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Stability of LM2940 LDO with Output Inductor

Seokkwon Kim
Prodigy 20 points

Hi.

As described in the datasheet, output capacitor at least 22uF is necessary for stability.

If an inductor is inserted between LM2940 and output capacitor, I wonder that the condition for output capacitance and ESR changes or not.  

Thank you.

  • 0
    TI__Mastermind 22610 points

    If an inductor is inserted between the LM2940 output and the output capacitor the question is no longer about esr, the question is about control loop stability.

    Since cout and esr are required to give adequate phase margin to the control loop, inserting any inductor in series, with it's competing phase shift and impedance, will NOT be useful for maintaining control loop stability.

     (edited)

  • 0 in reply to Donald Jones
    Prodigy 20 points

    Thank you for your reply.

    Do you mean that the LM2940 may not be stable when an inductor is inserted between the LM2940 output and the output capacitor (with proper capacitance and esr) ?

    Could I know how to check stability for this situation?

    If pspice simulation is possible, please let me know.

  • 0 in reply to Seokkwon Kim
    TI__Mastermind 22610 points

    " ... the LM2940 may not be stable when an inductor is inserted between the LM2940 output and the output capacitor ..."

    Yes.

    "... If pspice simulation is possible ..."

    I'm not aware of any pspice model from TI for an LDO that will include the capability for accurate phase margin simulation. The standard TI LDO pspice models are behavioral.

    "... how to check stability for this situation?.."

    Build the circuit and test it. Load step transient response is the simplest way to check LDO control loop stability. Two related application notes that describe this are :

    Simplifying Stability Checks ...

    http://www.ti.com/lit/an/slva381b/slva381b.pdf

    ESR, Stability, and the LDO Regulator ...

    http://www.ti.com/lit/an/slva115/slva115.pdf