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LM431: Vout oscillation

Jason Fan
Intellectual 1705 points
Part Number: LM431
Other Parts Discussed in Thread: TLV431A, TL431,

Hi Experts:

i designed a reference voltage for the signal detection with LM431B , the schematic is below: 

 

Because the Cload selection could affect the Vout oscillation , i choose the 1nF for the Vout  (remove the 1uF ) , however, i still find the Vout oscillation like below: 

In the description of the datasheet, the Vout is in the stable range: 

When i replace the LM431B with the TLV431A in the same circuit , the Vout is stable , no oscillation . Please help me what's the reason , thanks!

  • 0
    TI__Mastermind 19370 points

    Hi Jason,

    The stability boundary conditions chart is a typical graph that can vary from device to device and lot to lot.

    I cannot recommend to use 1.001uF on the output because it will be in the instability region of both device.

    A much smaller much larger cap is recommended. I would check out the app note below to understand stability charts.

    While this TL431 might not oscillate, other TL431 can oscillate.

  • 0 in reply to Marcoo Z
    TI__Intellectual 1705 points

    Hi  Marcoo:

    Thanks for your reply.

    Yes, we are currently using the 1nF in the TL431 with no issue .  However, i don't know why the oscillation is still existing on the LM431 ? 

    if this TL431 oscillate and other TL431 can't oscillate , how can we make sure the capacitor is Ok in the mass production stage even it could pass in the evaluation stage.

  • 0 in reply to Jason Fan
    TI__Mastermind 19370 points

    Hi Jason,

    Can you verify if it is 1nF or 1uF total cap?