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TL431: regulator with MOSFET

Ann_Lien
Genius 12500 points
Part Number: TL431

Hello expert,

My customer would like to use TL431 as regulator with MOSFET. We have two questions for it,

  1. From datasheet, regulator with BJT is shown in figure 34. But I would like to know if we have to change anything if we want to use MOSFET to replace BJT?
  2. From Figure 34, it mention Rb should provide cathode current >= 1mA to the TL431. May I know how can I calculate this current?

Best regards,

Ann Lien

  • 0
    TI__Mastermind 19550 points

    Hi Ann,

    1. The main reason it works with BJT is because it only needs a VBE voltage drop to function properly. This is possible on a MOSFET but you would need a voltage higher than VI(batt) for NMOS to work properly.

    2. For this configuration IKA = (Vi(batt) - Vka) / Rb. Typically Vka in this configuration is 0.7V higher than Vout.

  • 0 in reply to Marcoo Z
    TI__Genius 12500 points

    Hello Marcoo,

    Thanks for your checking! For answer 1, I will send the mail to you for customer's schematic and could you kindly help to check if it is okay for it? Since they didn't use voltage level higher than Vi(batt) and they just change BJT to NMOSFET. Thanks a lot!

    Best regards,

    Ann Lien