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Needed: 12-24V to 5V regulator @ 60-100mA

Discussion in 'Electronic Design' started by Piotr Biernat, Dec 12, 2006.

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  1. Hi!

    I need a power supply for a circuit that is supposed to work in a car/bus.
    Requirements are as above - from 12V or 24V regulate 5V @ 60-100mA. (I
    already know what to do with load dump, reverse polarity etc., so this
    topic is not relevant now).

    I'm just wondering if it is still ok to use linear regulator for such a
    thing - quick calculation gives (24V - 5V) * 100mA = 1.9W of power to
    dissipate. Quite a lot. 5V/24V = 20% efficiency.

    On the other hand, most easy-to-get switching regulators (e.g. National's
    SimpleSwitcher) seem optimized for higher currents, so efficiency is not so
    good at lower current. I've found some chips from Linear, like LT3470 or
    LT1394, but they seem hard to get here (unless obtained directly from
    Linear's page).

    Also I'm just wondering about the switching regulator reliability for such
    an application. Maybe it's just my lack of experience with switching
    supplies, but I "trust" LDO linear regs a bit more :)

    Any suggestions welcome :)


  2. linnix

    linnix Guest

    Linear for car perhaps, but not bus. Even a less efficient switcher is
    better than linear. My guess is more than 50% for 24V @ 100mA.
    It depends on the designer/manufacturer.
  3. Looks nice, dirt cheap. MC33063 is probably better for my needs, as it is
    supposed to operate from -40 celsius. Gonna give it a try probably, thanx !

  4. budgie

    budgie Guest

    At those current levels, a switcher using an MC34063 is suitable. I use them a
    lot at that sort of current level. In fact, most of the el cheapo mobile phone
    in-car chargers use it. DIP-8 chip, one diode, four resistors, a cap and an
    inductor. Pretty simple, schematic in the data sheet.
  5. jasen

    jasen Guest

    car's aren't nice 12V supplies, your finished device should be able to
    withstand various fault conditions

    eg: a load dump where the battery goes flat and then opem-circuit,
    and everything takes a hit from an altenator with the field current wound
    all the way up, upwards of 80V can be developed, this'll destroy lm317 and
    7805 regulators.
    auto electrics are often horribly inneficient 20% is no big deal,

    that bus will be closer to 27V typical by the way.
    the commercial units I've seen used switchers of some type

    if you only need one it's easiest to just buy a "cigar-lighter to usb" adaptor.
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