Recommend part? USB LiPo charger that also operates w/o battery - Page 1
Forum section: electronics / projects

author: sebmadgwick
author_rank: Regular Contributor
timestamp: August 03, 2014, 04:55:24 pm
content: My device is currently powered by a lithium ion cell which is charged via USB using aMicrochip MCP73831. This works well but the current is limited to 500 mA and [unrelated to the 500 mA limited] the device cannot powered reliably if the battery is not connected.I effectively want to bypass the charger (and power directly from USB) if a battery is not connected, or switch between the battery+charger and a direct USB connection if USB power is detected.This is a very small device and so I'm hoping there is a single chip solution available. I guess it would be a combined charger/UPS.Can anyone recommend a device? If not then I would be interested in a discrete solution. Thanks!

author: zapta
author_rank: Super Contributor
timestamp: August 03, 2014, 05:13:13 pm
content: Check this one (and similar):http://www.ti.com/product/bq24192I was able to operate a single cell device that uses this charger with battery removed (current was ~1.2A).

author: sebmadgwick
author_rank: Regular Contributor
timestamp: August 03, 2014, 05:37:39 pm
content: Wow, that chip does everything! I'll definitely look into this.I was curious how to make a discrete solution and drew the attached. Just sharing fyi.

author: Dago
author_rank: Frequent Contributor
timestamp: August 03, 2014, 05:52:56 pm
content: You also might be able to use something like a TPS2115A:http://www.ti.com/lit/ds/sbvs044f/sbvs044f.pdf

author: sebmadgwick
author_rank: Regular Contributor
timestamp: August 03, 2014, 06:17:07 pm
content: Thanks, Dago. Thebq24192offers so much functionality and it would be great to support USB OTG in the future (e.g. writing to thumb drives). However, I may not have time to get to grips with this more complex part. TheTSP2114/5does exactly what I requested.Thebq24192uses a 1.5 MHz switching frequency. I presume this introduces much greater EMC risks. Can anyone comment on this?

author: jeremy
author_rank: Super Contributor
timestamp: August 03, 2014, 06:26:44 pm
content: Usually the higher switching frequency (>1M) is to allow the use of ceramic caps instead of electrolytics or tantalums. Electrolytics are big, and tantalums can be a bit too explode-y sometimes.I use them all the time, and as long as you follow the manufacturer specs for the layout and capacitor dielectric, I don't see why you'd have a problem.

author: andyturk
author_rank: Frequent Contributor
timestamp: August 03, 2014, 06:45:30 pm
content: You might also consider the big brother to the *831:MCP73871It has more capacity than what you're using and will automatically switch between available power sources (and tell you which one is active). It's not I2C and just has some gpio lines for control.I used it on a 9-axis IMU project. ;-)

author: zapta
author_rank: Super Contributor
timestamp: August 03, 2014, 07:00:57 pm
content: Quote from: sebmadgwick on August 03, 2014, 05:37:39 pmCan't the parasite diode of Q3 feed the usb power directly to the battery? Do you assume higher voltage on the battery side?Another option is to have an 'or' circuit using Ideal Diodes or a power switching circuit like this onehttp://www.ti.com/product/tps2113a

author: sebmadgwick
author_rank: Regular Contributor
timestamp: August 03, 2014, 07:29:20 pm
content: Quote from: zapta on August 03, 2014, 07:00:57 pmYou are right. Another diode would be needed as with Q2. The TI part you and and Dago suggested is a far better solution.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 03, 2014, 07:45:30 pm
content: Quote from: andyturk on August 03, 2014, 06:45:30 pmI've used that part - works nicely, much easier than messing about with external switching

author: RiverTown
author_rank: Regular Contributor
timestamp: August 03, 2014, 07:51:25 pm
content: Quote from: andyturk on August 03, 2014, 06:45:30 pmI also can recommend MCP73871. I've used it in my latest ESR meter project. Charging current can go up to 1A.Another good thing is reasonable price of around two bucks a pop.Only disadvantage can be QFN package if you are using MCP73831 in SOT23 package.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: August 03, 2014, 08:01:04 pm
content: Thanks andyturk and others for theMCP73871suggestion, this looks ideal. QFN isn't a problem, I will use theevaluation boardfor dev.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2014, 08:41:12 pm
content: Quote from: sebmadgwick on August 03, 2014, 05:37:39 pmFlip Q3 around and take out Q1, Q2, and R2.