BM2257 required in my circuit - Page 1
Forum section: electronics / projects

author: HeMan
author_rank: Contributor
timestamp: September 23, 2025, 09:10:39 pm
content: I was after a little help and guidance if I may...I am building a battery powered LED strip using a 12V COB LED strip. Because I am powering it from a 3.7v LiPo I was experimenting to find the best boost module from a heat point of view... In the end I settled on a TPS61088 based module, because it ran at the lowest temp (65C), and was the closest to 12v (and therefore the brightest). I was using my BM2257 to read the input and output current of the module (3.3A in and just over 500mA out, all well within the specs of the module).Here's the odd part. Whilst it worked with the BM2257 as part of the circuit, as soon as I soldered everything up, the TPS61088 module quickly ran up to over 90C and the thermal cut out kicked in! Or maybe it was the batteries thermal cut out? Either way, it seems the BM2257 is required!So, why is Schrodinger's cat haunting me here? Is it the shunt resistor in the BM2257 that is making everything work nicely? If so, do we know what resistance it is and what power dissipation it needs to handle, as it seems I might need to buy one because it isn't very convenient to leave the BM2257 in the circuit!

author: thm_w
author_rank: Super Contributor
timestamp: September 24, 2025, 10:30:17 pm
content: Is the lipo capable of supplying >3A?Does the lipo have a protection circuit?Does it work if you desolder and put in the BM2257 again?Some of these boost modules can lock up at low input voltages. Maybe the additional resistance was reducing inrush and preventing it from entering some bad state.

author: Smokey
author_rank: Super Contributor
timestamp: September 24, 2025, 10:35:04 pm
content: This doesn't exactly answer your question, but it might help anyway.How about switching to 5V LEDs. The 12V ones just run multiple in series to drop the voltage anyway. Use the 5V ones and then your boost requirements are much easier.

author: jwet
author_rank: Frequent Contributor
timestamp: September 25, 2025, 07:19:24 am
content: The BM2257 has enough resistance to limit the peak current to something that your battery can deal with. The standard design for the TPS61088 has a peak current of about 10 amps (r4=100K), this is a lot for a single cell. The chip has a I limit resistor (R4) to limit peak current- . The default value in the reference design is 100K which gives a 10 amp peak current. These modules usually follow the simplest data sheet example. Look at figure 6-5 in the DS, increase this R to lower the peak current and you can get your meter back.Good luck.

author: HeMan
author_rank: Contributor
timestamp: September 25, 2025, 11:18:14 pm
content: Hi everyone, thanks for helping so farThe battery is a 146074, 10,000mAh that says it is 3.85v. I can't find a datasheet, but piecing together the specs from the various places (eBay, AliExpress etc.) that sell them, I believe it can supply 3A. Some say 4 (maybe the constant limit) and some say 16 (maybe a burst limit), but the lack of an "A" after these numbers worries me a bit. It does seem to have a protection circuit that I can see under the kapton tape.Yes, I had considered 5v LEDs, but I had quite a lot of the 12v ones around so wanted to use those if I could (I have quite a few to make for the local Scout group, who need some lit up signs for an event they are running). I did buy a small 5v length from Amazon to test with yesterday, but it is noticeably dimmer than the 12v ones, which is probably due to the one I ended up getting rather than anything inherently wrong with 5v ones (as you say, they are mostly all sub 5v LEDs just in series anyway). But it feels like it will be a gamble to keep buying more until I find a good (and budget friendly) type and I have all these 12v ones just sat around...The TPS61088 module I am using is a 12v one of these:https://www.aliexpress.com/item/1005009191176179.htmlwhich has a voltage adjuster resistor, but nothing I can see to change the current limit. There are tabs to solder to change from PFM to PWM, but I am not sure what this does (I know what the acronyms mean, but not how they apply to this) or if it would help. And I don't think I can change r4, as if it is in there somewhere (I don't have a schematic), it is a tiny SMD resistor I would not be able to remove, let alone replace!I tried de-soldering and putting the BM2257 back in and yes, it behaved again. But then it got me thinking, is it the resistance of my tangle of test wires and JST connectors also effecting it? And sure enough, if I add about a metre of a high gauge wire to the circuit, then that also seems to fix the issue (and the LED is very slightly dimmer). The BM2257 measures the wire as 0.1 or 0.2 Ohms, and it seems that is all that is needed to limit the current in to the TPS61088 module to keep it under the thermal cut out. But this doesn't fill me with confidence; surely there is a better way of doing this (I guess this is where PWM could come in? The COB LEDs say they are dimmable)?I also notice that if I run this on my bench power supply at the full 4.2v, it is drawing much less current that when I drop it down to 3.2v (i.e. a discharging battery under load) at which point it is drawing over 6A from the battery. So I suspect the variation in the battery, and how it behaves under load, is also effecting my results...

author: thm_w
author_rank: Super Contributor
timestamp: September 25, 2025, 11:37:07 pm
content: You can measure what ILIM resistor they used and try adjusting that as per thedatasheet.If your output is 12V 0.5A, 6W, then your input is going to be 6W / efficiency = ~7W7W at 3.2V is going to draw more current than 7W at 4.2V.

author: jwet
author_rank: Frequent Contributor
timestamp: September 26, 2025, 02:27:33 pm
content: Quote from: HeMan on September 25, 2025, 11:18:14 pmThis is a switching supply so as the Vin goes down, the current goes up, after some efficiency losses of course, power in will balance power out. As someone mentioned, the ESR of the cap bank enters into this too- its not simple thevenin type source.You'll have to look at the DS for the converter and look at your board and find the Ilim resistor and raise it. These little modules genereally use the most generic circuit from the DS. You need a tweak.

author: mikerj
author_rank: Super Contributor
timestamp: September 26, 2025, 02:58:11 pm
content: Has your lipo got a protection circuit in it? You may be tripping an over-current protection at startup and adding in the extra resistance in takes you under this threshold.