Powering the Arduino Nano 33 IOT directly at 3.3V pin?

[Peabody]

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/2090

I 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?

[tooki]

I’m sure some more experienced people will chime in, but my hunch is that nothing bad will happen. If you look at the block diagram, the only significant path for current into the output is up through the body diode of the upper MOSFET, through the current sense resistor to the input. (See the current arrows I drew in the screenshot attached.) So your 3.3V will drop by 0.7V to 2.6V on the input. If that’s even enough to trigger the EN input, the internal LDO that powers the chip’s logic will drop at least another 0.2V probably. Whether the circuitry can even power up at that voltage I don’t know, but I doubt it, since the minimum input voltage is 4.5V.

The other thing that makes me doubt it would be damaged is that it’s not at all unusual to have a substantial capacitor on the output, which would maintain the voltage for a while if there’s no significant load. That’s a common enough configuration that I would think it would need to survive that happening regularly.

Finally, the absolute maximum ratings state a maximum voltage on the output pin of 6V, with no caveat of that applying only when it’s on.

[Eraldo]

I found this thread from the arduino forum:
https://forum.arduino.cc/t/arduino-nano-33-iot-direct-3-3v-power-supply/891441

This should answer most of your questions i believe

[mariush]

Any wire, any connector, will have some resistance and some inductance.

If your device has a burst of power consumption, like for example jumps from 5mA to 100mA, there will be some voltage drop on the wires between your external regulator and your components (or up to the first capacitor on the input path), so the voltage may go down below 3.3v

The voltage regulator powered from 5v basically makes sure there's always as close as possible to 3.3v, and this 3.3v is produced very close to consumers, for minimal losses.

[Peabody]

I found this thread from the arduino forum:
https://forum.arduino.cc/t/arduino-nano-33-iot-direct-3-3v-power-supply/891441

This should answer most of your questions i believe

Yes, that's the thread where the Arduino Tech rep says not to do it, that it could damage the circuit.  But another guy says he's been powering his IOT that way with no problem.  The thread never really comes to a conclusion.

[Peabody]

Any wire, any connector, will have some resistance and some inductance.

If your device has a burst of power consumption, like for example jumps from 5mA to 100mA, there will be some voltage drop on the wires between your external regulator and your components (or up to the first capacitor on the input path), so the voltage may go down below 3.3v

The voltage regulator powered from 5v basically makes sure there's always as close as possible to 3.3v, and this 3.3v is produced very close to consumers, for minimal losses.

The biggest spike will come from the radio on the Nano going into transmit mode.  But a big capacitor can probably deal with that pretty well even if the external regulator is not close.

Anyway, the basic issue is that this is supposed to be a battery powered button box, with close to 100 LEDs, some considerable portion of which could be lit up at the same time.  I don't really know what a button box is - just trying to help with the circuit.  But it just seems to me that if everything in the circuit can work at 3.3V, then they could reduce the power lost in the LED current limiting resistors by about a third, or even more for blue, versus powering at 5V.  Also, it might permit staying with a 1S battery setup and a buck converter down to 3.3V, which would simplify charging.

It turns out that Adafruit sells both 3.3V and 5V modules using the MPM3610.   So it may be possible to test one and see what happens.  My guess is it would work fine.   But I don't want to recommend something I'm unsure of.

[Peabody]

I’m sure some more experienced people will chime in, but my hunch is that nothing bad will happen.

Thanks, tooki.  That was my thinking too, but in this area I'm not one of those more experienced people. So maybe I can find someone who has done it this way, and will swear to me that it works fine.  :-)

[thm_w]

I've done it many times for other regulators, but that is not saying it will work in all situations.

You can see some recommendations here for a specific IC: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/357881/back-feeding-tlv62130
- keeping reverse current limited (which it should be if no large loads are connected to the 5V, or you don't have a massive amount of capacitance there).
- might blow up if forced PWM mode is used

From MPM3610 datasheet:

An external AAM pin provides selectable power-save mode and forced PWM mode.

Advanced Asynchronous Modulation. AAM sources a 6.2μA current from an internal 4.9V supply. Float AAM or drive AAM high (>2.5V) to force the MPM3610 to operate in continuous conduction mode (CCM). If AAM mode is required under light load, connect a resistor to ground to program AAM voltage in the range of 0V to1V

Other discussions:
https://electronics.stackexchange.com/questions/365058/do-i-need-to-protect-buck-converter-when-switching-to-alternate-power-source


If you have a current limited supply I would try applying the 3.3V and measuring Vin (maybe increase it to 3.5V too), see if anything odd happens there.

[Peabody]

If you have a current limited supply I would try applying the 3.3V and measuring Vin (maybe increase it to 3.5V too), see if anything odd happens there.

Thanks very much.  I think if I can't find someone who has done this with this particular part, I should probably get the Adafruit module and test with that.  Just to be sure.

[tooki]

How about just contacting Monolithic and asking? They make the regulator and thus might have some idea about what does and doesn’t damage it.

[Peabody]

I'm not an engineer, and have never had any luck trying to do that - except on the TI forum when that was still available to hobbyists.  But I did find the Monolithic forum, and a thread on this very subject:

https://forum.monolithicpower.com/t/mpm3610-in-arduino-nano-every/1284

But it doesn't look good.  The poster reports that when applying just under 5V to the output, he measures over 5V on the input pin.  I think that means it is switching in some backward way.

But it might be different at 3.3V since the converter should be shut down at that voltage.  Anyway, I think it's clear I need to get the 3.3V Adafruit module and experiment with it.