Inductor selection for high effciency LM2765

Discussion in 'General Electronics Chat' started by spinnaker, Dec 31, 2012.

  1. spinnaker

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    Oct 29, 2009
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    I am selecting components for a LM2765 buck regulator. Since the power supply will be supplying 5V for a battery operated Pic, I want the supply to be as efficient as possible.

    For a high efficiency supply, the data sheet reccomends a Schott 67143980.


    http://www.schottcorp.com/products/...y&cat_id=162&highlight=1&product_number=34539

    But when I check my regular sources Mouser, digikey and Jameco , I can not find that inductor.

    Looking the datasheet, it has a very low resistance and has a ferrite EP core.

    My input voltage will be from 11 to 13.5 volts. Output voltage will be 5V @< 75ma.

    Any recommendations on a coil are appreciated.
     
  2. takao21203

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    Apr 28, 2012
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    Does not matter too much for this IC.

    As the frequency is 52 KHz.

    Any 220uH (or 100uH etc.) shielded ferrite inductor will be fine.

    Toroids also can be used, but for the 75mA you don't need a large toroid.

    Just make sure the current capability of the coil is at least 2x your max. current.

    I have observed most coils somehow work with this IC, only if you increase the current, some will have problems.
     
  3. bountyhunter

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    Sep 7, 2009
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    since the LM2765 is a boost (voltage doubler) I assumed you meant the LM2675 which is a buck converter?

    The differences in inductors will have a negligible effect on efficiency. What will is the use of a sync rec type buck instead of one that uses a diode. Going from 12V to 5V means the catch diode conducts more of the time than the switch. A switcher that uses two external FETs will significantly improve efficiency.
     
    Last edited: Dec 31, 2012
  4. spinnaker

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    Thanks for the quick replies!

    What about in regard to coil resistance? Does it not matter that much?

    The catch diode I was going to use is a standard 1n5819. Would that be a decent choice?


    I would have to select a different chip to use external FETs? I am interested from a learning standpoint but I might avoid it as I want the circuit to be as compact as possible.
     
  5. takao21203

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    you can use 1n4148 :)
     
  6. spinnaker

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    Slightly smaller forward voltage?

    Can I get by with a 185817 or is that stretching it at my input voltage level?
     
  7. takao21203

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    the 1n4148 is not bad at low currents.

    Difference between a good and a very good diode are neglible, unless you have so large currents you need to cool the diode.

    If you want very high efficiency, use one of these modern ICs with integrated MOSFET, and high frequency.

    These LM2576 etc. can at best approach 80%, due to the transistor.
     
  8. THE_RB

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    The 1N5819 is a good choice and will give you 2% better efficiency than a 1N4148 as its forward drop at 75mA will be about 0.3v vs the 0.7v drop of the 1n4148.

    Inductor choice won't be critical at 75mA as the I squared R losses will be very low due to the low current. Generally an inductor with low DC ohms is better but for such a low current as running a 5v PIC <75mA (and more likely <20mA?) you can just pick a convenient inductor for availability and size etc.

    Try a few inductor values and sizes in the 150-470uH range, and see what you think performs the best. There are a few projects here with 5v buck converters and photos of the inductors for different currents, it might give you some idea;
    http://www.romanblack.com/smps/smps.htm
     
  9. takao21203

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    OP could use 2x 1n4148.

    For these low frequencies, nearly any switching diode will do.
     
  10. bountyhunter

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    It's a 1A reg, it will smoke a 1N4148.
     
  11. spinnaker

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    No kidding? Did you read the design requirements I posted?
     
  12. bountyhunter

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    Answer this: when you first turn on the buck and the output cap has zero volts, what does that look like?

    How much current is the reg going to pump into the cap during turn on?

    How long will it take a 1N4148 to blow getting a couple of amps through it?

    Your design, if you think a 1N4148 is OK, use it.
     
  13. takao21203

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    I have used 1n4148 sometimes for small regulators.

    Since they are not expensive you can even use 2 or 3.

    Typically you would also only use a small output capacitor, some 100uF not some 1000uF.

    Not that I needed to use these or had nothing else available.
     
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