That MAX17220 would be interesting but it doesn't have any package which is hand solderable... That MCP16252 is very interesting. If I understood correctly I could in bypass mode have only the wide operating voltage PIC to work and then the PIC could pull the EN high when wake up from sleep and other devices could operate as well. I'll order bunch of these. Thanks everybody. There are so many IC's that it is impossible to just search something you didn't know exists.
Maybe I have a hard time understanding you... but I'm not sure that's what bypass mode does.
I think the idea is that in bypass mode, the chip just passes the input voltage to the output and consumes very little current doing that (600uA or thereabouts)
For example, you may have a microcontroller that can run on 1.8v or 3.3v at a lower frequency like 1-4 Mhz / stand-by / sleep but which may need 3.3v or 5v at higher frequencies.
So you could enable the regulator to boost the lower voltage to your desired voltage, wait a few ms (or however much it takes, see datasheet) for output to ramp up and then wake up the microcontroller and push the clocks higher to do the job fast and when done, disable the regulator again.
So you could have a 3v...4.2v lithium battery and give whatever voltage when running at low frequency / standby or boost to 5v as needed.
Or maybe have 2 AA batteries ( 2v .. 3v range, depending if you use alkaline or rechargeable) and want to boost to 5v only when needed and your micro can work with as low as 2v otherwise (lots of PICs for example work with even 1.8v at low clocks)