I have a circuit, Arduino Leonardo clone, which I'm attempting to power with a buck regulator. Specifically, I'm using a DSN MINI 360. It's based on the MP2307 buck regulator. If I power my board through the buck regulator, the board works fine. However, if I disconnect the regulator from input power and then try to power my board with USB, the board seems to draw too much current until the PTC kicks in. With the regulator connected to the +5v on my board, there isn't enough power supplied through the USB circuit to enable the circuit to run.
If I disconnect the regulator and power the board through USB, the circuit works normally.
I do not have a schematic for the regulator, but it seems to follow the typical application circuit described in the regulator's data sheet.
Below is the complete schematic for my custom circuit based on the ATMEGA32u4. The circuit, as built, is complete, except U2 (SPX3819 Linear Regulator) has been removed. All other components are present on my circuit board.
My question is why does the buck regulator seem to be drawing so much power through its output, and is there any way to prevent this from happening without having to disconnect the buck regulator when using USB? Would a diode be appropriate in this situation?