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TPS61022: Phase margin and Cff connect or not

Satoshi
Guru 19645 points
Part Number: TPS61022

Measured FRA data for TPS61022 is attached below,

Phase margin is 0°or less on low frequency range, is this problem for unstable and should improve?

When add Cff: 100pF (referenced by Cff equation), phase margin is increase(improve) but input current become large, I guess that the same pattern for below thread.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1108222/tps61022-input-current-abnormal/4112132

If possible I want not Cff, or change small Cff value (lower than correct Cff equation result) for reduce input current.

I want to judge above points: problem or not, and Cff need to add or not need. 

Best regards,

Satoshi

  • 0
    TI__Genius 12505 points

    Hi Satoshi,

    By definition, PM is the phase difference between 180 and the phase at Gain=0dB. And more, I don't think phase < 0 on low frequency range is a problem for unstable. PM >= 45 and GM >= 10 is enough, so Cff is not needed according to your bode plot.

    Best regards,

    Nathan

  • 0 in reply to Nathan Ding
    Guru 19645 points

    Hi Nathan,

    Thank you for reply,

    I think although there have a limit, phase become <0° and not reach -180° on low frequency range will be stable, correct?   

    Is there any reference document?

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Guru 65470 points

    Hello Satoshi,

    What exactly are you measuring? Phase or phase margin? You could check if there is an issue if you apply the frequency where you see a value below 0 to the feedback node and check the output voltage.

    Best regards,
    Brigitte