Trouble during powerup on Photon

I was powering my project with a 3.0V LDO which works and then redesigned the system to run with a buck regulator at 3.3V, but the power up is inconsistent. If I press the restart button, the system always starts. Otherwise, if I just power up and I measure the power at the 3.3V input, I get 1.89V and on occasion, the system does boot and the regulator provides 3.3V. What am I missing?

Are you powering 3V3 or VIN? VIN has a voltage range of 3.6V to 5.5V, so 3.3V is not enough.

If you are powering 3V3 directly, you need to be very careful doing so, as you cannot connect to the USB serial port normally if you power 3V3 because the 3V3 rail will be powered by both your power supply and the internal regulator powered by USB, which may damage the internal regulator or your buck converter. You can either use a special unpowered USB cable, or disable your 3V3 supply when power is present on VIN, such as with a combination of an N-MOSFET and P-MOSFET.


solved my own problem. I was using an inexpensive power recharging board in a manner that when the power was turned on, I assumed the power board was inactive; not so. Isolating the power board such that it is only used when charging and at that point making sure the Photon was off appeared to fix the problem.