I prototyped a project using the Mega2560 and have based the PCB off the Mega (partially). Anyway, I've used the same 3v3 regulator (LP2985) and long story short, it's dying on me.
My complete circuit draws less than 130mA from a 12Vdc supply. but when powered from either USB or DC, the regulator drops way below 2.5V and sometimes 1V.
It's only being used for SDcard power, voltage translator (GTL2000) and VDD for Newhaven LCD (using the ST7565R controller).
The prototype worked perfectly!
However, whenever the PCBd circuit powers up and starts to do it's thing, the 3V3 dies (which is probably why my LCD isn't working as it's now operating at below the recommended specs). Input voltage remains at 5V.
I've since learned (the hard way) not to copy verbatim parts of arduino circuit as I did not have correct cap values for the 3v3 regulator according to the datasheet. (Note, they're not present on the mega 2560 schematic). I now have 2uf at the input and 2.1uf (should be 2.2) at the output and yet, it still dies. Scoping the output also seems to pull the output further down.