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TPS54824: Output Capacitor Selection

Hirotsugu Kobayashi
Mastermind 9540 points
Part Number: TPS54824

Hi team,

I am considering making TPS54824 from 12V to 1.35V at 700kHz.

I want to use 220uF MLCC for the output capacitor, but the resonance frequency is about 300kHz.

Is it possible to use an MLCC whose resonant frequency is lower than the switching frequency?

The capacitor we plan to use is GRM31CR60J227ME11L.

https://www.murata.com/en-global/products/productdetail?partno=GRM31CR60J227ME11%23

Best Regards,

  • 0
    TI__Mastermind 35185 points

    Hi Kobayashi-san,

    There is no issue using an output cap whose resonance frequency is lower than the switching frequency. I have used similar 220uF ceramic caps with this part before. When using these 220uF ceramic caps, the one thing I have found to be careful of is to keep the loop crossover frequency well below the resonance frequency of the cap. If the crossover frequency is too close to the resonance frequency of the cap, it can result in low gain margin in the loop. That should not be an issue here as the crossover frequency should be <100 kHz.

    As these caps will be inductive at the switching frequency, I would recommend including some smaller valued capacitors with a resonance frequency above the switching to help reduce the output ripple.

    Anthony

  • 0 in reply to Anthony F
    Mastermind 9540 points

    Hi Anthony,

    Should the ESR value used in the ripple calculation be the switching frequency value?
    A D-CAP2 topology product has a crossover frequency equivalent to several hundreds of kHz. Is it unstable?

    Thanks for the advice, I'll use it in combination with a capacitor with a small capacitance.

    Best Regards,

  • 0 in reply to Hirotsugu Kobayashi
    TI__Mastermind 35185 points

    For the equivalent ESR, I recommend using the impedance of the capacitor at its resonance frequency. Below the resonance frequency it is mostly capacitive and above the resonance frequency it is mostly inductive.

    It may or may not be unstable with several hundreds of kHz crossover frequency. It's just something to be careful of as it can cause the loop to have low stability margin. If there are some small capacitors in parallel it would help avoid the risk of instability as they would reduce the total impedance of the output caps near the crossover.