Battery charger/power supply

I like to check out sparkfun’s website from time to time, not because I buy a lot there, but because they do a lot of work filtering the the thousands of ICs out there down to a handful that cover many of the things you’d want to do in a small project.

The other day I came across a pretty nifty board there that they call a LiPo Charger/Booster.  It basically allows you to charge a lipo in circuit while supplying either 5v or 3.3v.  The thing I found odd was that the power supply IC used (TPS61200)  seems okay for 5v output (it’s a boost converter with a second mode that allows it to down regulate) but it’s only about 70-80% efficient at 3.3v out(fig 1).  Contrast that with a LDO linear regulator and your efficiency is 80-95%(fig 2) depending on state of charge(weighted average is roughly 90% if the back of my envelope is correct).

Figure 1: a lipo spends >80% of its charge in the 3.5-4v range (when being discharged at <1C)

Figure 2:A LDO linear regulator is actually very efficient when vout just below vin.

The next thing I thought was odd about the charger/power supply was that Sparkfun‘s low voltage shutdown was set at 2.6V, which is extremely low.  A lipo is usually considered dead below 3V and damaged below 2.5-2.6 V so while this is a technically okay shutdown voltage the battery could easy self-discharge below that if left alone for too long (especially if left plugged in since the shutoff circuit needs a tiny amount of current too).   As you can see from figure 3, a reasonable shutdown voltage can be as high as 3.3-3.5v while still discharging >90% of the battery’s capacity.

Figure 3: A typcial Lithium Polymer discharge curve

 

Because I want one of these boards and because of the problems I described above, I figured that it would be best if I made my own.  Here’s what I came up with.

 

 like the Sparkfun part, it can charge off of USB or from a header.   Unlike the SparkFun part it only outputs 3.3V (all I need) and is more efficient, which means longer battery life and less heat generated.  Also, I managed to route the entire PCB on one side of the PCB, which means that  it could me safely glued to the back of a battery pack or case without worrying about weather or not you’re damaging traces.  That also means that you can, in principle make this PCB at home (though it uses some small SMT parts) without having to line up two layers on a PCB.

I’ll probably add a few mounting holes and change the design around but I thought I’d put it up now or else I may never.  I’ll try and keep this page updated when I change the design.  You can download the eagle files here.

 

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