LI-ION phone charger

[Makaram]

Hey guys looking to make a portable phone charger running off a 6v battery pack and was wondering what would be a good regulator for this job?
Im looking at a sepic dc dc converter for its ability to both buck and boost (is this correct?)
as i would like to maintain a constant 5v even when my supply is below.

Firstly is this correct that a sepic is what im looking for rather than a standard low dropout regulator or just a buck converter
and secondly does any1 have recommendations for a specific sepic i could use?

 

[bountyhunter]

Sepic converter is what you need. Depends on what load current you want. I recommend perusing the product lines of Maxim and Linear Technology.

 

[Makaram]

thanks for the quick and straight forward reply! ill look into those recommendations

 

[WTP Pepper]

Sepic is correct, but can be hard work to make yourself as it's better to wind inductors on the same core. Lots of fiddling when you can buy these....

**broken link removed**

I have used them without issue.

 

[Makaram]

ooo thanks wtp! wasnt planning on making but thanks alot for the link.
Is something like this https://www.linear.com/product/LT1308A also what im after?

 

[MrAl]

Hi,

Dont you need a 5v output? So from a 6v input this would mean a step down. Probably a linear regulator could be found with low drop out that would do the job without too much waste power. I dont think you want your 6v battery running down too low anyway. Are you talking about lead acid or just some cells strung together?

 

[Makaram]

https://www.linear.com/product/LT1308A

 

[Makaram]

I was thinking some LI ION button cells just to be compact.
If I just use a step down from 6v and my input drops to 4.4 it wont charge the phone will it?

 

[WTP Pepper]

Yep that device looks like it will work. Beware, even though many of Linear Tech's power supply ICs that are excellent they come is some pretty small packages that will more than test your soldering skills to the max. They also require a thermal pad underneath as a heatsink.

As for Li Ion button cells, I doubt they will supply the current to charge a phone battery via a SEPIC or directly.

 

[RCinFLA]

Look at your portable charger's battery power capability compared to cell phone battery. If for emergency situation then it can make sense. A SEPIC converter is usually less efficiency in power conversion then a straight buck converter.

Take four alkaline batteries. They are about 2000 mAH's drawn down to 0.9 vdc. Let's say average cell voltage is 1.45 vdc with 2000 mAH's. Time four, that is 5.8 vdc at 2000 mA hours, 11.6 WH's. A smart phone battery at 3.7vdc average voltage at 1500 mAH is 5.5 WH's. Your power converter will have loss. The cell phone translation from 5 vdc input to 4.2 vdc peak voltage has loss. The set of alkaline batteries will provide about 1.8 full recharges, a rather expensive recharge cost.

 

[Makaram]

to be honest practicality isn't a problem I do these things to learn but thanks for the sound insight into the batteries. I plan on making it rechargeable also.

 

[Makaram]

so batteries connected in parallel doesn't give a maH boost as well as voltage?

 

[alec_t]

Batteries in parallel give only a maH boost. Batteries in series give only a voltage boost.