USB/Bat dual input power regulator choice - Page 1
Forum section: electronics / beginners

author: Kaisha
author_rank: Contributor
timestamp: July 08, 2021, 11:28:29 am
content: I'm looking to power a small hand-held personal project from two sources. Either a USB input, or a pair of AA batteries in series. I need a regulated 3.3v output, and (possibly) a 2nd 1.2v output.I've been trying to find a suitable IC for this, preferably something I can source from LCSC because I plan to use the JLCPCB/SMT service, or at least something that could be handsoldered (and I'm not that great at soldering).Here's the problem. I would love to support any AA sized battery, from NiMH all the way up to lithium ion 14500s (not true AAs, but rather 3.7V Li in an AA sized package), but this means I would need a buck/boost that supports a voltage range from 1.8v (low end of NiMH) to just under 9v (fully charged 14500 Li). I couldn't find a single one that even supports that with a single input, let alone dual usb/bat inputs.If I can't support any AA, then at least I would like to support the lithium ion 14500s (ie. 9v top end). I don't really need charging capability, but I would like support for dual input with some sort of smart switching between bat/usb, and proper USB current limiting. A power-good pin or similar would also be really useful.Any ideas? I've spent nearly 5 days and looked over 100s of datasheets from TI, Analog, and Maxim, and have come up with nothing that fits the bill. This seems like a fairly mundane set of requirements... and yet I've come up short every time.Any thoughts or ideas?

author: MathWizard
author_rank: Super Contributor
timestamp: July 14, 2021, 03:22:45 pm
content: What kind of current will the 3.3V circuit need ? What about just using some 5V to 3.3V linear regulator for USB, at least it should be just 1 additional part, maybe a cap, and a switch.I have no idea on a buck/boost IC that would help, it does sound odd you can't find 1 though

author: mariush
author_rank: Super Contributor
timestamp: July 14, 2021, 04:45:08 pm
content: You could just have a slide switch on the side of the project box, to switch between the USB input and battery input, if you go with AA batteries. 3 x rechargeable 1.2v..1.35v alkaline , followed by a ldo to 3.3v and you're good to go. Or use 4 AAA batteries for a voltage closer to usb 5v , and use a buck regulator to do 5v->3.3vIf you go lithium batteries, you could simply use a battery charger IC which can also supply power while charging the internal battery.For example, see the Typical Application Circuit on page 2 of the Microchip MCP73781 :https://ww1.microchip.com/downloads/en/DeviceDoc/MCP73871-Data-Sheet-20002090E.pdf(note you can't use this with AA batteries)If your product uses a low amount of current, you could use a basic LDO to get 3.3v from 3.3v...4.2v you get from battery or usb. If you can find LDOs with voltage drop as low as 50mV-100mV ... so there's plenty of power from 3.4v all the way to around 4.2vFor higher currents, you would use either a regular buck regulator or a buck-boost if you must squeeze the last bits of juice from the battery.

author: Kaisha
author_rank: Contributor
timestamp: July 15, 2021, 11:25:44 pm
content: Quote600mA peak with an avearge draw of 150-ish mA.QuoteWell that certainly would be the easiest method, but part of the point was to learn how these sorts of things are done 'properly'. No one would release a real device like that, and even though this is a hobby project I like to at least pretend otherwise...QuoteThe enclosure is already chosen. It's 2 AA batteries in series.QuoteOnly if I it's a single lithium battery, 2 in series requires special ICs which I can't get (as far as I can tell on TI makes anything close to what I'd be looking for). There are a number of devices like:https://www.ti.com/product/BQ25672,but again, I don't have access to them.As far as I can tell an IC the likes of which I described doesn't exist... So I'll have to modify my requirements.

author: mariush
author_rank: Super Contributor
timestamp: July 16, 2021, 07:42:49 pm
content: Well, I'd recommend limiting the battery selection to the ones that don't go over 5v, the usb voltage.It allows you to use a much wider range of dc-dc converter chip, a lot of them have the highest input voltage at 5.5v-6vYou can pick synchronous rectifier regulators to save on parts and to make it easier to make SEPIC / buck-boost circuits, allowing you to boost 2v to 3.3v or step down 5v to 3.3vexample of such buck-boost regulators :TPS6300x :https://www.digikey.com/en/products/detail/texas-instruments/TPS63001DRCRG4/1672389RT6154 :https://www.digikey.com/en/products/detail/richtek-usa-inc/RT6154BGQW/5880469ISL9110x :https://www.digikey.com/en/products/detail/renesas-electronics-america-inc/ISL91107IRTAZ-T/5765690There are chips that auto select between two inputs, some chips output the highest voltage between the two, some chips have main and backup and auto switch to backup voltage if main drops etc etcDigikey has them in a category with ideal diodes and other such chips :https://www.digikey.com/en/products/filter/pmic-or-controllers-ideal-diodes/758See for example TPS211x (but note minimum voltage is 2.8v, max voltage is 5.5v) :https://www.ti.com/lit/ds/symlink/tps2110a.pdfor FPF3040 if you can handle bga chips :https://www.digikey.com/en/products/detail/on-semiconductor/FPF3040UCX/3724956

author: viperidae
author_rank: Frequent Contributor
timestamp: July 17, 2021, 07:05:26 am
content: Have a look athttps://www.ti.com/product/TPS63070I just put in 2v to 9v input, 3.3v output in to ti's webenchEdit: maybe not, after reading the data sheet it needs 3v for startup, 2v once it's running

author: Kaisha
author_rank: Contributor
timestamp: July 26, 2021, 04:21:34 am
content: Quote from: viperidae on July 17, 2021, 07:05:26 amLOL, yeah, I saw that one too. I was thinking 'yeah, found a good one', then read a little further... damnit!!!