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TL431 Current question

Derek Xie
Genius 12255 points
Other Parts Discussed in Thread: TL431

Hi Team:

My customer is using TL431 in their application to avoid high voltage.

Please see the schemacti below:

The question is that if they remove the TL431, the idle current 85mA.

If they add TL431, the idle current will ve 146mA+-3mA.

The question is that why add TL431 will have around 60mA current increase.

Other question is that why the current is not stable, +-3mA is normal current difference, sometimes it will become 10mA difference.

Could you also let me know what's relationship between the Ika and Vka? How to calculate it? Thanks.

  • 0
    TI__Mastermind 24485 points
    Hello Derek,

    Can you help me to understand what "idle" currents you are talking about?
    Case 1 without the TL431: What is connected? Are you just connecting the 5V directly to the linear regulator?
    Case 2 with the TL431:
    If I look at just the TL431 alone, I see that you are setting the cathode voltage to 3.75 based on the 5k, 10k resistor divider.
    The TL431 will fix 2.5V at the reference pin if the current into the cathode is >1mA and <100mA
    Given that the cathode voltage is being set to 3.75, and it looks like the resistance between your 5V and 3.75V is 13.7||13.7 =~ 7ohms, you are actually pulling somewhere close to 180mA through those resistors. Depending on the loading from the regulator, this may be too much current for the TL431.

    If the resistance between the 5V and the 3.75V is actually just 13.7 Ohms then you should have somewhere close to 91mA. The TL431 will be able to maintain the 3.75V at the cathode as long as the linear regulator does not draw more than 90mA, making sure that the TL431 has at least 1mA to set the reference voltage.

    In general, the TL431 is going to sink whatever current is available to fix the cathode voltage to 3.75V.
    Does this make sense?

    Best,
    Michael