I'm using a LP2981 low drop out 5V voltage regulator along with a dsPIC30F3012 for a couple small (in size) applications and I seem to be having a powering problem.
For the first application I tried using 2 x 2032 button batteries (adding to ~6 volts). For some reason this didn't work, yet when I connected it to 5.5V power supply it did work. I had to move up to a 7.2V lithium polymer battery which was a lot bigger (basically the same size as the PCBs themselves) doubling my overall project size.
For another project, I tried powering the project with that same 7.2V lithium polymer battery and it didn't startup the pic at all but started up the X-Bee connected to a 3.3V LP2981 on the same PCB. And again, for some reason when I connect it to a 5.1V power supply (on the same Vin pin) it powers up fine.
I have a 10uF over the Vin and a 0.1uF and 0.01uF on the Vout on both the 5V and 3.3V regulators (all across to ground). The current is <100mA as required by the data sheet. I even tried placing a large Cap at the Vin incase there was a large initial drain, but that didnt' work either.
Anyone have any ideas?