There are quite a few [similar posts](http://electronics.stackexchange.com/questions/38782/regulated-3-3v-from-a-lithium-ion-or-lipo-battery) on this subject but I'm a little confused about the best way to proceed in my specific project. All my previous regulators have been linear types (eg: 7805, LM317 at best) - nice and simple. For me anyway.

The project I'm working on has a ARM based SoM (system on module) with a 3.3V requirement. Probably up to 500mA. (that bit is fine - stackexchange has some great posts on that). But this SoM connects to great big greedy sensors which have 5V and 12V requirements. 12V @ 250mA. And 5V at a huge 2.5A (!). Those likely need to be powered off and on independently.

This is all driven from LiIon or LiPoly which needs to be recharged. I can use one or two cells although it might not be up to me. Two cells might be easier given that I can boost to 12V easily and then derive 3.3V and 5V from the 8.4V line.

I'm not sure about the best way to proceed as there's a blizzard of chips out there. I was hoping someone has used a nicely behaved one that combined charging and regulated outputs. For example, Microchips MCP7383 can charge LiIon fine and a chip like the [TPS63020](http://electronics.stackexchange.com/questions/26941/how-to-design-switching-regulator-in-battery-operated-device) is a buck/boost regulator to give you 3.3V but other chips combine the charger and the regulator which seems a better idea.

If I use a single cell LiIon I get 3.4/3.5V to 4.2V. Is that too tight for 3.3V, especially given the power draw when I turn on the 5V boot circuit. Should I first boost to 5V and use that to get 3.3V? And then use another boost to 12V?