James,

Here's a link to a Linear Technology buck controller with the power switch inside. Simply add an inductor, filter cap, compensation network and divider network on the output and you're home.

I apply this chip in a similar circuit to convert 24VDC to 7.5 VDC followed by a linear regulator to 5VDC for a low noise supply.

http://www.linear.com/pdf/1956f.pdf

Good Luck,

Bruce