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LMR23630-Q1: About feed-forward cap setting

Charles Jeon
Expert 2975 points
Part Number: LMR23630-Q1

My customer would like to apply the LMR23630-Q1 and modify only the Rfbt value to change the output.

Vin : 25V    /    Vout : 5V or 3.3V(Modify the Rfbt)    /    Iout : 3A

Cout and Lout proposed calculating the minimum value in the datasheet and applying an integer higher than that value.

But I do not know how to suggest Cff.

Datasheet is shown in "CFF is not needed when COUT has very high ESR and CFF calculated from Equation 19 must reduced with medium ESR.".

If I use a high ESR alu-cap, do I not need CFF? 

If I use a low ESR Ceramic CAP, do I must need CFF? 

If I use a small CAP with ESR and do not apply Cff, what kind of problems will I encounter?

  • 0
    TI__Expert 4100 points
    Hello Charles,
    Feed forward capacitor is used to optimize system transient response and in essence to provide phase boost.
    In general it is good practice to leave place holder on PCB for feed forward capacitor in case transient response need to be optimized. Device datasheet provides description of selection criteria for Cff. It could be assumed that CFF should be selected such that the bandwidth of the control loop without the CFF is centered between fZ-CFF and fP-CFF. The zero fZ-CFF adds phase boost at the crossover frequency and improves transient response. The pole fP-CFF helps maintaining proper gain margin at frequency beyond the crossover. Device datasheet provides equations for calculating these values.
    Regards
    Brani