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current limit problems

Discussion in 'Electronic Design' started by IanM, Oct 18, 2007.

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  1. IanM

    IanM Guest


    I am limiting the current in a cct to ~60mA by using a 10ohm resistor in
    the load current path to turn on a pnp transistor which turns off a mosfet.
    It works but I cannot afford the voltage drop across the 10ohm resistor.
    Normal max operating current is 30mA and the 300mV drop is causing problems.
    Any solutions have to be upower as the cct spends most of its life drawing
    about 24uA.

    any ideas?


  2. D from BC

    D from BC Guest

    Maybe use one of the many various mosfet driver IC's..

    D from BC
  3. me

    me Guest

    The best bet would be to eliminate any conditions in the circuit that would
    cause it draw over ~60 mA and do away with the current limiting. Without
    more information it would be hard to give a better answer.
  4. Fred Bloggs

    Fred Bloggs Guest

    Is this a series pass MOSFET? If so then a current limit does not turn
    it 'off', it should be reducing the gate drive to maintain 60mA.
    You obviously need to reduce the current sensing resistor and amplify
    the voltage drop to a working magnitude. It will be easier to use an
    amplifier with an input range that includes the positive rail, and input
    offset will be a consideration if you are to avoid trimming.
  5. use a 1 Ohm resistor, followed by a 10x CMOS opamp?
    Or use a germanium transistor, it would trigger at about 150mV.
  6. Hammy

    Hammy Guest

    Why dont you look at a HIGH-SIDE CURRENT MONITOR IC like the Zetex
    ZXCT1009 (CHEAP and small SOT-23) you have an increased error with a
    reduced sense voltage less then 100mV,but worth looking at.Sense your
    current on the input of your regulator . You can easily rig a OVP,as
    well as UVP.I'm using one for this purpose.
  7. Phil Allison

    Phil Allison Guest

    "Jan Panteltje"

    ** I was frightened to even suggest that kind of " horse and buggy " idea.

    Glad you did - but.

    ........ Phil
  8. I used a ge transistor in something similar.
    The Ic temp sensitivity (Icb leakage) caused it to shut the thing
    down at about 75 degrees C, which was good, as it needed temp protection :)
  9. Phil Allison

    Phil Allison Guest

    "Jan Panteltje"
    "Phil Allison"

    ** I know that is simply no joke - as the very famous 1970s " Phase
    Linear" 700 watt and 400 watt audio power amplifiers use Ge devices as
    output Ic sensors in their ( VI) current limiting circuits.

    Types used were the TO5 pack 2N1304 and 2N1305.

    ........ Phil
  10. Ian, there's a simple scheme using two resistors that can
    gain you a factor of two reduction in the sense-resistor
    voltage drop. Namely, you add a resistor in series with
    the PNP transistor's base, and tie another resistor from
    the base to ground. This reduces the CL threshold roughly
    by the resistor ratio times the input voltage. You should
    confine yourself to a factor two because of the PNP's Vbe
    tempco and part-part uncertainty. You also have consider
    the PNP's base-current needs, as it shuts off the MOSFET.

    Vs Rs 4.7 60mA CL, 140mV loss at 30mA
    ---+-----/\/\----+---, ,----------------
    | | _|__|_
    e 20k | ,--- Vbe - Vx - R1 Ic/beta
    b -+- R1 --' | I_cl = ---------------------
    c | | Rs
    | chose R2 |
    | | | where Vx = (Vs-Vbe) R1/(R1 + R2)
    +---- | ----------'
    | | 0.65 - 0.32 - 0.05
    | GND I_cl = ------------------- = 60mA
    0.25 mA 4.7

    I chose R1 to drop 50mV, max, by assuming a 0.25 mA
    MOSFET gate-drive at cutoff, and a minimum PNP beta
    of 100. You chose R2, based on your supply voltage,
    to develop I = 0.32/R1 = 16uA through R1 (is that low
    enough micro-power?) to get a 320mV drop across it.
    For example, for 15V you'd chose R1 = 875k.

    There's a slightly more complicated scheme using five
    resistors and a 2nd PNP transistor that allows you a
    factor of three or four reduction. This scheme, which
    I first saw used by Bob Widlar in one of his IC designs
    35 years ago, obtains the reference voltage for the
    CL-reduction from another PNP wired as a Vbe multiplier,
    rather than using the power supply. This allows you
    to cancel out the BJT's Vbe tempco and part-to-part
    uncertainty issues. However, if you're going that far,
    I'd consider one of the other suggestions made here
    instead, such as the Zetex ZXCT1009 or a small opamp.

    I'll bet if Tony Williams was here, he'd have something
    interesting to say.
  11. Jamie

    Jamie Guest

    In the old days, using an incandescent bulb worked :)
    as current increased, so didn't the R.
    I suppose one could use a CLD (current limiting diode).
    they are actually FETS back to back internally but maybe
    a worth to look into.

    In any case, it sounds like a good candidate for a signal
  12. Fred Bloggs

    Fred Bloggs Guest

    Yes, without a doubt. It's impossible to digest the fact that we'll
    never be hearing from him again.
  13. Winfield

    Winfield Guest

    Another sobering fact to digest is that one of these
    days it'll be us.
  14. IanM

    IanM Guest

    I think I saw this in your book Win, AofE is at work so can't verify it at
    the moment.
    Thanks for this. It looks like a solution to me.
    I spent a good while researching the sci.electronics groups before I posted
    this question and Tony Williams posts came up a lot.
  15. JosephKK

    JosephKK Guest

    Winfield posted to
    So twit Horowitz and get the next edition of AoE out.
  16. Hey, we spent over half the days working on it so far...
    Got any good capacitor-dieletric tempco data you'd like
    to see us include in our plots? Other capacitor stuff?

  17. Zetex makes good stuff.

  18. Everybody has to go sometime. After seeing enough deaths in one's
    family, it almost becomes welcomed.
  19. John Larkin

    John Larkin Guest

    Are you going to venture into the great Bypassing/ PowerPlanes/
    ReturnCurrent debate? I might have some interesting TDR stuff.

  20. JosephKK

    JosephKK Guest

    Winfield Hill posted to
    I do not have any to hand any more. Maybe i can get some from
    previous co-workers, that make not pan out or take some time however.
    I was part of a test group space qualifying MLC capacitors about 20
    years ago.
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