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I have a board which has a microcontroller powered by a 3.3V buck and I need to know if it will harm the buck to power the net that the buck usually powers with a bench-top power supply.

A bit of context: I'm running into this issue because in order for my battery to charge, my microcontroller must contact the battery management board via I2C and tell the board that it's ok to charge the battery from wall power. However, when the battery fully runs down, the microcontroller cannot tell the battery management board to charge the battery because it's not powered (oops). This issue will be fixed in the next iteration of boards and is not the subject of this post, just wanted to provide some context.

So, in order to get the microcontroller to talk to the battery management board, I want to power the net that powers the microcontroller externally with a power supply, which should allow the microcontroller to tell the battery management board to charge the battery and turn on the buck. To prevent two power supplies from powering the 3.3V net simultaneously, I want to use the body diode of a MOSFET between the external power supply and the 3.3V net, so that when the buck kicks in, the potential on the cathode of the body diode will be higher than the anode, and the external power supply will immediately stop providing power. To ensure the potential is greater on the buck side, I'll set my power supply to ~2.9V (my microcontroller accepts voltages as low as 2.0V).

TLDR;

I have a board which has a microcontroller powered by a 3.3V buck and I need to know if it will harm the buck to power the net that the buck usually powers externally. Here is a high-level schematic of the buck:

3.3V Buck diagram

Will applying external power to the output of this buck harm the buck?

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It's a synchronous buck, so it may get confused and try to power the circuitry that's powering it. If it does that it'll become an unregulated boost converter.

I would put a voltmeter on the PVIN lead, an ammeter on the lead to the 3.3V net you want to power, and carefully bring the voltage up. Or keep the voltmeter and try to bring it up through a series 47\$\Omega\$ resistor (or whatever value you think is appropriate). That way if anything tries to suck too much current it won't act like a short.

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