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Reference Impedance Test Circuit in TL431 Datasheet

Christian Grau1
Prodigy 20 points
Other Parts Discussed in Thread: TL431

In the TL431 datasheet (SLVS543M) there is a test circuit to measure the Reference Impedance.

 

Can anybody explain how that measurements works? I miss the ac coupling...

 

Thanks for any suggestion.

 

Chris

  • 0
    TI__Guru*** 149666 points

    Christian,

    AC coupling is not needed here becuase VIN[dc] is 0V. 

    The circuit works fine for simulation, but it not the most practical setup in reality.
    The 50 resistor is pointless because the simulated AC generator has zero impedance; a real source will likely be 50 ohm internal. The voltage source (set to get required cathode current) is also simulated as zero impedance, a real supply would be a complex impedance, so bypass caps (plural) are highly recommended.

    The basic theory is that the 1k resistor and the TL431 impedance form a voltage divider. The AC voltage on the TL431 vs. the input AC voltage and 1k fixed resistor is mathematically converted into TL431 impedance value.

  • 0 in reply to Ron Michallick
    Prodigy 20 points

    Ronald,

    Thanks for your explanations. I got the idea.

     

    Kind regards

     

    Christian