I have a micro and RF module both which I'd like to power from either 3 AA NiMH cells or a single 18650 Li-ion cell. Haven't decided which I'll use yet. My current requirements aren't that much (~2mA or so) but I'm looking to achieve a fairly long runtime (30+ days between recharging).

Both the micro and the RF module have a fairly wide input voltage, however the RF module has a max input of 3.6V. So the issue is with fully-charged batteries either of these set-ups would easily supply more than the 3.6V max: more like 3.8-4.2V I'm guessing.

My gut tells me that the drop from, say, a 1N4001 diode (~0.6V) should do the trick, at least initially but as the cells discharge I'll no longer *need* that 0.6V drop and will then just be a waste.

My other thought was use a high-efficiency buck/boost converter ([such as this one](http://search.digikey.com/us/en/products/AS1337A-BTDT/AS1337A-BTDTCT-ND/2469606)) with Vout set to 3V or so. I don't necessarily *need* a fixed 3V output since both the micro and RF module will work down to 1.9V.

What's the most efficient way to drop the output so that either of these battery set-ups will provide 3.6V or less?