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Shunt regulator TLV431

Discussion in 'Electronic Design' started by j, Oct 7, 2008.

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  1. j

    j Guest


    I'm using the TI TLV431 in the shunt configuration with two external
    resistors (see schematic figure link below) to output +2V (vO), and an
    Input of +5V. I've determined that the R1/R2 ration needs to be 0.613
    (given that VREF=1.24V) for this to happen.

    However, I'd like to determine sink resistance (output resistance?) of
    this circuit but I'm not 100% sure that I'm doing it correctly.

    My first take on this is that the output resistance would be
    determined by shorting the output to ground and determining the short-
    circuit current (isc), which would be completely driven by the +5V
    input and the resistor Rin, thus the output resistance being: Rin. Is
    this the correct approach?

    Schematic figure:
    Datasheet for the TLV431 can be found here:

    Any help would be appreciated.

  2. Palindrome

    Palindrome Guest

    It really depends on why you want the "output resistance".

    Normally, you will use this circuit to supply a range of output current
    to other circuitry. I would measure the output voltage when sourcing the
    min and max designed output current and derive the "output resistance"
    from that.
  3. j

    j Guest

    I need a low impedance voltage reference for the circuit linked below
    (the red arrow in the image). I need a +2V since the output of the
    AD629 has to be withing 2V of either rail. Do you know of a better
    way to achieve this? I'm rather unfamiliar with voltage references/
    voltage regulators. Note: +Vs = 5V, -Vs = GND in the attached
    image. VREF would be the circuit referenced in my original post.

    The datasheet for the AD629 can be found here:

  4. j

    j Guest

    awesome! thanks Jim!
  5. Palindrome

    Palindrome Guest

    At first glance, it looks like a less than an ohm source impedance with
    an over ten thousand ohm load impedance. So, I wouldn't be too worried
    about the impedance issues.

    I think that you have misunderstood the significance of the "2v". This
    chip, with a 10v supply, is incapable of producing an output voltage
    lower than 2v or an output voltage higher than 8v. Without the
    application of a 2v Vref, if used as a unity gain device, you would get
    a constant 2v output for any input lower than 2v. Only after the input
    exceeded 2v would the output start to change.

    With 2v Vref, the output is still 2v when a zero input is applied, but
    will start rising as soon as the input does - so, by the time the input
    is 2v, the output will be 4v.

    Obviously, once the output reaches 8v (with a 10v supply) it will stop
    going up - no matter how much the input increases further.

    So, it all looks fine. The only change I might make is to up the supply
    voltage a tad. 5v is the minimum. I'd probably go for a 10 or 12v
    supply, if one was available. I am always suspicious when manufacturers
    quote one voltage range for operation, but use a rather narrower one for
    important performance figures, like power supply rejection ratios.
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