Power Muxing for USB and high voltage

[boguesuser]

I am working on an ESP32 dev board. I want to power the board via either USB or 30v connector.

How does one typically prevent the 30v from going to the USB if both connectors are connected at the same time?

[ledtester]

You could get some ideas from this design:

https://oshwlab.com/Miraculix200/esp32-s3-mini_copy



The diode D1 will protect the USB +5V rail. The external power, if applied, should always be at least 5V.

For the scenario where the external power is a lower voltage battery and you want to have the battery charged when USB is applied, have a look at the schematics for the Adafruit ESP32 Feather:

https://learn.adafruit.com/assets/110822

[boguesuser]

I'm just a bit slow.

Is a schottky diode all that's needed to protect the USB device?

And is a schottky used here due to lower forward voltage drop?

[ledtester]

Another diode could possible work depending on the headroom required by the 3.3V regulator. The current the MCU needs could also be a factor.

[boguesuser]

Ok. Thank you for your help.

I'd at most be pulling 3.3 watts and can tolerate 4.2v of input voltage to the regulator.

[Salitronic]

I'd at most be pulling 3.3 watts and can tolerate 4.2v of input voltage to the regulator.

If you are expecting to draw 3.3W (I'm assuming this is on the 3.3V) through an LDO, that translates to 30W on the 30V input, with over 26W wasted in the LDO.
You need to use a buck converter.