Metro Battery Power

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njdevi11
 
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Metro Battery Power

Post by njdevi11 »

Can I run the metro boards at 3.3v direct from a battery @~3.7v? I love the charger/battery backpack for the pro trinket, will this work with the metro mini as well?

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adafruit_support_mike
 
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Re: Metro Battery Power

Post by adafruit_support_mike »

Yep. The Metro boards are specifically designed to run at 3.3v.

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njdevi11
 
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Re: Metro Battery Power

Post by njdevi11 »

I looked at the pinout, I don't think this will work to be directly connected to the metro, correct?
https://www.adafruit.com/products/2124

But if I hook up to the 5v bus (is this available?) from the usb connector and the VIN and GND with wires it should work. It's a shame I can't just solder it on like the trinket. A variant that could hook up directly to the meto [mini] would be the perfect companion.

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Re: Metro Battery Power

Post by adafruit_support_mike »

njdevi11 wrote:I looked at the pinout, I don't think this will work to be directly connected to the metro, correct?
Correct.
njdevi11 wrote:But if I hook up to the 5v bus (is this available?) from the usb connector and the VIN and GND with wires it should work.
It should, but your mileage may vary.

The Vin pin goes to an NCP1117 5v voltage regulator. Obviously the regulator can't deliver 5v out when you send 3.7v in, but the regulator will let as much power through to the 5v rail as it can. The NCP1117 is a linear regulator though, so you'll get at least 0.6v of voltage drop across it. I just connected a nearly-full LiPo measuring 4.0v to Vin, and measured slightly over 3.3v on the 5v rail.

The 3.3v regulator is an MIC225 low-dropout regulator, and takes its power from the 5v rail. The voltage on the 3.3v pin was 3.28v, which reflects the small amount of current going to the microcontroller.

A LiPo-powered Metro is actually happier if you bypass the NCP1117 and connect the positive terminal to the 5v pin. Cutting and bridging the IO voltage jumper to set the Metro for 3.3v operation more or less isolates the 5v rail, and everything on the board takes its power from the 3.3v rail. Removing the NCP1117 from the power path gives the MIC225 more headroom, and guarantees a solid 3.3v until the LiPo is nearly depleted.

Obviously you don't want a LiPo connected to the 5v rail when you connect a USB cable, and connecting the LiPo's positive terminal to the 5v rail eliminates the option of using a charger. Connecting to Vin costs you some headroom, but leaves the option to hot-plug a USB cable and charge the from the 5v rail.

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