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TL431: Delay timer circuit

Muhammad Fariz Bin Nor Azhar
Intellectual 480 points
Part Number: TL431


Dear TI,

Currently I'm designing a delay circuit using TL431. I have some question regarding the reference circuit in the datasheet as below:

1. Can I know what is 'In' referring to? 

2. If I want to use 24V input, how do I recalculate the 650 Ohm and 2k Ohm resistor?

Thank you.

Best regards,

Fariz

  • 0
    TI__Genius 9375 points

    Hi Fariz,

    For your first question, although it looks like In (like a current value), this represents ln (natural log). This is derived from the generic RC time delay equation. For your second question, I'm not 100% on why the 650Ω and 2kΩ values were chosen. Let me get a product expert for the TL431's opinion on this, but I believe the 650Ω resistor is just used in conjunction with a standard LED while the second is used for intensity control.

    I will report back with more conclusive info on this hopefully by the end of this week.

    Thanks,

    Abhinav.

  • 0 in reply to Abhinav.Sharma
    Intellectual 480 points

    Dear Abhinav,

    Thank you for your feedback. Kindly please help to ask the product expert.

    Thank you.

    Best regards,

    Fariz

  • 0 in reply to Muhammad Fariz Bin Nor Azhar
    TI__Mastermind 19580 points

    Hi Fariz,

    As Abhinav mentioned, the In is the natural log symbol. The equation is based off of the discharging RC circuit equation.

    For this explanation

    R1 = 650 Ohm resistor

    R2 = 2k Ohm resistor

    R1 is chosen to provide current into the LED for light intensity. It needs to be chosen such that Ika(min) < ((Vs - 2V) / R1) < Ika(max). In TL431, Ika (min) = 1mA, Ika(max) = 100mA.

    R2 is used to provide early current while the LED is turning on. R2 = VLED / 1mA. VLED is the forward voltage drop across the LED

  • 0 in reply to Marcoo Z
    Intellectual 480 points

    Hi Marcoo Z,

    Thank you very much for the explanation. 

    Best regards,

    Fariz