DC-DC Converter

[simcop2387]

I know this is probably a pipe dream but I've been looking through some DC-DC converter designs and I think I can do this with a SEPIC converter.

I'm going to be powering a circuit off of a battery and would like to be able to drain the battery as much as possible when running (This will be Lead-Acid or NiMH, not a Lithium), so I'd like to keep running after the voltage drops below my target 5V and even when the battery drops below. I know that this will result in the battery having more current being drawn and will likely mean that I can't drop below maybe 4-4.5V depending on the current. I haven't finished building the rest of the project yet but I expect 500 mA-750mA draw on it, it's going to be just a simple marquee display with an Xbee radio.

Has anybody done this before and knows of something already made or knows a set of plans for building one already?

[simcop2387]

Since I couldn't find a way to edit, I would love the input to be able to handle up to 15V if I wanted to use a lead acid, but from what I'm seeing googling around that likely won't happen and I'll be going with the NiMH battery packs at 9.6V

[adafruit_support_mike]

Just to hit the basics, a SEPIC is a boost-buck converter. Its claim to fame is that it reduces switch losses by using a single mosfet to do all the switching. In the literature, some people love it and others hate it.. about par for the course on any design. Boost-bucks are usually meant for applications where the battery voltage crosses the target voltage.

If you're looking to roll your own, I can suggest a couple of classic references:

"Switching Regulators for Poets A Gentle Guide for the Trepidatious":
http://cds.linear.com/docs/Application% ... an25fa.pdf

"Building a DC-DC Power Supply that Works":
http://pdfserv.maxim-ic.com/en/an/AN1897.pdf

If you're looking for existing circuits, Maxim's "Power-Supply Cookbook":
http://www.maxim-ic.com/cookbook/powersupply/

has a couple of SEPICs.