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LM5010A ripple rejection

Atsushi Yamauchi
Genius 4810 points

I would like to confirm the operation of the ripple rejection in the datasheet Figure 17.

1. Does the ripple at FB point increase by the increase of output ripple?

2. Can we use the ceramic capacitor of output capacitior for Figure 17 (C2)?

Best regards,

Atsushi Yamauchi

  • 0
    TI__Intellectual 2320 points

    Hello Yamauchi-san,

    The ripple injection circuit in Figure 17 is a simple integrator circuit. While the RA, CA is the RC network which creates the required ramp voltage, the capacitor CB ac couples it at the FB pin. The ramp voltage only depends on the VIN voltage.

    If the capacitors are sized properly, the ripple at the FB pin remains unchanged and does not change with any increase in the output voltage ripple. However, if the CB, and CA are not selected properly (If CB and CA are too big leading to much reduced impedance), the ripple at the FB pin may get coupled to the output node. Please see the attachment to select the correct values of the RA,CA and CB.

    Ripple Injection Circuit Summary.docx

    You can definitely use a ceramic capacitor for the output capacitor. The ripple injection circuit will take care of the required ripple needed at the FB pin. There is no need for an extra ESR or/and electrolytic capacitor in this case.

    Regards,

    Sourav