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5V to 5V regulators

Discussion in 'Electronic Design' started by panfilero, Jan 14, 2013.

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  1. panfilero

    panfilero Guest

    I have 5V from a regulator that's not very well regulated that I would liketo feed into a LDO regulator to clean it up a bit.... but I'm not sure howI can do this since I would have to be consistently above 5V in order for the LDO to do its thing.... does anyone know a good way to do this?

    much thanks
     
  2. If he can afford a diode drop, then a capacitor multiplier will clean
    up the noise.
    (Assuming not too much current draw.)
    Yup cascading regulators seems a bit silly.

    George H.
     
  3. OK, that makes sense.

    Though I'm not sure I'd call the output from some LDO's 'really
    clean'.

    I made this 'low noise' power supply that's an LDO regulator followed
    by a cap multiplier.
    (Sorta ~2-3nV/rtHz at moderate currents (100mA) (Noise almost all from
    the LDO.) I later needed a higher current (low noise) power supply
    for the same project (~0.4A).
    So I used a voltage reference into an opamp->transistor pass element-
    hadn't used the LDO, but it was too late by then.

    George H.
     
  4. panfilero

    panfilero Guest

    I can't touch the first regulator, it's regulation can be off by 8%, I can only add something to it's output, the cap option sounds interesting I'm going to look into that

    thanks!
     
  5. panfilero

    panfilero Guest

    If I'm able to get my first regulator up to 5.5V is that good enough for anLDO to clean it up to 5V or am I too close to 5V? My load is pretty light.... 13mA or so... I've found some LDOs that have a dropout voltage of less than 100mV

    I'm thinking this part lp2950

    http://www.ti.com/lit/ds/symlink/lp2950-n.pdf
     
  6. Guest


    maybe one of the numerous boost converters meant for LiPo batteries
    something like a tps62100, add a diode or two to keep the Vin
    under the needed Vout

    -Lasse
     
  7. panfilero, Is your problem that the 5V is not constant, it droops
    under load? Or is it the noise on the 5V line? The cap multiplier
    will do nothing for the droop.. in fact it might make it worse.
    (Higher output impedance.)

    George H.
     
  8. Joerg

    Joerg Guest

    Easiest might be John's 2nd suggestion. A simple boost conversion that
    adds 20% or so to the voltage, then a linear regulator to make a clean
    5.00V. If it doesn't have to be so clean use only the step-up switcher.
    If necessary with a Schottky up front to drop 400-600mV in case there
    are phases where the voltage coming in is too high.
     
  9. Guest

    You mights as well suggest he improve the stabilzation control algorithm of SpaceX.
     
  10. Joerg

    Joerg Guest


    C'mon, this ain't no rocket science :)
     
  11. rickman

    rickman Guest

    I'm not following what you are describing. Can you explain in a little
    more detail? I guess I'm not getting how you use a cap multiplier...
    oh! I guess you are saying a cap multiplier rather than a cap *voltage*
    multiplier! Is it that difficult to just use a large cap rather than a
    cap multiplier? Or do I completely not understand what you are saying?

    Rick
     
  12. rickman

    rickman Guest

    I'm not sure that works so well. Most LDO's have noise rejection that
    rolls off somewhere in the 10's to 100's of kHz which is typically below
    the noise range of the switcher. So they don't really do much at all to
    clean up switching noise. Much better to minimize the noise in the
    first place with a proper layout, etc.

    Rick
     
  13. rickman

    rickman Guest

    TI used to make a switched cap regulator that was pretty sweet. It was
    good up to 100 mA or so and regulated the switches like an LDO to
    maintain a given output voltage. But I don't think they make it anymore.

    Rick
     
  14. Tim Williams

    Tim Williams Guest

    Cap multiplier: an emitter follower where the base is supplied by
    RC-filtered power. Effectively, the capacitance as seen from the emitter
    is C 1* hFE, which improves HF performance considerably. It also acts as
    a reversed grounded-base amplifier, so the C-E isolation is large (only
    Early effect working into the equivalent emitter impedance plus additional
    bypass).

    I recall Phil Hobbs had an application that required two or three stages.
    Nice part about that is, subsequent stages can be biased from the
    already-filtered base supply, which means an additional Vbe + Vce(sat) is
    not incurred, only another Vce(sat).

    Tim
     
  15. Oh Sorry, the cap multiplier is just a RC and transistor
    circuit fragment,
    http://en.wikipedia.org/wiki/Capacitance_multiplier
    (R2 is part of the load.)
    I learned it from Phil H.'s book.... (For years I called it a Hobbs
    filter.)

    George H.
     
  16. panfilero

    panfilero Guest

    I've found a part that appears to reccommend a way of doing what I'm tryingto do, but I'm confused...

    the part is DCP0105

    the datasheet is here

    http://www.ti.com/lit/ds/symlink/dcp0105.pdf

    on page 16 it starts talking about using an LDO after the 5V to 5V isolator

    "It is possible to post regulate the 5VOUT DCP0105 and still guarantee a minimum VOUT of 4.75V. This still gives the benefits of isolation in reducingthe power supply noise to 5V digital circuitry."

    I don't understand what they mean... guarantee Vout of 4.75V? That's terrible... that's 0.25V away from 5V.... are they talking about something else?I thought adding a post regulator requires a higher votlage than 5V at the input of the regulator and that the point is to have the 5V LDO provide anice clean 5V at its output.... will this do that?

    thanks!
     
  17. panfilero

    panfilero Guest

    much thanks for all the replies, this is the solution I'm going with, I'm going to end up just boosting up the voltage and then regulating after that
     
  18. Jamie

    Jamie Guest

    You should try a 3 phase version of that going into a single DC node
    point. It gives a nice DC output.

    Jamie
     
  19. Jamie

    Jamie Guest

    Where I come from, there is only one inventor and
    many copy cats.

    Of course, if you talk to a lawyer, they may have a different
    view of things.:)
    Jamie
     
  20. Guest

    What if the cats didn't copy anyone?
    There's a statement!
     
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