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

Discussion in 'General Electronics Chat' started by tom66, Oct 8, 2010.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
    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.
    Last edited: Oct 8, 2010
  2. Zez

    New Member

    Apr 29, 2009
    Perhaps LM317 in TO-263 package would be the right solution.
  3. bertus


    Apr 5, 2008

    Can you order at conrad?
    Then take a look at the part in the datasheet.

  4. tom66

    Thread Starter Senior Member

    May 9, 2009
    @Zez: TO-263 would be an option, but it still has 50°C/W thermal resistance, and at 4.175W that would be 209°C above ambient.

    @Bertus: I don't really want to use a module, because it restricts me to a single source, and because they are quite big. Thanks for the link though.
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010