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Any good resources for DC power supply design?

Discussion in 'Electronic Design' started by Mike Noone, Feb 7, 2007.

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  1. Mike Noone

    Mike Noone Guest

    Hi - I end up making a lot of simple power supplies for my PCBs. Most
    everything has been DC to DC - lots of linear regs, a couple dc-dc
    step downs, even one step up. But I normally just choose a part and go
    by what its datasheet says. Are there any good resources for what
    sorts of considerations should go into choosing parts to give a clean,
    stable, accurate power supply for a PCB? I'm talking about low
    voltage, low current stuff. Normally I have a somewhat noisy 12V
    signal coming into a PCB and I'll need 3.3V or 5V at a couple hundred
    ma from it - so supplies for that sort of situation are what I'm most
    interested in.

    Any books/websites/app notes/etc.?

    Thanks!
     
  2. Have a look at the HP Power Supply Cookbook:
    http://www.slack.com/pdf/HP-AN90B.pdf

    Dave :)
     
  3. If you are really after a "nice, clean and stable" supply it's hard to
    beat some of the "low noise" linear regs available.
    It's common to use say a normal DC-DC converter to get your base
    efficiency, and follow that with a nice low noise linear reg for
    powering sensitive stuff like high end ADC's. The linear reg need only
    drop a volt or two, and current requirements for parts that require
    low noise supplies are usually small, so the linear reg doesn't affect
    your overall efficiency much.

    Dave :)
     
  4. PeteS

    PeteS Guest

    All the usual suspects have good app notes.

    Linear tech
    TI
    Analog Devices
    Maxim (but see the old thread about availability)
    National Semi
    On Semi

    and more, of course.

    Each manufacturer naturally targets their own devices, but as a good
    general view, Linear Tech does the best (imo, ymmv) app notes on such
    things.

    Cheers

    PeteS
     
  5. Mike Noone

    Mike Noone Guest

    Hi Dave - so for example on a current project I have a 12V power
    source. It's a li-poly battery - so it fluctuates around that a bit.
    It has a widely varying load on it, so it will not be particuarly
    clean. I need to use this to power a 16b ADC board that has many
    sensitive analog sensors on it. The board needs a 5V supply. It
    shouldn't draw more than 200ma peak. I think 50-100ma will be the
    normal load. I also need to maintain as much efficiency as possible -
    I'd really like to see 90% or better. I'd like noise on this supply to
    be in the single digit micro volt range, ideally. (being that it's
    powering a 16b ADC)

    So are you saying that the ideal solution would be to use a DC/DC
    converter to bring the voltage down to, say, 5.5V, and then use a
    linear regulator to bring that down to 5V? Can DC/DC converters just
    not be accurate enough?

    Thanks,

    -Mike
     
  6. Yes. It might be overkill, or it might not, depending on your
    application. Lots of system factors are involved, but it can't hurt to
    start with a nice low noise regulated supply.
    I have done this with high end 24bit sigma-delta converters using a
    low noise low dropout reg like the Micrel 5255:
    http://www.micrel.com/page.do?page=/product-info/products/mic5255.shtml
    30uVrms noise.

    In one case, I used a high efficiency custom designed 70-90VDC to 5V
    DC-DC converter, and then the extra low-noise regs from the 5V rail
    for the 24bit converters which didn't comsume much power on their own.
    The 5V powered some higher power digitial stuff where the noise was
    not critical. I still got >90% system power efficiency.

    As a rule you would not power a 16bit ADC directly from a regular DC-
    DC supply, you are begging for trouble.
    They can be, but other system factors come into play. For instance,
    PCB layout can be critical. Get this wrong and it doesn't matter how
    low-noise your power supply is. You can blow it all with one bad
    ground track placement.

    Dave :)
     
  7. Terry Given

    Terry Given Guest

    alas poor femptoFarad, it buggered him, Horatio
    Cheers
    Terry
     
  8. budgie

    budgie Guest

    I'm currently looking at some integrated DC-DC stepdowns for both final 5V load
    and also as pre-regs to some linears.

    RECOM (in deutchland) make some TO-220-pin-compatible switchers. I'm hoping to
    find other sources for these, as I also have a couple of 12V->5V linear regs in
    need of thermal relief.
     
  9. Guest

    Hi Mike,
    check this link http://www.emittsol.com/section/market-analysis/
    ma100002.html

    Srinivas
    EMITT Solutions
     
  10. colin

    colin Guest

    I always seem to be doing the same thing, my solution of choice now is to
    use integral swith type lm2594, and lowest esr caps you can and get the
    layout extremly tight, a second LC filter next to this helps a lot too and
    another LC next to the adc or as already suggested use low drop/low noise
    linear regs, but be aware linear regs arnt that good at rejecting switching
    frequency noise.

    For higher power/efficiency a synchronous rectifier type is good such as
    dual mosfet driver type tps40054.

    I always seem to need far more different voltages than I would like.

    Colin =^.^=
     
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