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What's the function of the Diode D3 in the internal circuitry of TL431 ?

Jankel
Intellectual 731 points
Other Parts Discussed in Thread: TL431, ATL431

The  internal circuitry of TL431 is as follows.

The picture is from the book "Switch-Mode Power Supplies, SPICE Simulations and Practical Designs, Christophe P. Basso", Page 327

Thanks very much

Jankel Cao

  • 0
    TI__Expert 7395 points
    Jankel,
    I've contacted the product group here. I believe the highlighted refers to D2. Are you asking about D2 or D3?
    In general, you may also want to look into the much improved version of this device the ATL431.
    Regards
    John
  • 0 in reply to John Stevens
    Intellectual 731 points

    Hi, John,
    Very nice to receive your reply.

    oh, i'am sorry, the highlighted is only my note, it does nothing with this topic. Effective information for this topic is only the schematic in the picture.
    I guess that D3 plays a protective effect of proventing the 'BE' junction of Q1 from reverse breakdown,

    and i guess D3 may be a schottky diode ?

    Best Regards.

    Jankel

  • 0
    TI__Mastermind 22610 points
    Since this is a generic behavioral model that does not represent the actual TL431 internals, and D3 appears to be 'ideal' diode other than 'N=0.01' (Emission coefficient), I would guess that D3 is strictly to promote convergence by providing a current path to clamp G1 output at start-up when the Ref input is less than V1 (2.49V).

    This question would probably be better directed to the author and/or publisher of figure 3B-1.