The Nano 33 IOT is all 3.3V on the inside, but is powered with 5V from USB or an external 5V supply. On board is a fancy regulator - the MPM3610. It's a synchronous buck converter. The relevant part of the IOT's circuit is shown below.
But the idea is to power the IOT, plus a number of other things, directly with 3.3V provided by an external regulator. So I would be applying 3.3V to the MPM3610's output, with its input open. I have done this routinely with linear regulators, but don't know the consequences if a buck regulator is being used.
https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MPM3610GQV-Z/document_id/2090I can't find anything in the datasheet that addresses this question, so I'm wondering if anyone has experience with this regulator and could answer the question. I don't have either the regulator or the IOT to experiment with.
I can't disconnect the regulator because it would be needed when flashing new firmware. But in trying to think this through, the following options occur to me:
1. Power with 3.3V at the 3.3V pin, and let her rip.
2. Power with 3.3V at Vin, hoping it will just get passed through.
3. Power at 3.3V pin, but add a diode back to Vin to kinda equalize things in some way to prevent damage.
In general, is option 1 just something you don't ever do with switching regulators, or possibly something you can do with a boost regulator (because the diode is in series), but not with a buck regulator? Or is it just not a problem at all?