AVRXMega
- Exact same regulator setup. 5V in, 7833 SOT-223 3.3V regulator, and identical bypass caps
- Both boards draw approx. 5mA unprogrammed, and 30-50mA when programmed.
- Both are 2-layer standard 1.6mm 1oz PCB with a thorough application of ground stitching vias
- Board A has no issues with regulation. Regardless of the load current (up to 500mA), the output is rock solid at 3.3V.
- Board B will not regulate to 3.3V unless the input voltage is dropped below 4.5V, and even then it will only output 3.4V. Above 4.5V input, the output voltage tracks the input with an offset of 1V. So 5V in is 4V out.
- Adding a 100Ω load resistor to Board B improves the output, it will only track up to 3.6V, and will only output 3.4V once the board has been programmed, drawing approx. 60-80mA.
I have tried half a dozen different regulators, even swapping the ones on Board A and Board B. I have used several different bare boards of A and B, even with just the regulator circuits populated.
I have even tried 100mA (TC1015-3.3), and 10mA (MIC5232-3.3) regulators, they all do the exact same thing.