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TL431 output voltage in comparator mode

ravi kumar89
Prodigy 80 points
Other Parts Discussed in Thread: TL431, ATL431

Hi,


we are using TL431 in comparator mode. It is only mentioned that VKA is ~2V.

1. I would like to know the contributing factors of VKA variation.(in comparator mode with Vref>2.5V)

2. when the Vref > 2.5, the NPN transistor conducts and draws more current from the Vref pin. How is this going to impact the output voltage?

  • 0
    TI__Mastermind 24485 points
    Hello Ravi,

    Will get back to you soon.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    TI__Mastermind 24485 points
    Hello Ravi,

    1. The output voltage is typically in the range of 1.8 to 2.2V. As the reference current increases and the cathode current increases, the output voltage will increase, but not significantly so. The temperature has more of an effect on the output voltage. As the temperature decreases the output voltage typically increases.

    2. You are correct. In order to control this there should be a resistance into the reference input because the reference current increases when the input attempts to exceed 2.5V.
    The forward bias from the reference to cathode and the cathode circuit headroom set the cathode voltage.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 80 points

    Hi Michael,

    Thanks for the information. I have few more questions on the same topic.


    1. You have mentioned that VKA will vary from 1.8V to 2.2V. Is this only from Temperature contribution? i.e. 1.8V @ 125C and 2.2V at -40C? Can you provide a graph regarding the same. I would also need the absolute Max for this VKA at any temperature.

    2. I would like to understand how the VKA will vary with reference voltage. Here i am providing my circuit details. Application is: If the input voltage(V1) is higher(>17V approx), the MOSFET VGS to be pulled Low. i.e. < 2.1V(VGS threshold).

    In this circuit, at V1=18V,  183uA will flow through R4. If 166uA sinks through R5, a n approx 17uA will flow through the Vref of TL431. 

    With VKA at ~2V, VGS_FET will be 1.76V. This is enough to turn OFF my NMOS.

    In this circuit, i would like to understand the VKA variation with higher I reference current.

    Thanks for your explanations

    Regards

    Ravi kumar

  • 0 in reply to ravi kumar89
    TI__Mastermind 24485 points
    Ravi,

    I will get into the lab hopefully tomorrow to further characterize this as we currently do not have any data specifically for this device.
    In general, we expect that it will be between 1.8V and 2.2V across temperature.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    TI__Mastermind 24485 points

    Hello Ravi,

    I have attached data that I have taken for the TL431 across cathode currents for different temperatures and reference currents.

    I hope this helps. It looks like for 10uA Iref and 125C, the output voltage exceeds the estimated 2.2V stated before. Please note that the original estimate was based on information that we had for another reference.

    The attached data is accurate for the device which you are talking about. Note that the higher curve for each temperature corresponds to Iref=10uA.

    /cfs-file/__key/communityserver-discussions-components-files/196/8371.TL431ComparatorData.pdf

    Is this going to be acceptable for the application?

    If not, I can gather the same data for the ATL431 which I expect to perform better, but would have to take additional data to verify.

    Best,
    Michael