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Confusion about LM431 - Adjustable Precision Zener Shunt Regulator

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by eyAyXGhF, Sep 10, 2011.

  1. eyAyXGhF

    eyAyXGhF Member

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    Hey all,
    I've recently used an LM431 because I needed a 5.00V reference voltage. According to the datasheet (http://www.national.com/mpf/LM/LM431.html), to set the voltage output of the LM431, there is basically a voltage divider.

    I'm wondering if the stability of the LM431 is basically dependent on the stability of my power supply, since the voltage divider that sets the output voltage hangs off the positive rail of my power supply.

    Maybe this isn't the correct application for the LM431, but I would like the 5.00V stability to be better than my powersupply. Otherwise, why wouldn't I just set a trimpot to 5V and send it through an op-amp buffer for my 5V reference?

    Attached is a screenshot of the LM431 in my schematic.

    Mike
     

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  2. simonbramble

    simonbramble Active Member

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    Do you have a schematic of your power supply? With many power supplies these days, the reference is internal. If you do need an external reference, then yes the stability of your reference is the main dictator of the stability of your power supply. Your reference drifts, your power supply output drifts.

    The 431 works as follows: you set up a voltage divider and connect the junction of the resistors to the control pin. The 2 end terminals of the Zener go low impedance when the control input reaches 2.5V (or whatever the datasheet says that pin regulates to).

    I am assuming you are generating the reference voltage from another (unstable) supply voltage. If you use a trimpot off this supply voltage, this will only pass any ripple from the supply through to the output of the trimpot
     
  3. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Unlike many series regulators you see that have a separate pin for the very output, the 431 chip is a shunt regulator. That means it shunts current to ground in order to regulate the output voltage (5v in your case). If the output is too high, it shunts more current to ground, and if the output is too low, it shunts less current to ground. This keeps the output (as shown as +5v on your schematic) at the required voltage.

    The 431 is basically a 2.5v shunt regulator, but if you connect equal value resistors to the sense input that doubles the output because while there is 2.5v from sense input to ground there is also 2.5v from output to sense input, and the sum equals 5 volts.

    I didnt check your resistor values but your basic connections look correct.
     
  4. dave

    Dave New Member

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  5. mvs sarma

    mvs sarma Well-Known Member

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  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The resistor dividers that determine the output voltage go to the 5V reference voltage. The resistor that goes to your power supply is there to limit the current through the shunt regulator and has no significant effect on the 5V output.

    Thus the stability of the 5V output is determined by the stability of the shunt regulator, not your power supply.
     
  7. eyAyXGhF

    eyAyXGhF Member

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    Thanks for explaining that, it definitely makes sense now.

    I think my issue with the LM431 might have something to do with the fact that I just noticed I had the pinout wrong in my notebook and on my PCB. I had the Anode and Reference swapped, woops! I've corrected it, but the LM431 still doesn't seem to be working, so I think I may have damaged it when it was connected wrong. Waiting on some replacement parts to try again.
     
  8. 4RESTER

    4RESTER New Member

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    You can use two TL431A.

    AIC431 take better precission.

    TL431 = ±1.6% min=2.455 typ=2.495 max=2.535
    TL431A = ±1.0% min=2.470 typ=2.495 max=2.520
    AIC431 = ±0.5% min=2.482 typ=2.495 max=2.508


    edefb05ff0fb1f6a4ba2265dd281b14e.png

    This is better that possible for low cost.
    To better precission use the LM4040-5 (precission 5.000V).
     
    Last edited: Nov 12, 2013
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  9. ronv

    ronv Well-Known Member Most Helpful Member

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    You don't say how much current you need from the 5 volts, but the 431 needs at least 1 ma to regulate. You have about 4ma so it only leaves 3 for your load. I would make R188 smaller and R189, 190 and RV20 bigger so there is more margin.
     
  10. ChrisP58

    ChrisP58 Well-Known Member

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    Another choice you have for a precision 5.00v reference is the LM4040-5.0. No resisters need, so you don't suffer the accuracy penalty from mismatched values or thermal drift.

    Available down to 0.1%
     

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