Li-ion battery charging chip as an LED driver - Page 1
Forum section: electronics / beginners

author: Recyclojunk64
author_rank: Contributor
timestamp: August 15, 2013, 09:45:13 am
content: EDIT: accidentally hit <enter> in the subject line before writing actually everythingI got hold of a few MC34673http://cache.freescale.com/files/analog/doc/data_sheet/MC34673.pdfLi-ion battery charging chips, they're very small at and compact and don't require any external circuitry other than a resistor to set the output current (it's not actually the shunt resistor that the charging current passes through) and maybe a few caps.I was thinking of using one of them as a make-shift LED constant-current driver for a small compact torch where there isn't enough space to fit a mosfet and all the other necessary circuitry to drive them, as these chips are only 3 x 2 x 0.5 mm in size. The LED would be connected to the battery output terminals, and the current set to something reasonable. The only problem though is that they have a 2.7v trickle charge cutoff, so if the voltage-drop across the LED is at any time below that, it will only be getting 10% current. And the LED isn't going to have any voltage across it either when the power is first applied to the circuit. Any ideas on how I could get around this? I was thinking of maybe a series diode or two to increase the voltage-drop across the LED, but then I'd need a higher input voltage to get around the extra voltage drop.

author: sleemanj
author_rank: Super Contributor
timestamp: August 15, 2013, 09:55:24 am
content: Quote from: Recyclojunk64 on August 15, 2013, 09:45:13 amFascinating.

author: MinK
author_rank: Contributor
timestamp: October 24, 2013, 08:26:08 pm
content: Not sure about situation you have, but you might try to put small resistor in series. It will reduce circuit efficiency, but it might overcome your problem

author: Zepnat
author_rank: Regular Contributor
timestamp: October 24, 2013, 09:46:40 pm
content: What current are you aiming for. There's amc7135 that output 350ma with no external components and are virtually bulletproof, cost about $1 for 5. But that maybe too much current for you. I think the input voltage might be a issue as some charger IC's have undervoltage shut down but I havent looked at that chip as its bed time now.

author: ConKbot
author_rank: Super Contributor
timestamp: October 25, 2013, 03:06:02 am
content: Quote from: Zepnat on October 24, 2013, 09:46:40 pmhttp://dx.com/p/amc7135-1050ma-regulated-circuit-board-for-diy-flashlights-10-pack-1885?tc=USD&gclid=CKSF38r-sLoCFc6j4AodpS4AWwis 17/18mm diameter too big? You can generally use them with 1S li-ion and you can get away with 2x CR123a (LiMnO2 primary, not Li-Ion rechargeable) in series if you load them heavily enough. Those are simple boards with just 3x of the above mentioned AMC7135 chips and a series diode to provide reverse polarity protection. You can parallel as many of the AMC7135s as you want, and you can get away without a PCB too. Ive seen copper rings with the tabs soldered to them with the IC leads hand wired, and a few layers tiered to make driver for a higher current LED.Just go to candlepowerforums and you'll be plenty inspired