Lithium Ion battery with buck converter

[electricianjay]

Getting the grips of things again, spent so long away from electronics (became a sparky).

Basically I want to design my own GPS tracker. Been done so many times and there would be much cheaper alternatives. This is a learning exercise for me.

So I thought of buying a 3.7V lithium ion battery from spark fun, designing a pcb with a buck-converter and micro+gps module. Are there switch mode power supplies which can provide a constant 3.3V output throughout the battery voltage range? I am guessing as the gps consumes more power the battery voltage drops to a point where a buck converter will no longer work. Then I have to use a booster converter? I'v looked at some linear voltage regulators and they seem to only work with input voltages of >5V far exceeding my battery voltage.

What is the usual way of supplying such devices?

[BravoV]

Google for "buck boost".

[digsys]

Use 2x 3.7V batteries in series ! It's far far simpler and better efficiency overall. If you're not using rechargeable, then  it's all
straight-forward. If you're going rechargeable, just include a simple Voltage balance monitor.

[electricianjay]

Thank guys.

Looks like I need to do some testing with batteries. I guess Daves constant current load will be a ideal job for this.

[NiHaoMike]

You could oversize the battery (just slightly) and just make do with a 3V or so low voltage cutoff.

Modern tablets have quite a bit of stuff that run on 3.3V yet almost all of them use a single (series) cell battery. Even if you want the low voltage cutoff to be 2.5V or so, a properly designed buck boost converter can be very efficient.

[minime72706]

Back when I did my college senior project in 2010, I used a TPS61132 to provide 3.3V out from a single-cell LiPo battery. It is initially in buck-mode until the battery voltage drops too low and then it switches to boost mode. Using two cells in series and using a simple buck converter is probably an easier bet, though.

EDIT: The reason I would suggest the two-cell buck-converter solution is that I found that prototyping with the TPS61132 had mixed results. It's really bizarre... My original prototype on a piece of RadioShack prototyping board using some DIP sockets and SSOP->DIP adapters (as well as some dead-bugging for the ganged inductor) STILL WORKS but the ones I made afterward, integrated into the project, failed. The same goes for a power supply I built with a MC34063 to convert 24VAC (HVAC/doorbell transformer output) to 3.3VDC output. The original prototype still works but every single prototype after that failed and sometimes with spectacular results. The TPS61132 and any chip similar to it is pretty much the perfect solution, however.
Tip you'll hear from many folks here: Don't use the MC34063 for anything - it's not worth it.

[sleemanj]

Some reading for you:
  http://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter

[minime72706]

Some reading for you:
  http://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter

Yes, the TPS61132 is a SEPIC converter. As I said before (in an edit), my original prototype still works, but others failed. One worked for a long time, but I think it eventually failed when the circuit switched from buck to boost mode. I had it sitting on a table in my room acting as a temperature sensor that I could just look at and one day it was just dead.

[minime72706]

If you want to give it a try, though it is going to be "fun" to prototype on your own, I can send you a couple of TPS61132s as well as a couple of the recommended inductors. I think I have two of each. Enough for one !@#$-up!

[electricianjay]

If you want to give it a try, though it is going to be "fun" to prototype on your own, I can send you a couple of TPS61132s as well as a couple of the recommended inductors. I think I have two of each. Enough for one !@#$-up!

Thanks for the offer but I have decided to use the TPS63001 buck-boost converter. http://www.ti.com/product/tps63001

I have free samples from TI. All other parts my parts are bought either from element14 and sparkfun. I have noticed that inductors are expensive!

[minime72706]

Chip looks like it would perform similarly - how much did the inductor cost you?

[electricianjay]

Chip looks like it would perform similarly - how much did the inductor cost you?

$2 each. Compared to the other components I bought its expensive.


Then I found this website:
www.coilcraft.com

Free samples, 15 day wait though. Looks like I cant finish before Christmas.