LDO vs Step-down buck vs Step-up vs Step-up-down

Hardware
1

Hi all,

I am working on battery powered ESP devices that need 3.3V. I’m experimenting with 2 x AA and single Li-Ion cells:

power

I’m currently using an MCP1700 LDO but understand any LDO will dissipate “excess voltage” as heat so not the most efficient.

I have seen folks using step-down regulators like the LM3671 but am a bit confused as it looks like it can work with an input voltage range -0.2V to 6V, which means it’s step up for < 3.3V and step down for > 3.3V.

Likewise I’ve seen the LM3525 step-up used but that seems to have a –0.3V to 6V range, meaning it’s both step up and step down.

If I want to provide an efficient 3.3V with batteries that will supply both below and above 3.3V as they discharge, do I need to use a step-up and step-down regulator or just stick a step-down or step-up regulator and not worry about it?

I guess I’m hung up on the “step-down” nomenclature as that suggests when a battery drops below 3.3V it won’t be very effective. Same for “step-up” and for an LDO - if the battery voltage is below 3.3v - the dropout voltage I would assume the regulator wouldn’t work very well?

2

What you are looking for is a buck-boost converter. e.g.

3

I use some el-cheapo chinese buck-boost (2.x–24V → 5V) modules plus a TP4056/DW-01-based LiIon charging module for my Wemos D1 Mini prototyping.

This allows me to feed it either from a 6V solar panel or a 5V Micro-USB phone charger, plus have the LiIon battery charged and protected.

Note: These are 5V output, but you can also get 3.3V output.