I have a Nano mounted on a carrier board. for a 2-wheel balancing car. The carrier board expects to receive 5 V from the Nano’s 5V pin.

The Nano has two possible power inputs:

1. USB 5 V
2. VIN (≈8 V) → on-board regulator → 5 V (but this regulator does not back-feed the 5V pin)

This Nano variant lets you short two pads so that USB 5 V is routed to the 5V pin. That works fine when USB is plugged in. But when powering the Nano only from VIN, the 5V pin is dead, so the carrier board gets no power.
I need the Nano’s 5 V pin to be powered whenever the system is powered by USB, by VIN, or by both at the same time.

A sketchy solution would be to jumper the pads so USB 5V feeds the 5V pin and add an external 5 V regulator on VIN and wire its output to the 5V pin. But that means two 5 V sources tied together, and I’m worried they may fight each other or back-feed, depending on which one is higher at any moment.

What is the correct way to design this so that:

• USB 5 V can power the 5V pin
• VIN → regulator can also power the 5V pin
• Both may be present simultaneously without damaging the Nano or the external regulator

Should I be using ideal-diode OR-ing, Schottky diodes, a power multiplexer, or is there a recommended method specific to the Nano’s power architecture?

 

My approach would be to use one of the tiny, ubiquitous 5V 3A buck converter modules and either run it in parallel with the \( \mathsf{V_{IN}} \), or in series with the the supply and plugged into the USB of the Nano, or in series with the supply and plugged into the \( \mathsf{V_{IN}} \) and parallel with the board supply.

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