I wish to power my circuit with a Lithium-ion battery LIR2032 (around 40 mAh capacity). These batteries have a voltage that goes from 4.2V to 3V typically during their discharge cycle.
My circuit (running at 3.3V) has a maximum current requirement of around 20mA -- although I should state that this is only the peak draw occurring in less than 0.1% of the time; the circuit draws below 1uA the remaining 99.9% of the time. It will be a sensor node which every 30s wakes up and emits data using nRF24.
What would be the best way to efficiently convert the (changing) output voltage of a Lithium-ion battery to less than 3.6V which is required for the MCU?
Efficiency is very important here, because of very limited capacity of the battery. Also I have no problem with varying voltage because the circuit can work with 3.0 or less, so it does not have to be precise 3.3V or similar, just need to be below 3.6 at all times.
EDIT: I come to an idea, I can find MCU which is capable up to 5.5V (so it can work directly from battery). So only for nRF24 module lower voltage is required. Please look at schematic. What do you think? The idea here is to turn on transistor only when required (transmitting). And even base current is used so only losses are in transistor (for voltage dividing).