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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.

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    \$\begingroup\$ +1 for going to the effort of calculating your temperature rise at maximum power! :-) Much better than the try/melt alternative! \$\endgroup\$
    – Fuzz
    Commented Oct 11, 2010 at 14:20
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    \$\begingroup\$ Thanks! I recommend this video on EEVBlog, it taught me everything I need to know: youtube.com/watch?v=8ruFVmxf0zs \$\endgroup\$
    – Thomas O
    Commented Oct 11, 2010 at 16:36

3 Answers 3

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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.

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  • \$\begingroup\$ Drop-in is a compromise between size and cost as we're trying to find the 'best' solution here. But, CUI drop-ins are certainly easy. \$\endgroup\$
    – Kevin Vermeer
    Commented Oct 8, 2010 at 22:49
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This buck converter is $1.19 in 1000 unit quantities. Input 4-40v, output fixed at 3.3v or three other voltages or adjustable from 1.3 to 37v, @500 ma.

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  • \$\begingroup\$ +1 This chip also requires a minimal amount of external components so space can be conserved. Still not as small as you can get a linear regulator though. \$\endgroup\$
    – Kellenjb
    Commented Oct 9, 2010 at 2:53
  • \$\begingroup\$ I'm wondering why they don't make those simple switchers go near 1 MHz and make the needed inductor smaller.. \$\endgroup\$
    – abdullah kahraman
    Commented May 14, 2012 at 15:35
  • \$\begingroup\$ Just found MCP16321. Great chip with small footprint and high efficiency for just about $1.6 \$\endgroup\$
    – abdullah kahraman
    Commented May 14, 2012 at 15:47
  • \$\begingroup\$ @abdullah - I guess not everybody would be happy with the RF noise. \$\endgroup\$
    – stevenvh
    Commented May 15, 2012 at 4:46
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You could use a mc34063 chip as a step-down converter. There is a calculator here which you can fill in and get the right parts out of it. Note that you need a good diode, i.e. one that is fast enough to switch your operation frequency, and is able to handle the peaks (Schottky). So is for the inductor. Those will probably be the 2 major expenses, as with any switching supply.

I think you should be able to design such a circuit for under $3. It's possible to get that chip for under $0.30 at digikey (at quantity)

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  • \$\begingroup\$ Thanks for the suggestion. I was considering an MC34063, but size is a concern, and the chip doesn't make the best use of inductor + capacitor size. \$\endgroup\$
    – Thomas O
    Commented Oct 8, 2010 at 21:09

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