First: Welcome to elektroda! This forum is a nice collection of EEs and other people involved with electronics, and we try to help each others here
(And please remember to read rules and announcements so you get the full picture of the stuff here. Your first post seems to be correctly placed - not like some other newbies manage) :wink:
About your question:
I do not want to go deeply to any supplier's specific chips and applications, just give an example of one part, which might be close to your needs. I rather want to give some general advice what to look at:
1) The steady state consumtion of regulator chip is of course one essential parameter on so low power budget. So Be careful about the data there - because most of the chips are optimized for much higher power levels.
2)You need to see efficiency on very light loads, and for micropower regulator a bit high input voltage. The performance for your application is not always easily readable on the data sheet - so you should be prepared to test several parts before final desicion.
3) I guess you need a voltage lower than the input, and only one voltage, non-isolated. In that case buck regulators tend to be the best and simplest to implement. If you need step-up, inverted, or isolated topology, you have thougher task to have ultra low power and high efficiency!
4) In case the chip needs an external switch (for example Linear Tech LTC1771), be aware that the gate (or base) drive of the switch transistor needs some power, too! For a MosFet low gate capacitance, and, if possible, reduced operating frequency, helps. LTC1771 data sheet says for example that 80% eficiency should be feasible on a few mA loads and 15V in. But, it would perform much better on lower input voltages, and unfortunately you are stuck with a relatively high fixed operating frequency if selecting that part.
Good luck,
Ted