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TL431A: AC simulation using TINA-TI

K.Yaita
Guru 29720 points

Part Number: TL431A

Hi Team,

There is TINA-TI AC Reference Design as the following.



I download the design and tried changing schematic as the following (VKA=5V, IKA=50mA, CL=20nF) then run simulation, however phase margin is around -20deg (unstable).





I believe the schematic is stable condition because left side of curve B in Figure 16.



Should we consider AC simulation can't be used for TL431A?
If my simulation is incorrect, your advice is really appreciated. 

Best Regards,
Yaita

  • 0
    TI__Mastermind 19580 points
    Hi Yaita,

    I tested the model and I also measured the same situation. From my understanding from looking at the model, the model is very conservative with its phase margin. While the figure 16 is accurate, it is a typical graph and there will be deviation from device to device.
    The TL431A AC model compensates for this by having a larger unstable region which is why 20nF does not work but something like 1nF will work. The AC simulation works and it should be used.
    If you decide to use 20nF, you can run into stability issues from lot to lot variation. I recommend staying at least half a decade to a decade away from the boundary region. I would not use 20nF, it is to close to the border.

    -Marcoo
  • 0 in reply to Marcoo Z
    Guru 29720 points

    Hi Marco-san,

    Thank you for your kind support!
    >I recommend staying at least half a decade to a decade away from the boundary region. I would not use 20nF, it is to close to the border.
    It is blue dot when VKA=5V, IKA=50mA and CL is around 50nF on B curve.
    I understand it is recommended set to CL less than 10nF in this case.
    Is my undersytanding correct?



    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 19580 points

    Hi Yaita,

    Yes 10nF is better. If you are going to use this TL431 in extreme temperature deviations, then I would decrease the value further. Temperature does affect the stability of the device.

    Also why is the bias current of the TL431 50mA? that is a lot of power being used. Can I propose to use figure 34 to have a better Ika? If you decide to use figure 34, I would highly recommend to use the TL431LI as the stability region is much better.
     
    -Marcoo

  • 0 in reply to Marcoo Z
    Guru 29720 points

    Hi Marcoo-san,

    I understand!

    >Also why is the bias current of the TL431 50mA?
    It is just I want to confirm AC simulation by TINA-TI is usable or not to judge stability.
    Thank you for TL431LI (preview) information too!
    Do you know when TL431LI will be released?

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 19580 points
    Hi Yaita-san,

    The TL431LI will be released in Q4 2018. You can sample now and by the time production starts, the full device will have been released.

    -Marcoo