I'm looking for an elegant way to power a project, something that doesn't take up too much board space ... It's an LED controller that's powered from 12V normally, but also has a USB interface - planning on the uber-cool V-USB stack to drive it. The Tiny861 will run at 3.3V (or maybe 3.6V if that works out better ?)
The box will normally be powered by 12V coming into a switching regulator solution, but has to be able to accommodate being powered by USB for configuration. Code will detect USB commo and cut power to the LEDs so we don't try to pull too much power from the USB line.
There are a couple of ways to do this. I suppose the simplest is to use a typical 3.3V buck switcher to handle the 12V supply, and a couple of diodes in the USB power pin to drop that to 3.6V, followed by a 3.3V zener. Does it need that zener ? Would there be problems with a 3.3V switcher and the 3.6V from the dropping diodes at the same time ? Could use an adjustable switching regulator to put out 3.6V, but that takes more board space from the samples I've been looking at from Linear and National.
This feels kludgy - there must be a better way. I remember seeing several regulators in EDN or somewhere that take multiple Vin inputs, just haven't found the blurb articles yet. Ideally, I would like something that can take +12V on Vin-1, +USB5V on Vin-2 either individually or both at the same time, and spit out 3.3V.
Any good ways ? Anything to avoid ?