I've asked on a couple of other forums so thought I'd try here as well.

I would like to be able to run an AVR at either 3v3 or 5v (don't mind either way but it probably makes no difference to the problem) with an input range to a regulator of 1v5 to 15v.

There are buck/boost regs but AFAIK they all have a max of 5v.

The best idea I can come up with so far is a linear or buck reg AND a boost reg, parallel them and diode OR the outputs. Another way is to buck down to ~2v and internally connect that to a separate ~2v input and boost back up to 3v3 or 5v.

I don't mind having two separate physical inputs, but one would be better.

Of course I want it to be free and use no components

Why? I'm designing a dongle that can connect to batteries in a battery bank (as in large solar power banks) and the three common battery sizes are 2, 6 and 12 volts.

6 and 12 are no problem, but 2 is a PITA and I don't want to have two versions of this gadget.

What is the current requirement?.

If the current is low then something like this for the boost side:
http://ludens.cl/Electron/ledlamp/ledlamp.html

Probably only about 30-50mA so not much.

You can do the boost with a single 8-pin chip, a coil and a cap or two these days, and the buck is easy as well. What I would like to do is get both and I can't figure out how except for just having both and using one or the other

Actually, have a look at this little number

http://www.ti.com/product/reg711-5

Doesn't even need a coil.

I was thinking that you would design it for the 2V battery and then just suffer the loss when running on the higher voltage batteries. So something like the LP2951 low dropout regulator to drop the voltage to say 1.5V then boost from there.
Its not as if you are concerned about the current draw like you might be if running of drycell batteries.

What a cool little chip
But you've got the same problem if the battery is greater than 5.5V.

I think you're going to be stuck with some sort of combo of buck then boost anyway.

Yep, that's what I've got in mind. The boost chip can handle from 1v8 to 5v5 input so as I drop down to that range when using the larger batteries I should be OK.

It means having two separate inputs, one for 2v and the other for 6/12v, but that's not a big deal. It would be nice to have it auto range but an extra wire is a lot cheaper , just make sure you use the right one. I may have to add some protection on the 2v input.

Here's the current circuit.



I've used the 2951 before, good chip and I was going to use it here but then I found that MAX regulator, it can handle up to 45v in a load dump situation which is nice.

Also I'm not sure if D2 is needed, I don't know if the MAX reg likes having Vs on it output when it's not powered. I might have to ask Maxim.

Oops, I just noticed that the MAX output feeds back to the R1/R2 voltage divider, I'll have to think again there.

I was thinking more like the attached file. (Dont know how to add it inline)

Hmmm, what happens when the input voltage drops below 5 + headroom, does the output just follow it down. If so that's a neat solution. But maybe it shuts down if Vin is too low.

I'll have a close look at the data sheet again.