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TLV431: TLV431: TLV431 voltage comparator in open-loop configuration issue

Thibaud Jenny
Prodigy 10 points
Part Number: TLV431
Other Parts Discussed in Thread: TLVH431, LMV431

hello everyone,

I use the following design from SLVA987A with a TLV431: 

I have the following issue, this circuit work very well with TLVH431 but not TLV431.

When the Vref voltage is lower than the internal 1.24V reference, the cathode voltage is set to high level (~Vsup).

But when the Vref voltage is higher than the internal reference, the cathode voltage go to the low Vka voltage and increase rapidly with an unstable transition.

I measure the following oscillograme of cathode voltage when it was out of expected situation:

For my application, I want to detect under voltage on power supply. With the TLV431, I have a flag when the power supply voltage (Vin) is under a certain value. I have a second threshold when the power supply voltage is upper another value (It's not expected for under voltage detection).

However, my application works well with TLVH431 variant.

Where does this difference in operation come from?

Thank you in advance.

  • 0
    TI__Expert 6640 points

    Hello,

    These two parts may have slightly different stability regions when used in this configuration. This could cause the TLV431 to be unstable and oscillate while the TLVH431 is stable.

    Adding an output capacitor in the stable region might improve the TLV431 oscillation. 

    Best,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 10 points

    Hello,

    Thank you very much for your answer. I added a 3.3µF capacitor at the output and i no more see any oscillation.

    But I still have a problem :

    On TLVH431 / LMV431, when I increase the voltage on ref pin, the cathode voltage is limited to the Vka(min). (and ~3.3V when it is lower than reference voltage)

    For the TLV431, when I increase the voltage on ref pin, the cathode voltage is not limited and increase significantly. It is the same for reference voltage, and it is not limited to the internal voltage reference.

    I don't have the behavior for a comparator with the TLV431. In this exemple I juste simulate power down (See the difference between LMV431 and TLV431)

    Rin = 10k, Rsup = 1k, Vsup = 5V.

    Is there still a difference I can't see in the documentation?

    Thank you in advance.

  • 0 in reply to Thibaud Jenny
    TI__Expert 6640 points

    Hello,

    Are you sure LMV431 causing the issue or TLV431? I have seen this issue where Vout is not limited with LMV431 but I have not seen this issue with TLV431. I will try to recreate your results and get back to you tomorrow.

    Thank you for your patience,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 10 points

    Hello,

    It is the TLV431 that's giving me trouble (Left oscillogram is the LMV431, right oscillogram is the TLV431).

    For variants like LMV431 or TLVH431, I haven't seen any problems.

    Thank you,

    Thibaud

  • 0 in reply to Thibaud Jenny
    TI__Expert 6640 points

    Hello,

    I tested TLV431BQDBZT and LMV431BIMF/NOPB with this schematic and values Rin = 10k, Rsup = 1k, Vsup = 5V.

    LMV431

    TLV431

    Unfortunately I was unable to replicate your test results. LMV431 had a slight rise in the low output voltage. 

    Have you tried repeating your test with multiple parts? 

    Thanks,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 10 points

    Hello,

    Thank you for your test.

    I will check correctly with several parts if I can get your results.

    Thibaud 

  • 0 in reply to Walter Zollinger
    Prodigy 10 points

    I reproduced your test for TLV431AILPR and TLV431IDBZR and LMV431AIZ/LFT3.

    For the LMV431, I obtained the same result as you (with a slight rise in low output voltage).

    For TLV431x, I got the same result as before,

    The cathode voltage "follows" the reference voltage. (With a lot of noise / oscillation in the transition if I don't use an output capacitor).

    So I don't know where the problem could come from, I have the same problem on the different references of the TLV431 family.

    Thanks.

    Thibaud

  • 0 in reply to Thibaud Jenny
    TI__Expert 6640 points

    Hello Thibaud,

    I will consult with other members of the team to try and find the cause of the issue by the end of the week. Thank you for your patience on this issue.

    Thanks,

    Walter

  • 0 in reply to Thibaud Jenny
    TI__Expert 6640 points

    Hello,

    If there is no issue with TLVH431 then I would just continue to use that part.

    Thanks,

    Walter