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Opamp linear current regulator must come quickly out of positive saturation

Discussion in 'General Electronics Chat' started by Flyback, Sep 16, 2017.

  1. Flyback

    Flyback Well-Known Member

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    Hello,

    We have a linear LED current regulator involving an opamp which drives into the base of a FET and regulates the current in a sense resistor in the source of the FET (thers a reference voltage into the noninv input of the opamp). At first the opamp is powered but there is no voltage to power the LEDs….therefore the opamp current regulator is “hard-on” with its outputs banged up to its positive rail (in positive saturatrion). Then suddenly the LED driving voltage comes on and so the opamp has to slam very quickly out of positive saturation and quickly bring the LED current into regulation without LED current overshoot.

    ..In order that the opamp can more quickly come out of positive saturation, and start regulating the LED current, should we add circuitry to prevent the opamp’s output going any closer than about 2V away from its positive rail whilst it is in positive saturation?
     
  2. AnalogKid

    AnalogKid Well-Known Member

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    Only you can say. Do the LEDs mind a brief blast of unregulated current?
    Also, what is the opamp? Maybe a faster part will take less time to come out of saturation.
    Also, how long does it take for the circuit to stabilize now?
    Also, can you post your schematic?

    ak
     
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  3. Flyback

    Flyback Well-Known Member

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    Thanks , We really need absolute minimum LED current overshoot....the supply goes on and off regularly....we really need to limit or preferably get rid of any LED current overshoot.
    The schem and offending led current overshoot waveform is as attached.

    There is also an LTspice simulation attached.
    This shows the exact situation that we have, but for copyright reasons, i unfortunately cannot tell or show the actual application.
     

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  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Try this:
     

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  6. AnalogKid

    AnalogKid Well-Known Member

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    In addition to ron's compensation, try this. You already are committed to 0.9 V drop across R1. Use that to turn on an NPN transistor in the classic current source/sink circuit, with the collector tied to the M1 gate and emitter to GND. No matter how fast the opamp is, a single 2N4401 probably is faster.

    For adjustability, a 100 ohm trimpot and 100 ohm resistor in series, in parallel with R1.
    Pot wiper goes to NPN base.
    Collector to M1 gate.
    Emitter to GND.

    For more speed, add a small capacitor from the top of the trimpot to the wiper. Caution, this might cause a bit of ringing at the turn on transition.

    ak
     
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  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    If the current over shot by 200%, this is a good idea.
    The current over shot by only 20%, so I think the current limit transistor will never turn on.
     
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  8. AnalogKid

    AnalogKid Well-Known Member

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    I don't know about never. But I agree that relying on a p-n junction's V-I curve as a comparator reference is not a precise thing. That's what the pot is for.

    ak
     
    Last edited: Sep 17, 2017
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