LM2576 datasheet questions

Discussion in 'The Projects Forum' started by tracecom, Dec 7, 2011.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    I am confused about the Micrel LMN2576 data sheet attached; on page 8, Figure 1, there are two schematics and two parts lists. In both schematics, L1 is shown as 330 μH, but in the parts lists, one is shown as 330 μH and the other is shown as 100 μH, yet they both have the same part number. Can anyone explain this discrepancy, or is it just a "typo," and the correct value is 330 μH (which seems probable)?

    In either case, I cant find the exact part at DigiKey, nor am I finding any reference to AIE (which I presume is the manufacturer.) I did find
    this: http://www.inductech.com/RelId/60684..._Inductors.htm
    However, I contacted "Inductech" and was told that they don't sell through distributors and don't sell to individuals, so I can't get the part there.

    Will any 330 μH, 3 amp inductor work, or is there something unique about the part specified by Micrel? Or can someone supply a link to the correct part?

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Why don't you look at the National Semiconductor datasheet?
    http://www.national.com/ds/LM/LM2576.pdf
    See page 14.
    LM seems to be the normal P/N prefix used by NS. Micrel usually uses MIC for their p/n prefix. So, that's an indication (to me, anyway) that they copied the design from NS.
    Most of NS's examples show a 100uH inductor and a 1000uF cap; while Micrel uses a 330uH and a 330uF cap.

    I'm sure Bountyhunter can give you the "inside scoop" on that if he chooses to do so.
     
  3. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    Thanks. That clears it up. It looks like the 100 μH reference in the Micrel datasheet is an unintentional holdover from the National application design. It also helps me understand why Micrel specified a part from an obscure supplier (who apparently is no longer in business.)

    ETA: When I look up the PE-92108 at Digi-key, it comes up as a 67 μH part. Go figure?
     
    Last edited: Dec 7, 2011
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Not all inductors are created equal. The 2576 usually specd the cheapest junk inductor available, which is a "stick" shaped core of powdered iron. Those have horrific EMI. A toroidal shaped core keeps the flux contained so it shoots out less EMI. Also, different materials have more or less switching losses. Ferrite cores have lower losses than iron. Inductors are all different.

    A larger uH value will reduce the inductor ripple current (which is good) but is bigger and costs more.

    Attached is a mini ref on inductors.
     
    Last edited: Dec 7, 2011
  5. SgtWookie

    Expert

    Jul 17, 2007
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    If you have a source of junked PC power supplies, you can find all kinds of goodies in them; at least several toroids, too.

    If you want to experiment with them, have a look at Ronald Dekker's "Flyback Converters for Dummies" page:
    http://www.dos4ever.com/flyback/flyback.html
    It's a really good resource, because it's written in laypersons' language.

    It's preferable to stay with lower voltage versions of his supplies, though. You can reduce R4 to ~47k to keep them in a safer range; and in figure 16; reduce the turns ratio to 1:3 instead of 1:10.
     
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