Current limiter IC needed - Page 1
Forum section: electronics / beginners

author: Teun
author_rank: Regular Contributor
timestamp: July 17, 2019, 11:46:17 am
content: Hi all,I'm working on a project which has 5 parallel outputs for charging handheld devices. I need to limit the current of every channel to +/-3.5A at 5V. Or 2A at 9V (roughly 18W). Can anyone recommend a good IC for this? Preferrably with little other components needed.I tend to get a bit confused searching for these things. Do I need a "load switch" or "current limiter" or any other name these things might have?

author: ogden
author_rank: Super Contributor
timestamp: July 17, 2019, 01:41:26 pm
content: What is voltage of power supply? 18W is too much for simple linear current limiter because such would dissipate lot of heat. It means you need solution based on buck switching converter (voltage regulator) with current limiting function.What you are looking for is something likehttp://www.ti.com/product/LM20343which have both - adjustable output voltage and current

author: exe
author_rank: Unknown
timestamp: July 17, 2019, 02:40:38 pm
content: Do you need to limit current (CC mode), or to disconnect DUT from the power supply (e.g., e-fuse or something)?

author: Teun
author_rank: Regular Contributor
timestamp: July 18, 2019, 10:11:18 am
content: There is a big 90W power supply which will be divided in to 5 channels. So this is more of a distribution board.Preferably I would like it to work in CC mode. But a disconnection with some sort of auto reset after a few seconds would be ok as well.

author: exe
author_rank: Unknown
timestamp: July 18, 2019, 10:32:29 am
content: Not exactly what you asked, but may be still useful:https://www.st.com/en/power-management/e-fuses.htmlI didn't do much research if there are fuses with controllable re-arm time. In the worst case, you can add a 555 timer (monostable operation) to reset it after failure. I'd just use a power supply with CC mode, 90W on 5 channels doesn't sound much. Two three-output supplies should cover you.

author: ogden
author_rank: Super Contributor
timestamp: July 18, 2019, 10:50:21 am
content: Quote from: exe on July 18, 2019, 10:32:29 amE-fuse is useless in battery charging because you may get continuous fuse tripping for days w/o actual battery charging.  No offense, but you shall NOT suggest such a BS w/o being absolutely sure that you are right.

author: exe
author_rank: Unknown
timestamp: July 18, 2019, 11:03:05 am
content: Quote from: ogden on July 18, 2019, 10:50:21 amCalm down, dude, you see what you want to see, not what I actually said. I said "may be useful" because the problem described very vaguely. I don't know what's exactly the use-case, nor you know to do any judgement. Please don't add more noise.

author: ogden
author_rank: Super Contributor
timestamp: July 18, 2019, 11:26:10 am
content: Quote from: exe on July 18, 2019, 11:03:05 amWho is adding noise . If you need more detailed description as "current limiter for charging handheld devices" then please do not blame OP here.

author: exe
author_rank: Unknown
timestamp: July 18, 2019, 02:36:10 pm
content: May be this will help:http://www.ti.com/lit/an/slva859b/slva859b.pdf"adjustable current limit of smart power switches". Different vendors sell similar devices under different name. Afaik, I bought something like "rugged mosfet". Beware of Rds(on), it's often surprisingly high for claimed current rating. It may need a heatsink, particularly for CC operation.

author: ogden
author_rank: Super Contributor
timestamp: July 18, 2019, 03:39:25 pm
content: Quote from: exe on July 18, 2019, 02:36:10 pmThose also are just adjustable circuit breakers which are not suitable for battery charging. OP need current limiter which is *limiting* current, not interrupting it. Other name for current limiter is constant current (CC) source.

author: exe
author_rank: Unknown
timestamp: July 18, 2019, 04:03:13 pm
content: Quote from: ogden on July 18, 2019, 03:39:25 pmThat's exactly the opposite datasheets say. E.g.http://www.ti.com/lit/ds/slvse42a/slvse42a.pdf:QuoteI never used one myself, even though I have a similar part in my parts bin. So, don't know if it is suitable or not.But if it is for charging batteries, then I wouldn't use it because limits are most of the time not precise. At least calibration is needed. I'd buy a charger, or charging ic controller. But chances are such controllers are already built-in in the "handheld devices" (whatever they are). Esp. if they use lipo batteries. My guess current limit is needed for protection.

author: bdunham7
author_rank: Super Contributor
timestamp: July 18, 2019, 05:19:43 pm
content: Quote from: Teun on July 17, 2019, 11:46:17 amWhat do you want it to do if the device being charged draws more than it's allotment? Shut off, shut off temporarily and reset, limit the current to exactly the specified amount or foldback? Why the different voltage specs--are these phones or what? What will be the input voltage from the power supply? If you don't know the answer to those questions, can you tell us exactly what type of handheld devices you will be charging?

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 18, 2019, 07:33:32 pm
content: ^^^Above asks good questions.It's certainly possible to design a circuit to limit current to 3A when the input is 5V, or 2A when Vin is 9V, but it will either require a bunch of components (op-amp based control loop) or drop a good deal of voltage (~1V or so) even when current is just under the maximum permitted (e.g. - the classic two-BJT current limiter circuit).A foldback current limiter would likely be a better choice here, but the OP didn't specifically say why current needs to be limited, or what the downstream load actually is - presumably a battery charger, and not a battery itself, because there aren't any batteries that will be happy havingeither5V or 9V applied to them. Given that, it makes more sense to use an efuse rather than try to turn a voltage source into a current source, especially at 2-3A, as that could result in 15-20W of dissipation per channel in the event of a hard and persistent short.

author: wraper
author_rank: Unknown
timestamp: July 18, 2019, 08:09:39 pm
content: Quote from: Teun on July 18, 2019, 10:11:18 amSuch approach is simply wrong. Half-decent charger will have individual buck converter for each channel.

author: ogden
author_rank: Super Contributor
timestamp: July 18, 2019, 08:59:29 pm
content: Quote from: wraper on July 18, 2019, 08:09:39 pmRight. You usually do not build 3.5A chargers using linear components, not to mention smart fuse ICs. "Disconnection with some sort of auto reset after a few seconds" means circuit breaker. Do you need circuit breaker (fuse) or battery charger? If battery chargers are built-in (into handheld devices) then obviously fuse is all you need and mentioned chip is good enough:http://www.ti.com/lit/ds/slvse42a/slvse42a.pdf

author: Teun
author_rank: Regular Contributor
timestamp: August 07, 2019, 07:32:53 am
content: Sorry for late reply. Been on vacation.Quote from: wraper on July 18, 2019, 08:09:39 pmI totally agree but unfortunately that's not up to me. So this is just the way it has to be done.The different voltages are for different devices some at 5V some at 9V. I just need the board to be easily adaptible to these voltages.Most devices wil never draw more than 2.5A. But if you had a charger that keeps shutting down on over current it wouldn't charge at all. So CC mode is preferred.Exe's datasheet seems like the sort of device I had in mind.Thanks for all the help so far guys.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: August 07, 2019, 08:08:40 pm
content: stm L200 - both adjustable,or something as Lm317 current limiter, but on LT1083\4\5; no idea actually if it need linear and a power dissipation ...or this:https://www.analog.com/en/parametricsearch/11471#/