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LM3481SEPICEVAL: Adding a feedforward capacitor in feedback loop

Kevin Schmidgall
Expert 3860 points
Part Number: LM3481SEPICEVAL

Howdy,

Is there any reason you could think of where putting a Cff would cause problems on this EVM? I have an engineer trying to improve the transient response of the EVM by adding a Cff and, in their own words, it is causing "a very squirrely response" on the output. Even with a 100pF Cff it was having problems. With 100pF Cff and Rfb being 59k I'd think it would be putting a zero far enough out to not really be doing anything but wanted another opinion.

Regards,

Kevin

  • 0
    TI__Guru* 78060 points
    The FB is a high impedance pin, and the switching noise on the Vout line will easily attack the FB through the Cff coupling. If you want to add Cff, please also remember to add a series resistor with it to filter the bad noise out. This series resistor introduce a pole for low pass filtering.
  • 0 in reply to Youhao Xi
    TI__Expert 3860 points
    Sorry for taking a while to get back to this.

    By adding that series Rff to Cff where does that place the extra pole and zero? SLVA466A Gives some equations for the pole and zero added with just Cff but how do those change with the added resistor? Is the pole you mention an additional pole, or is the same as the pole that gets added by Cff but with a different equation?


    Thanks,
    Kevin