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LM25117-Q1: The crossover frequency setting range

Kai Lai
Expert 5836 points
Part Number: LM25117-Q1

Hi Team,

My customer are designing LM25117-Q1 & LM25119-Q1, We want to know is there have any theory that we usually selected between one twentieth and one fifth of the Fsw ?? (Source from LM25117-Q1 datasheet page.23)
Why is larger than 5% Fsw ? 

I know larger than 20% Fsw will cause more unstable and too much noise on the Vout.

And does this "usually" selection crossover freq. between 5%~20% Fsw also apply to LM25119-Q1 ?

You can feedback me by mail kai.lai@ti.com directly, if you need more information from me! Thanks!

  • 0
    TI__Mastermind 49597 points

    Hi Kai,

    Thanks for the question. 5-20% is a general guideline in terms of loop crossover frequency for most buck controllers, not just the parts you mentioned. Not too high to approach the Nyquist frequency (Fsw/2) and not too low to be super slow.

    While there is no theoretical minimum, consider the LC input filter to manage EMI and input voltage transients. The loop manages such transients up to approximately the crossover frequency where the gain rolls off, and then the LC filter provides attenuation. So if the crossover frequency is super low, it's possible some input variation/disturbance gets through to the output at frequencies below the LC filter cutoff.

    Regards,

    Tim