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High eff. automotive buck SMPS chip?

Discussion in 'Electronic Design' started by [email protected], Aug 2, 2013.

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  1. Guest

    Howdy,

    I'm trying to save some heat in an existing product,
    automotive application.

    Anyone have a fave buck synch. switcher, Joerg-cheap?
    Vin: +12-28v
    Vout: +5v
    Iout: 4-5A
    n: >=90%

    <$2 for the semi's is the ballpark.

    The unit has a nice Simple Switcher already, but the schottky
    catch diode burns about a watt. I'd like to save that. My
    attempts to sketch in a synch rectifier are partsy, pricey,
    and ugly.

    TIA,

    James Arthur
     
  2. Guest

    Hi Jim!

    There is no .ASC. It's a simple LM22676, nothing special.

    http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lm22676&fileType=pdf

    I'm looking for either another part, or a cute way to bodge in a sync.
    rectifier to replace the catch diode.

    This is an old trick from the Tek 7104 (IIRC) 'scopes:
    L1 _ _ _ _
    ----+---' | ' ' '-----+----> Vout
    | | |
    | |/ --- C1
    '---| Q1 ---
    |<. |
    | ===
    ===

    Q1 is the synchronous rectifier. Cute, but you've got to tap L1,
    which means a custom-wound inductor. You'd also have to invert Q1,
    so that a modern BJT wouldn't break down b-to-e.

    Cheers,
    James Arthur
     
  3. Guest

    Probably. I spent some time searching, but the S/N is terrible--
    DigiKey has >18,000 SMPS chips.

    This one looks like a "maybe":
    http://www.aosmd.com/res/data_sheets/AOZ1231QI-01.pdf

    It's pretty nice, but not as rugged as the Simple Switcher's
    43V Vcc(max).

    Here's my modernized version of the Tek technique:

    =======
    .-.-.-. L1
    .--+----+---' ' ' '----+----+---> Vout
    | | | | |
    | ,-' | | |
    | ^ D1 | | |
    | | | | --- C1
    --. | === | ======= | ---
    | | |<' .-.-.-. L2 | |
    Lx|--+-|>|-|-----' ' ' '----' |
    | D2 |\ Q1 ===
    --' |
    ===

    It uses a standard low-sat BJT and standard inductors, but
    it shoots-thru badly at Lx turn-on.
    Decent. That, in concept, was what I'd hoped someone had handy
    up their sleeve.
    The LM22676 runs near 500KHz, so it's rather zippy for a SS.

    The best circuits I've sketched so far sense the inductor current
    via a tap, such that when you drive L1 the auto-transformer action
    cuts off the sych. rectifier automatically. Slick, but custom.

    The smart thing is probably to use the AOZ1231 and be done with it,
    but then I've got to add load-dump over-voltage protection--Vcc(max)=28v
    is not enough. Ick.

    Cheers,
    James
     
  4. Joerg

    Joerg Guest

    Assuming automotive means several 100k units/year, talk to Linear about
    their LTC3810. Comes in a 100V edition which is great for automotive.
    Unfortunately around $4 but that is the price for 100. If they can't
    come down enough, look for something similar. If you don't feel like
    sloshing through all the mfgs price lists tell your facvorites
    distributors "Find me something similar for a buck fifty". The external
    FETs are cheap. It allows higher frequencies so you save on the inductor.

    Then there is this one, but haven't tried it myself:

    http://www.micrel.com/_PDF/mic2176.pdf
    (but check for stock)
     
  5. Guest

    That's a beauty Joerg. Stock is a bit scary, but it's a good prospect.
    Thanks!

    Cheers,
    James
     
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