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TL431: Rsup resistance

Miranda Gu
Genius 16632 points
Part Number: TL431

Hi team,

My customer plan to use TL431 in 12V to 3.3V @50mA application. They use below calculator to determine external components. 

However, the calculated Rsup (R1) is quite large, generates about 0.45W power loss, at least using 1210 package.

My first question:  Can customer use smaller Rsup resistance? What's the minimum value of Rsup?

I tried to change IDiv/IRef from 200 to 10, but the calculator reported below error message:

My second question:  Why IDiv/IRef  should be greater than 100?

Look forward your answers, thanks!

 

  • 0
    TI__Mastermind 19580 points

    HI Miranda,

    The excel is a good starting point for the calculations.

    My first question:  Can customer use smaller Rsup resistance? What's the minimum value of Rsup?

    Rsup (R1) shown in the excel is the biggest value the system can have. It should not be any bigger than this but it can be smaller. A smaller R1 will cause the TL431 to sink more current. If your system can handle the power loss then it is acceptable. Keep in mind that the TL431 also has its own thermals.

    My second question:  Why IDiv/IRef  should be greater than 100?

    The excel has limitations in the way it creates designs due to it being in excel. It assumes the largest resistor R3 sets the limit in the design so it uses Idev/Iref to set the limits. I do not see the purpose of the IDEV/IREF scaling factor. I would get the ratio of R2/R3 and scale the resistors to the value you need for Iq purposes. The calculator is great for R1 but R2/R3 can be improved.

    What is the purpose of needing 50mA at 3.3V?

    How is this being used? Feel free to email me if its proprietary.