Single-cell LiPoly to 3.3V regulator - Page 1
Forum section: electronics / projects

author: LukeW
author_rank: Frequent Contributor
timestamp: February 11, 2013, 02:24:51 am
content: OK, so here's the problem I want to solve:- A single-cell lithium polymer, which maybe freshly charged, or slightly discharged, so let's say the cell voltage is somewhere in the ball park of 3.5-4.0V.- Regulation down to an output of 3.3V, with a reasonably large output current capacity, say 1A if I can easily get it that high.- A digital enable line to shut down the regulator would be nice.- I have considered very-low-dropout LDOs such as the LP2980, however their output current capacity is a bit low, and the use of a linear regulator seems like it's inefficiently wasting a bit of battery energy.- A buck regulator that can accommodate a very low input-output voltage difference of only a few hundred mV seems like the way to go.- The UCC3954 seems like it's an attractive candidate but it's obsolete and I can't find anything similar as a currently-in-production alternative.Does anyone have any ideas or suggestions? Thanks!

author: mamalala
author_rank: Unknown
timestamp: February 11, 2013, 02:44:58 am
content: You could use a SEPIC converter. This will give you 3.3 Volts output with an input that can be above, at, or below the output voltage. TI has a wide range of converter chips that can be used in a SEPIC configuration. Linear has a lot of them as well (for example the LT1307).If you want to add charging, you can use something like the MAX1811 as a single-chip charger to have the LiPo charged from USB.Greetings,Chris

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 11, 2013, 04:09:47 am
content: I don't have any particular suggestions but if you do go with a switching buck regulator be careful to get one with a duty cycle high enough to allow for such a small worse-case difference between Vinand Vout.A regulator with a max. duty cycle of 90% won't work, for example, since the output voltage for a buck regulator is basically determined by something like the following formula:Vout= D * Vinwhere D is the duty cycle.

author: cyr
author_rank: Frequent Contributor
timestamp: February 11, 2013, 12:32:31 pm
content: I've used one of LT:s buck-boost converters for just this purpose, I'm not quite sure of the model but it might have been this one:http://www.linear.com/product/LTC3441

author: digsys
author_rank: Unknown
timestamp: February 11, 2013, 01:14:32 pm
content: Is it really worth it ... at ~1A your LiION will spend MOST it's life at app 3.8 - 3.70V. If you check the curves, they don't hang aroundabove 3.8V for long at all. So, 3.70V - 3.3V ~ 89% efficiency !! Power loss ~ 0.4V x 1.0A = 0.4W PLUS you really don't want to beusing your LiION below 3.3V anyway (even so, LDO will "follow" it down ... So, is it worth it?

author: ConKbot
author_rank: Super Contributor
timestamp: February 11, 2013, 05:58:44 pm
content: Quote from: digsys on February 11, 2013, 01:14:32 pm+1 for that, under heavy load, ( > 0.5C or so) cutoff at 2.9-3.0V and the open circuit voltage recovers to a safe voltage. Under a light load, cutting off at 3.3v will have used most the capacity in the battery anyway. I pulled the battery out of my phone after it shut down due to low battery to see where it cut off, and it was at ~3.5V open circuit IIRC. For a 3.3v rail a buck-boost or SEPIC are definitely overkill, and not doing any favors for your cell life. If you can find a buck which can go to high, or even 100% duty cycles with a low on voltage drop, you should be good to go. Or as suggested, a LDO will be quite efficient once the battery voltage drops off its initial 4.2v

author: mariush
author_rank: Super Contributor
timestamp: February 11, 2013, 06:28:46 pm
content: I was thinking the same thing, the battery will stay around 3.8-4v most of its life.Most buck regulators will only give you 80-85% efficiency, which is not much better than just using a ldo to get 3.5-4.2 down to 3.3For example, MIC2941 will do 40mV at low loads, up to 350mV at 1A.. and it even has shutdown pins :http://www.digikey.com/product-detail/en/MIC2941AWT/576-1138-ND/771607Or you can get the fixed version even cheaper :http://www.digikey.com/product-detail/en/MIC2940A-3.3WT/576-1134-ND/771603http://www.digikey.com/product-detail/en/MIC2940A-3.3WU/576-1135-ND/771604You can just save money on extra parts for a buck circuit and get a bigger battery