Power management with LiPoly batteries
I'm a huge fan of single-cell Lithium Polymer batteries; they're small, relatively cheap, and they come in a wide range of different capacities. However, their output range of 3.0-4.2V from discharged (with protection circuitry) to fully charged can be a bit of a hassle, especially if working with 3.3V devices. Of course there are plenty of power management ICs out there, from very bare-bones to extremely complex, but the majority of them are really not that well suited for the average home PCB fabrication set-up, whether it be due to their exceedingly tiny surface mount packages or there large number of required external components.
The majority of devices I make consist of some combination of 3.3V and 5V components. To power these with a LiPoly I need three things: a consistent 3.3V supply, a consistent 5V supply and a charging circuit. Of course there are many great modules and breakout boards that accomplish these tasks and more, for instance from Adafruit and SparkFun, but I'm not generally a fan of the different-breakout-board-for-every-component look, so I've put a fair bit of time into searching for the simplest to use PMICs, and I've discovered a few good options.
By far the easiest LiPoly charging solution I've found is Maxim's MAX1555, which can be purchased at SparkFun. It comes in a friendly SOT-23 package, and the only external components it requires are a few decoupling capacitors. To quote the MAX1555 datasheet:
The datasheet shows the typical configuration, which can be modified to use an LED as a charge indicator like this:
The great thing about this IC is that it is an in-system charger, meaning it will happily power your circuit while charging the battery (unless of course the circuit draws more than the charging current, in which case there will be a net drain on the battery!).
Getting 5V from a LiPoly battery is not all that tricky either, especially using an IC like the MCP1640 adjustable boost regulator. Available in a SOT-23-6 package, the MCP1640 can supply 5V at up to 300mA from a single-cell LiPoly at a little over 90% efficiency, using only a few external components, like so:
(This circuit is straight from the MCP1640 datasheet)
So in the case of a circuit which only needs a 5V supply, the output of the charging circuit can be routed straight to this boost circuit, and all is well.
This is where it starts to get a bit tricky. As the maximum voltage of the LiPoly is greater than 3.3V, and the minimum voltage is less than 3.3V, neither a regulator nor a boost circuit alone will work. One obvious option is to simply stick a 3.3V linear regulator on the output of the above boost circuit, but this is not generally the best idea. For one, if the boost circuit is 90% efficient, followed by a voltage regulator that might be 80% efficient or less, you're looking at an effective efficiency of 72%. That means that if the circuit draws all its current from this 3.3V supply, 28% of the power into the circuit is wasted!
The cleanest and most efficient way to get a 3.3V supply is straight from the battery using a buck–boost converter. The problem is that the majority of buck-boost ICs come in very small packages. Not only does this make them very hard to solder without stenciling and cooking in a reflow oven, but it means it is extremely difficult to etch your own board if it uses one. After hours of catalog searching, however, I have found a few buck-boost ICs that seem suitable for DIY use.
Most recently I stumbled across the MAX710 and MAX711. I don't know much about them yet, but judging by the datasheet they seem a bit outdated. They both have an input range of 1.8-11V, and the MAX710 has a selectable output of 3.3V or 5V, where as the MAX711 has an adjustable output range of 2.7-5.5V. Both can source a maximum of 700mA. Their efficiency is not all that impressive, and they sell for a whopping $8.61 on DigiKey, but they are the only buck-boost I've seen that come in the lovely SOIC package, which makes them super simple to etch for and solder.
I think the best buck-boost option I've seen is the LTC3440. Like many buck-boosts, the LTC3440 was designed for single-cell Lithium batteries, meaning it is optimized for an input voltage of ~3.7V. It has a maximum continuous output current of 600mA, and has an efficiency of ~95% when supplying 3.3V at 100mA with a 3.7V input. What's great about the LTC3440 is that it comes in a 10-MSOP package. Though the 0.5mm lead pitch doesn't leave much room for error, having exposed pads makes soldering pretty easy using a solder wick (great tutorial on skywired.net), and it shouldn't be too hard to etch.
I'm working on developing breakout boards for a few different combinations of these components, which I'll post write-ups for as I get them done.