Electronic – Best solution to get 3.3V from 5V-20V @ 250mA


For my Super OSD project, I need a +3.3V supply at 250mA from a variable input of 5.5V to 20V. Cost is a concern, as is size. Originally, I considered an LM317 in a SOT-223 package, due to the small size. Unfortunately, it's difficult to get a heatsink for SOT-223 packages.

Then I did some calculations; the LM317 would be dissipating up to 4.175W with 20V input and 250mA of load and with a θJA of 140°C/W, the LM317 would cook along nicely at 584°C above ambient. So not practical.

The next solution would be a little buck converter, but I'm looking to get a small design and a cost of <$3. Does anyone know of an ideal chip for the job? Or is it still possible to use a linear regulator? I'd prefer to do so, but getting rid of the wasted heat is really a problem. Power efficiency is not critical for this application, as an electric motor will be drawing 10's of amps, compared to the few hundred mA for this module.

Best Answer

There are number of companies that offer drop in switching regulators that might meet your needs. I'd look at some of the electronics distributors out there with good parametric search capabilies like Digikey, Newark, or Mouser that might have something already designed that will work for you.

Although it's a little more expensive than your $3 cost target. Look at this drop-in switching regulator.

It handles the output current and input voltage range you've specified. It's $5.10 in quantities of 100. Take into consideration the time saving cost benefit of not having to design one yourself.