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TPS62821: Feed forward value

Pjs
Intellectual 880 points
Part Number: TPS62821

Hi, 

I'm using a potential divider that has R2=10k. The feed forward capacitor equation (5) in the datasheet suggests that this would require a feed forward capacitor of 1200pF. In the past, I've only seen feedforward capacitor values in the low 100pF ranges so this seems quite large - is 1200pF a sensible value to use here?

Is there a feature in Webench where I can specify my potential divider and have it re-calculate surrounding components such as the feed forward capacitor? If I update the resistor divider, then nothing else updates to reflect these changes.

Many thanks

  • +1
    TI__Mastermind 32361 points

    Hi Philip,

    As a rule of thumb, by decreasing the lower FB resistor (R2) by 10x, the feedforward capacitor (Cff) should be increased by 10x to maintain the same loop response. Hence, a combination of R2 = 10k and Cff = 1.2nF is fine.

    Webench is not capable of adjusting the other passive components automatically (R1 - upper FB resistor; Cff) when R2 is changed to 10k. Instead, all the component values (R1, R2, Cff) need to be modified and re-stimulated the circuit.

    I hope I was able to answer your queries.

    Best regards,

    Excel

  • 0 in reply to Excel Regidor
    Intellectual 880 points

    Thanks very much Excel - most helpful :).