I'm working on a setup where I'd like to drive a Mega32U4 with 3.3V. Looking at the datasheet, it says I should be able to get a stable 8MHz @ 3.3V, so that's good. I want to use a Li-Ion battery which tops out at 4.2V and discharges down to 3.0V, so I'm trying to figure out what DC/DC converter I need. The closest I could find is this one from Murata. It says input voltage of 2.3V-5.5V, and I can get a fixed output of 3.3V @ 300mA which should be enough to drive the AVR and an SD reader.
My question is: has anyone done anything with DC/DC converters, or have any ideas about them? Is the above linked one appropriate? I've never used one, so I'm totally in the dark here.
One small concern that I have. Reading through the datasheet, some of the graphs seem to indicate 5Vin for 3.0Vout and 3.3Vout. Being that those numbers are only on the graphs (particularly the efficiency ones), I wonder if that means it can only supply a stable 3.3Vout is it has 5Vin. What happens when the voltage drops below 3.3Vin as the battery discharges?
After talking to Murata, it turns out, that particular module won't work for my setup. In order for me to get 3.3Vout, Murata says I must provide Vin = Vout + 0.5V at least. So since I want 3.3Vout, I need at least 3.8Vin. They recommend 5Vin for both 3.0Vout and 3.3Vout. A Li-Ion battery tops out at 4.2V, 3.7V nominal and can be discharged down to 3.0V. So while I could get 3.3Vout at the beginning of a charge, once it gets below 3.8V, the module will cut out.
So what I really need is a Buck/Boost. Buck it down to 3.3V when the Vin is high, and Boost it to 3.3V when it's low. Back to the drawing board ... and searching. Urgh.
You are looking for a buck-boost converter. However, I have not found a reasonable module to do this without to design it every time. This still might be your best option, and its a great learning experience. I have used them but not designed them myself.
I did try copying Sparkfun's LiPo Charger (https://www.sparkfun.com/products/10217 ) and just strapping it into a project of mine, HOWEVER there is something wrong with it where it will randomly drop down to 1.2V and therefore not function properly. I may be over-taxing it however, as I'm driving a number of LEDs at the same time with it. I haven't had time to debug it recently, but that is a possible area of problem. The one disadvantage is I accept that my battery drops below a certain level and that it might slow down/stop then (I'm not worried about speed).
JB_AU:
Correct me if i'm wrong, but doesn't the Mega32U4 run 8mhz from 2.7-3.5v ?
You're not wrong, it can run 8Mhz with 2.7V. I'm just wanting to give it a constant 3.3V regardless of the battery's voltage. This is all still in design and research phase, so ... anything can change.
CrossRoads:
You are comparing a chip to an assembled board KirAsh4, not really an apples-to-apples comparison.
I realize that. But since I'm designing my own, I don't need a ready made board. I just need the chip and the three parts that go with it. Even with having a board made, it would still be cheaper than Pololu's.
Something else I forgot: if I power it straight from battery, which will fluctuate from a nominal 3.6V (or as high as 4.2V) down to 3.0V, I'm assuming that also means that the individual pins used for signals also fluctuate, yes? In other words, if the voltage is 3.6V, a pin set to HIGH would also measure 3.6V (or close to that), correct?