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TL431C voltage tolerance

Hello,

We selected the TL431C for our design.  However, we are having some issues in our system, and may have traced it back to this device.  It has 2% tolerance, but we need some clarification on what that means.  The output voltage range in the data sheet is 2440mV to 2550mV.  That is about 2% on either side of 2495mV.  However, does the 2% mean it could be 2% below 2440mV to 2% above 2550mV?

The reason we ask is that we are measuring a range of 2415mV up to 2496mV on a handful of units that we've tested. 

Please let us know what we should be expecting.  If needed, we may have to go to the TL431D with a tighter tolerance.

Thanks for your help.

  • Jamaal, 

    In your application is VKA=VREF or is cathode voltage higher or cathode current less than 10mA?
    There is a dVref/dVKA specification that lowers Vref as VKA increases.
    There is also a ZKA specification that lowers Vref as IKA decreases.

    2% does indeed mean +/-2% from 2.495V, but only when VKA=VREF and IKA = 10mA.

  • Thanks Ron,

    We know we have 17.3ma > Ika > 12.4mA.  The Vka in our circuit is 10V.  Below is our circuit.  We are using the output of the TL431C with a comparitor that measures out input voltage.  The variance in the output is causing some big issues in our system. 

    We are using a 4.3k ohm resistor in series with the cathode.  We've tried lower that to a 1k to increase the current.  We are using 1% resistors as the diver to Ref (15k and 4.99k).  Would changing any of these help?  We have ordered some 0.1% resistors and the 0.5% version of the part to try, but ideally, we can find a solution with the existing part.

    Thank you

     

  • Jamaal,

    The cathode current of 12mA to 17mA is high enough and will provide slightly higher Vref than data sheet's 10mA value.

    The 2415mV sample is lower than I would expect. So I suspect the 0.1uF capacitor makes the TL431C oscillate which will decrease the DC voltage you see on the reference and cathode. Check the cathode for oscillation (a small or large saw tooth wave).

    I suggest staying far away from the instability line "line C for 10V", Ideally 10X away, less than 6nF or greater than 6uF. (for line C at 10mA). Other caps can be used if a suitable series resistance is used.