I currently have a project the uses 3.3v and 5v rails of up to ~300mA of average power use each. They are fed by a design which uses two AMS1117 linear regulators that are provided anything from 7v to 12v depending on the power supply I'm using. Sometimes a wall wart, sometimes a bunch of 18650's.. It depends as I'm still in the prototyping stage of the power supply.
This solution as you can imagine is not very efficient, lots of power is lost as heat via the linear regulators.
So to fix this I was thinking of using a synchronous buck converter to take the 7v-12v down to 5v.. Then after that I'm not so clear what to do for the 3.3v rail..
Should I use another sbuck to take 7-12v down to 3.3v? or one to take 5v down to 3.3v or just use an AMS1117 to take 5v to 3.3v? Maybe there's a module or IC that does both voltages in a very efficient manner?
I can imagine this question has been asked a million times, but my Google fu found nothing definitive.
I need this to be as efficient as possible because eventually I'd like to run this from batteries.. So what's the best solution that is still cheap?