Electronic – LiPoly batteries and 3.6V digital ICs


Li-Poly battery voltage changes a lot while discharging — from 4.2V to 2.7V. Most modern digital ICs (I'm currently thinking of Cortex-M3 microcontroller and an RF module) require input voltage levels of 2.7V-3.6V.

So, I think that connecting them to 4.2V power supply will most probably result in a dead IC. How this problem is solved in actual devices?

I can only think of two solutions: the first is a buck-boost converter, which should certainly work, but is really hard to build (at least for me; I never had success with self-made DC-DC converters, not to say the ICs for them are hard to obtain in local Russian stores); and a linear LDO regulator, which will still incur some dropout, maybe as low as 0.7V, but it means that the smallest battery voltage with which the circuit is usable is around 3.4V, and quite a big part of charge is then left unused.

Best Answer

Linear has literally a few dozen buck-boost switchers which take 2.5V-4.2V in with 3.3V out, for example the LTC3534. This uses only common "chicken-feed" like Rs and Cs, a small coil and offers high efficiency. You may find the controller at the usual suspects, but it's not cheap. This is probably the best solution (apart from price).

The LDO is another option. NXP CortexM3 controllers like the LPC1343 work on voltages down to 2V, I don't know about the RF modules. LDOs with dropout voltages less than 100mV are not uncommon, so even if the battery's voltage gets as low as 2.7V you still have 2.6V left at the LDO's output.

A third solution is a switched-capacitor voltage doubler, followed by a buck switched regulator. This may look stupid at first sight, but you avoid the expensive buck-boost regulator, and will have much more choice for the buck (in both meanings of the word). Having the voltage doubler followed by an LDO is also an option, but then your battery will drain much quicker.