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Forum Index : Electronics : AMS1117 is garbage for battery operated projects

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CaptainBoing

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Joined: 07/09/2016
Location: United Kingdom
Posts: 2137

Posted: 07:12pm 24 Apr 2025

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This is just a cautionary tale of poor assumption on my part really - a heads-up if you like.

I think I have solved my problem but just in case anyone else wants to put this "in their pocket"...

I have a low power battery-operated requirement and in testing during bring-up, I was getting wildly inaccurate analogue input voltages reported by the '170. AVCC is connected to VCC as I usually do and it doesn't ordinarily result in any problems - I fully understand how to use AVCC if I need to operate at "odd" voltages but that is not the issue.

In part of my chase-down as to why the micromite thought a 50/50 potential divider (of VBAT) was resulting in 2.2V, operating from a trio of AA NiMH cells at a healthy 3.9V, I check VCC and it was 2.8V

Connect to a shore PSU and wound the voltage up and sure enough VCC popped right up to 3.3V

The board uses an AMS1117-3.3 to provide power but only when I start investigating and look closely at the datasheet do I see it can have 1.3V drop-out (!). It also has quiescent current of up to 11mA (double !) which explains why I couldn't get the board below 4mA with the CPU in SLEEP (which honestly isn't much of a problem as the 6V solar cell can push 50mA into the cells in a bright overcast). It never occurred to me that my go-to regulator is fairly hungry and not terribly sparing - and I always thought it was such a peerless little chap.

So I hunt around and I settled on AP7361C which is pin compatible in SOT223, has a dropout of around 90mV (at my voltage and current) and is 60µA quiescent. Just ordered a couple for this project only as they are quite expensive compared, but with the '170 in SLEEP (~40µA) and the Iq of the regulator, I might get standby current of sub 1mA and regulated operation down to an easy 3.5V on the battery (sub-nominal). Downside is maximum input voltage of 6V, which shouldn't be a problem. We shall see

h
Edited 2025-04-25 06:52 by CaptainBoing

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