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Low ESR caps for LDO regulator

Discussion in 'Electronic Design' started by Scott Miller, Jan 16, 2005.

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  1. I don't think so. If the other parallel capacitors added up to much
    more than the lumped capacitance at the regulator, they would begin to
    dominate the phase of the current at high frequencies.
     
  2. Andy

    Andy Guest

    Okay, how is one supposed to evaluate the stability / dynamic
    characteristics of a real circuit powered by a LM/REG/LD/LT/NCP/...1117?
    No model seems to be available from the manufacturers. Only through test
    and trial??

    Suppose I am powering by an LM1117 an IC, with the tantalum as required
    at the output of the regulator. The IC has 10 ceramic 0.1uF caps spread
    around, from the power rail to ground. Is there a minimum trace
    inductance to observe between the regulator and the first 0.1uF, that
    would preserve good dynamic response of the reg.?

    I understand that there may be no response easily available. If this is
    a really grey area, and everyone has to make his own tests, why not
    simply go with some AnyCAP from AD?

    Thank you.

    -- Andy
     
  3. I think that this is the big problem with LDO regulators. Their
    stability is highly load dependent. When I have used them, I
    performed stability tests (injected disturbances), varying all
    relevant component values over a wider range than tolerances would
    account for, in the actual layout, and using several different
    regulator units from different production batches. This means that
    the board layout had to include provisions for alternatives.

    But I always worry about the generality of the solution I arrive at,
    especially if the regulator or load undergoes any significant
    revision.
     
  4. Anders F

    Anders F Guest

    How would one determine the "mJ" capacity of standard 0603/0805/1210
    resistors?

    One could hope for a decent ramp-up of the LDO and put it in series with the
    load (?). Of course it will see the entire DC-load and drop the voltage a
    bit at high loads... But it won't get the "AC-kicks" and limit the caps.

    /Anders
     
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