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LM53603-Q1: Phase Margin

Koji Hayashi
Intellectual 955 points
Part Number: LM53603-Q1

Hi All,

TI's recommended phase margin is 45 degrees.
Is my understanding of this correct?
Does this take into consideration variations of external parts, IC and temperature?

The evaluation result at low temperature was less than 45 degrees.
Is it necessary to change the constant of the external device?


Best Regards
Koji Hayashi

  • 0
    TI__Guru**** 191445 points
    While I do not support LM53603-Q1 directly, I can make a general comment. The general rule of thumb for loop stability is 45 degrees of phase margin. Margin is in fact "margin", so you can generally get by with slightly less. I always pay close attention to the slope of the phase curve near crossover. If it id decreasing very rapidly then more phase margin is better. If it is very flat or in some cases increasing near crossover, then I am generally comfortable with a little less margin. You should always test to cover your expected operating condition. If you are uncomfortable with your low temperature phase margin, it may be possible to adjust it with a feed forward capacitor.
  • 0 in reply to JohnTucker
    Intellectual 955 points
    Hi John

    Thank you for your quick response.

    Does the phase margin of 45 degrees include "margin" due to variations in IC and external elements?

    In Sim, you can check with variations taken into account. However, it is difficult to confirm by evaluation.
  • 0 in reply to Koji Hayashi
    TI__Guru**** 191445 points
    Yes that is the purpose of margin, to ensure stability over operating conditions and component variation.
  • 0 in reply to JohnTucker
    Intellectual 955 points
    Hi John

    Could you provide me with the detailed break down of phase margin ?

    For example, 10 degrees in operating conditions, 5 degrees in component variation, 5 degrees in temperature, etc.

    Best Regards
    Koji Hayashi
  • 0 in reply to Koji Hayashi
    TI__Guru**** 191445 points
    I'm afraid it is not quite as simple as that. Basically you have two parts of the loop; the power stage with the output filter and the compensated error amplifier. These two combined form the complete loop. Each of the components that make up the circuit may have tolerances and temperature dependencies. In some cases these may be subtractive and cancel each other out. In other cases, they may be additive. I would suggest that you model your circuit in pspice. You can then vary the components individually or run Monte Carlo simulations to check the variability.
  • 0 in reply to JohnTucker
    Intellectual 955 points
    Hi John,

    Thank you for your quick response.

    I was understood.

    Best Regards
    Koji Hayashi