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TL431: Stability Boundary Conditions

T Kishi
Genius 4425 points
Part Number: TL431

Hi

My Customer has question.

1.Are there any differences in stability due to load capacitance between "TL431" and "TL431Q"?
⇒Customers use "TL431" and when load 1000 pF, It was confirmed that the output 　oscillated.
　It is known from the Application Note that this is due to the large capacity.
　Also, customers used "TL431Q" and confirmed that they do not oscillate even
　if 3000 pF is mounted on the load.

2. Does this Figure 16 "Stability Boundary Conditions" test stability?
　Are you conducting judgment on non-defective products?

3.If there is a Device that does not oscillate at 1000 pF, is there a possibility of oscillation due to aging degradation?

Since customers are very rush cases, please check them as soon as possible.

Regard

T Kishi

  • 0
    TI__Mastermind 19580 points

    Hi T Kishi,

    1. There is no difference in stability between the 2 devices but this is an AC parameter and can have lot to lot variation. No TL431 should oscillate with 1000pF, can you reconfirm this capacitance value?


    2. We should use figure 18 because it includes the Q-temp devices for TL431Q. Figure 16 and 18 are typical values of stability at 0 degrees and there is variation within manufacturing but it will be a minor difference. More details on this graph is in the app note which you referenced.

    3. I have no test data for in regards to aging vs  stability. But aging typically only affects long term drift and not the device functionality.

    Can you send us a customer schematic or anything else that can help us see if there if there is other problems that might of caused the oscillation?

    -Marcoo

  • 0 in reply to Marcoo Z
    Genius 4425 points

    Hi Marcoo

    Thank you  for reply.

    Customers are confirmed to oscillate in a 70℃. And customer also confirmed oscillation at 25 ° C / 1500 pF.

    The checked circuit is below.

    Does "long term drift" include internal parasitic capacitance etc?
    for example Is there a possibility that stability changes due to change in parasitic capacitance or change in leak current?

    Regard

    T kishi

  • 0 in reply to T Kishi
    Guru 140200 points
    Hi T Kishi,

    the 431 (no matter from which manufacturer) doesn't like load capacitance very much. Take care, this stability plots are only valid for typical devices and are only valid at room temperature! Even if you are slightly outside of the stability boundary lines of the plot, instability can occur and there's an increase of oscillating tendency. This has to do with the progressive loss of phase margin. So, concretely spoken there's no sharp limit between secure stability and oscillation. On the other hand, I don't think that the stability problem is a long term drift issue. It has to do with variations in the chip manufacturing, from lot to lot.

    There's a rule of thumb when using the 431: Stay away from the stability boundary lines by at least a factor of 10. So in your case even 1000pF load capacitance would be too much. 470pF would be adequate.

    There are some remedies when working with the 431. In my designs I follow these rules of thumb:

    1. Omit any load capacitance. Add a RC lowpass filter if noise reduction is necessary. Use a minimum R of 470R for this filter.

    2. Or use a huge load capacitance of >100µF. Important: Use a cap with a non-vanishing ESR. Or add an ESR of 0R5...1R.

    Kai
  • 0 in reply to kai klaas69
    TI__Mastermind 19580 points

    Hi T Kishi,

    Kai brings up good points regarding the stability of the 431 devices and their load driving capabilities.

    I would take an extra step and try to get new TL431 samples and test the circuit. With the schematic you showed me, I am surprised the device oscillates at 1000pF.

    -Marcoo

  • 0 in reply to Marcoo Z
    Genius 4425 points
    Hi Marcoo

    It does not necessarily oscillate with a load capacitance of 1000 pF.
    The customer says that it occurred in some of the mass - produced products.
    Even if you check with the sample, you may not see the phenomenon.
    However, it seems that it tends to be easy to confirm when raising the temperature.

    Regard
    T Kishi