3.3V constant from LiPo battery. Inexpensive and simple circuit / chip?

I have an application where everything on my board runs 3.3V and I'd like to power the assembly with a 3.7V LiPo. It is my understanding that these batteries can range from 4.2V to below 3.3V

I plan on incorporating this charging board:

https://www.sparkfun.com/products/10401

and taking power from the two +/- battery terminals on that board. So essentially with the USB connected I'll have 5V unregulated and battery alone anywhere from 4.2 to ~3V

I've been searching around the forums / net for an inexpensive or common solution to providing constant 3.3V. There have been a lot of numbers thrown around, certain chips and what not, but some of them are very expensive ($10 +). I've also seen suggestions for step up / step down regulators like this one from Pololu:

https://www.pololu.com/product/2122

I don't mind the Pololu board, but if there is a simple circuit to do this that I can add to my custom PCB (not surface mount) I would prefer that.

Any advice would be great! Also, I'm a mechanical engineer, not electrical so please be gentle :wink:

A simple LDO linear regulator will work, you just need one with a very low drop-out, 0.2V or so, to be useful. However as always with LiPo you have to think safety - you need the circuitry to prevent over-discharge too, there's probably a chip that does this and gives 3.3V out (if not there shoulld be!)

Question is -

Why do you think you want a constant 3.3V?

Considered running LiFePO4 cells unregulated? Very flat discharge curve centering at 3.2V

MarkT: A simple LDO linear regulator will work, you just need one with a very low drop-out, 0.2V or so, to be useful. However as always with LiPo you have to think safety - you need the circuitry to prevent over-discharge too, there's probably a chip that does this and gives 3.3V out (if not there shoulld be!)

I considered doing this, but what would happen to the output of that regulator when the voltage drops below 3.3V? (Or say 3.5V if the 0.2V drop out is considered)

I believe the LiPo's I'm looking at have an internal circuit that cut them off at a certain point to prevent over discharge.

Paul__B: Question is -

Why do you think you want a constant 3.3V?

Probably because I'm inexperienced and the data sheets tell me to use 3.3V so I'm assuming this is ideal. I've run my circuit on 3V with seemingly no negative consequence. However, I've read that if it dips too low it could "brown out?"

evildave_666: Considered running LiFePO4 cells unregulated? Very flat discharge curve centering at 3.2V

Have not considered them but I just looked. I was looking for something in the 400mAh range but all I can find are cylindrical batteries for LiFePO4 at that rating. Do you know if they have them in the flat style?

Thanks everyone!

doublec4: Probably because I'm inexperienced and the data sheets tell me to use 3.3V so I'm assuming this is ideal. I've run my circuit on 3V with seemingly no negative consequence. However, I've read that if it dips too low it could "brown out?"

Ah yes, I rather thought as much! :grinning: The "XY Problem" yet again.

So, let's get to work after all.

What is the "application" you first mentioned? You need to enumerate the various parts so that we can determine what actual limits there might or might not be in order to determine how to work within such limits.

I’m working with a microview:

and a RN-52 bluetooth module:

So essentially they are both capable of running in the 3.3V range.

Ideally I would like to be able to power the circuit with a LiPo battery. When the battery runs low I would also like to be able to plug in a USB cable and recharge the battery while still being able to operate my circuit (so power + charging simultaneously).

Thanks!

Found my solution:

https://www.sparkfun.com/tutorials/379