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simple negative rail gen.

Discussion in 'Electronic Design' started by Adam. Seychell, Jan 21, 2005.

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  1. I'm looking for a cheap simple method to get unregulated -5V 30mA from
    +5V supply, but output ripple must be < 10mV. I've read about a using a
    standard inductor based DC/DC stepdown and connecting the supply ground
    to the regulator's output. There devices like the LM2664 charge-pump
    voltage inverter, but switched capacitors are noisy (large ripple
    current in output capacitor). Is it possible to replace the capacitor
    with an inductor in one of these switched capacitor converters ?

    Adam
     
  2. Switched capacitor (charge pump) supplies are never very efficient,
    just very simple. I think you should look at an inverting supply
    based on a flyback inductor and something like the LM2594.
    http://cache.national.com/ds/LM/LM2594.pdf
    It can supply up to 200 mA of -5 volt with a 5 volt input.
    See figure 25.
    You may need a second stage of LC filter to get the ripple down to 10
    mV.
     
  3. Tim Wescott

    Tim Wescott Guest

    -- or just use an LC (or ever RC) filter following a switched cap filter.
     

  4. Thanks, I checked out Fig 25 in the LM2594 sheet. It looks like it'll
    fit the bill. But then I found the newer LM2734, which is much faster,
    smaller and doesn't cost any more. It seems manufactures of power supply
    IC are superseding their previous products at such a rapid rate that it
    has resulted in a obscene number of devices existing today.

    Adam
     
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