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External supply options for 3V (3.3V)
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External supply options for 3V (3.3V)

by fruitmonger on Wed Mar 17, 2021 2:53 pm

Hello!

I'm using the Itsy Bitsy M0 in a project that is powered by a pair of Li-Ion cells in series for a total of approximately 8V when fully charged. This setup was chosen due to the power requirements of other devices being used. Now I'm trying to decide the best option for powering the M0. The battery voltage will be too high for the M0's onboard regulator (max 6V), and tapping a single cell wouldn't be practical (see below).

There is a standalone 3.3V regulator in this circuit to power other 3V devices, because their total current demands exceed the onboard regulator's maximum rating (displays, sensors, etc.). It's in a TO-220 package rated for 1A. I've powered a few boards with external regulators using the 3V pin, but understand that this is not usually recommended since it's connected to the output of the onboard regulator. In theory this shouldn't be an issue if the onboard regulator is unused with no voltage at the input. Here are the options I've brainstormed so far:

Tie external 3.3V regulator directly to the "3V" pin
- This is also the output of the onboard regulator (unused)
- No other devices are connected to this pin, only M0 would be affected

Tie external 3.3V regulator into the BAT pin (onboard regulator)
- Uncertain if the onboard regulator is LDO (low dropout)
- Documentation states the minimum input is 3.5V
- Potential for unpredictable behavior

Add another regulator (such as 5V) and tie into the BAT pin
- Adds more parts and board space to the project
- Results in one regulator feeding another
- Still need a separate 3.3V for other devices

Tap a single cell to power the M0
- Adds complexity to the physical setup
- Results in uneven current drain of the two cells
- Skews monitoring of total current consumption

The batteries are not connected during programming, so there would be no issue with conflicting supply voltages.

Does anyone have other suggestions, or can confirm that feeding the 3V pin from an external regulator is okay?

Thanks for any advice offered!
Last edited by fruitmonger on Wed Mar 17, 2021 3:24 pm, edited 1 time in total.

fruitmonger
 
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Re: External supply options for 3V (3.3V)

by adafruit_support_bill on Wed Mar 17, 2021 3:23 pm

If there is no other power source to the board, connecting your 3.3v source to the '3V' pin will work fine.

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Re: External supply options for 3V (3.3V)

by fruitmonger on Wed Mar 17, 2021 3:39 pm

Thanks! That's what I would assume in most discrete circuits, but wanted to be certain since I'm not familiar with this board design.

Another benefit to that setup is that any battery surprises, surges from high current components, etc. will be handled by a large external regulator that's easily replaced, rather than risk burning the onboard one.

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Please be positive and constructive with your questions and comments.