This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TLV431B: Instability

Part Number: TLV431B
Other Parts Discussed in Thread: TLV431

Hello,

I am working with a customer who has this circuit on their board. We are seeing oscillations on the output and wanted to see if anything in the circuit is the culprit. It seems to be a very standard application of the TLV431B. 

Waveform generator is to replicate ~50mVpp ripple from an upstream switcher.

Regards,
Ryan B.

  • Hi Rayan

    Can you please tell the customer application in more detail? 1.1nF in the loop may cause pole shift and make the circuit unstable. I will simulate this and check.

    If customer wants to have 5V DC at the o/p you can prefer following circuit - 

    Regards

    Trailokya

  • Hi Ryan,

    In addition to what Trailokya mentioned. I would change C2 to be below 10nF or above 4.7uF because that might be affecting the stability region.

    -Marcoo

  • Hi Marcoo, Trailokya,

    Thanks for your help on this. Two more details that may help troubleshoot this further:

    1. Initially the resistor R2 which sets the cathode current through the TLV431 was larger (5kOhm). The cathode current was pretty low and increasing capacitor C1 did not help the instability and it would oscillate indefinitely. Removing C1 would change the oscillation waveform but not resolve it.
    2. Once we decreased R2 to 1kOhm and increased the cathode current, and removed C1, the device would not oscillate but the reference pin was regulating to 1.19V instead of 1.24V. Swapping in a new TLV431 showed the same results.

    Are there any other tests we could try given this information?

    Let me know if you need to know anything else.

    Regards,
    Ryan B.

  • Hi Ryan,

    In the original schematic, C1 is typically connected between cathode and ref pin.

    Did you perform any tests in regards to C2?

    Can you show me the updated schematic?

    -Marcoo

  • Hi Rayan 

    If we analyse small signal model of the circuit , It will introduce a poll at gm/2*pi*(C1 + C2) location and a zero at gm2/2*pi*c1 .

    IC = 1.24/R2 = 0.37mA

    gm = IC/nVT = 15ms

    Pole = 2.3Khz

    TLV has internal pole around 1kHz so in order to make the circuit stable you have to split the poles. You can split it by increasing the C2 to max possible value  or very low value (1n) both should help. 1n will be preferable because pole will move away by 2 decades. Please let me know if this resolves your problem.

    Regards

    Trailokya