Linear or switched mode regulator?

I have a custom Arduino that is poweredby a LiPo battery going through a buck/boost converter to give a constant 5 volts. The 5 volts powers some sensors while the arduino and tranceiver module run on 3.3 volts.

Being mindful of efficiency to conserve battery power; to step down from the 5v to the 3.3v should I use a linear or switched mode regulator? I read somewhere that at low voltage differences, a linear regulator can be more efficient that switched mode. (The arduino and transceiver pull 30-60ma.)

Well, consider (5V - 3.3V) * .06 = 100mW dissipated as heat only with linear regulator. These little guys http://www.pololu.com/product/2097 claim to be 90% efficient with 5V in and 3.3V out, so you would expect only 10mW dissipation loss and only cost a dollar or two more than a linear regulator.

An ultra-low-dropout regulator direct from the LiPo to 3.3V might be best, LiPo’s
are about 3.7V

You can measure the 3.3V rail w.r.t. the 5V rail to detect low battery too - the ratio
should be pretty constant until the LiPo voltage drops below what the LDO regulator
needs to maintain 3.3V.

lemming: I have a custom Arduino that is poweredby a LiPo battery going through a buck/boost converter to give a constant 5 volts. The 5 volts powers some sensors while the arduino and tranceiver module run on 3.3 volts.

I'd suggest running the arduino directly off the LiPo. You'll get ~4v at full charge dropping down to ~3.3 at 10% charge.

Thanks guys.

An ultra-low-dropout regulator direct from the LiPo to 3.3V might be best

As it is a remote solar setup, I want to extract the max from the LiPo if necessary. This means I will drain it down to 2.7 volts which is below the cutout for a regulator. By taking the 5v from the buck/boost coverter I can theroretically run the battery down to 0.9 volts (not advisable for LiPo).

I'd suggest running the arduino directly off the LiPo.

I still need an exact 3.3v for the transceiver. Any more will release the white smoke. I have to run the Arduino at the same voltage as the transceiver to avoid level shifters between the two.

I will follow CrossRoads advice and go with the Pololu regulator.

Regarding the theory; I am still curious as to whether others have heard of linear regulators being more efficient than switched mode regulators when there is a small difference between the input voltage and output voltage. I cannot recall where I read this in the past to check up on the theory behind it.

Re: Efficiency.

Sure, if an LDO is taking, say, 5.5V and regulating it down to 5V, the efficiency is very close to 90.9%. Slightly less because of the current the linear regulator requires.

However, it is tough to get a switching regulator to get better than about 85% efficiency, especially at low current draw when it must also occasionally supply higher currents. The switch mode circuitry typically requires a lot more current than a linear regulator just to run things.

I seem to recall some chips out there that are switch mode at higher current draw/greater voltage drop, but change to a low dropout linear regulator at very low current levels. Best of both worlds.