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TL431-Q1: TL431-Q1 Technical Consultation

Vayne Li
Intellectual 950 points
Part Number: TL431-Q1
Other Parts Discussed in Thread: TL431, TLA431

Tool/software:

Hi Teams,

The customer built a circuit according to the application circuit in the TL431 manual, as shown in the following figure,

They want to confirm three questions:

1. Is Iout in Formula  the same parameter as Io above the arrow?

2.If the input voltage VI (BATTA) is an unstable voltage value, such as varying between 9~32V, how should the resistance of R1 be determined? If the resistance values of R1 and RCL are both fixed, what is the output current of the circuit when the input voltage VI (BATT) varies between 9~32V? Is it constant?

3.Is this circuit a constant current source circuit? Or is it just a current limiter circuit? We want to use this application circuit to achieve a constant current output of 25mA. After reading some information, it is said that this is a constant current source circuit. The voltage between TL431 VREF and the anode pin is fixed, and a resistor can be connected in series between these two pins to output a constant current. For example, the RCL resistance is 100 Ω, Iout is 25mA, and according to the formula , R1 is 100k Ω when the input voltage VI (BATT) is 24V. When the input voltage VI (BATT) changes (between 9~32V), will the Iout current be affected because the R1 resistance is fixed?

  • 0
    TI__Genius 9640 points

    Hi Vayne, 

    Thanks for your question.

    Yes, Iout is the same as Io in the above illustration. 

    I would like to share one of our App note, Precision Current Sources and Sinks Using Voltage References, with you. Please find the link here: https://dms-prod.itg.ti.com/stage/lit/an/snoaa46/snoaa46.pdf

    If you check section 3.1 you can find a clear illustration.

    I would like to highlight Iout is depends on current setting resistor, Rs, and cathode current, Ika, 

    • Ika varies depending on the VBAT value. So you need the smallest Ika, which means you pick R1 to give you the smallest IKA at the worst case which is 9V.

    It's good to keep in mind to have it as low as possible for 9V but give it some margin due to temperature variation. You can pick a lower Ika device, like TLA431. 

    Hope this helps,

    Sila Atalar