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TL431: a question abou capacitor for TL431.

Kazuya Nakai59
Guru 21505 points
Part Number: TL431
Other Parts Discussed in Thread: TL432

 Hello guys,

 One of my customers is trying to use TL431 for their new products.

 The have a few questions as the follows.

 Could you please give me your reply?

 Q. The customer is trying to understand the following document.

       Understanding Stability Boundary Conditions Charts in TL431, TL432 Data Sheet (SLVA482A)

https://www.tij.co.jp/jp/lit/an/slva482a/slva482a.pdf?ts=1594993730225&ref_url=https%253A%252F%252Fwww.tij.co.jp%252Fsitesearch%252Fjp%252Fdocs%252Funiversalsearch.tsp%253FsearchTerm%253Dslva482a

  They have a few questions at Figure 5, ESRs @ Vka=2.5V graph.

  (1) They think this graph shows the minimum ESR of capacitance attached between the both terminal of TL431 for stable output voltage as a shunt regulator.

        Also Figure 1 in this document says output voltage is stable when 0.01uF is connected to TL431. But Figure 5 says the capacitor minimum ESR need to be 20~50ohm at the capacitance value.

        They think 20~50ohm ESR is very big and it is not reasonable. The Figure 5 is correct?

  (2) If figure 5 is correct, 20~50ohm resistor must be connect to the capacitor in series when the capaciance is 0.01uF?

  (3) ESR of capacitor changes due to frequency. Which frequency the ESR which Figure 5 says at?

 Your reply would be much appreciated.

 Best regards,

 Kazuya.

  • 0
    TI__Mastermind 19580 points

    Hi Kazuya,

    I will be glad to help.

    1. Figure 5 is created assuming that the whole entire region is unstable. I would not apply the ESR to a stable region of the TL431. This graph is only valid for a unstable region.

    Figure 5 was created with the equation of Fc = 1/(2*pi*R(ESR of the CL)*CL) and they vary the ESR And the CL. This equation does not take into account the stability region of the device.

    Also 0.01uF is stable for certain regions and not others. The boundary line on figure 18 of the TL431 datasheet is 0 degree phase margin. I would add margin to this value to ensure stability over temperature and lot to lot variation. Also the lines are typical and can vary between devices.

    If 0.01uF is stable for your system, I would not add ESR.

    2. I would not connect a ESR to a stable capacitor. If you were using a unstable cap such as 1uF that requires a smaller ESR for stability, then you would add a ESR in series.

    3. The ESR typically varies across frequency heavily for capacitors, I would just assume it is close to 0 because in practice when comparing a series 10 Ohms to 0.01Ohms the ESR of the ceramics are neglectable..