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To calculate the input capacitor for buck converter, I follow TI Application Report SLTA055–FEBRUARY 2006 "Input and Output Capacitor Selection".

It says to determine the input capacitor "75 mVpp is recommended for maximum input voltage ripple. This will yield approximately 22 mVrms of ripple voltage"

The question is how we select this value 75 mVpp?

thanks

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  • \$\begingroup\$ Depends on your input current, switching frequency and inductor, as well as input impedance. \$\endgroup\$
    – winny
    Oct 20 '20 at 18:34
  • \$\begingroup\$ how can input current, switching frequency and inductor, as well as input impedance effect on selection of input voltage ripple?? I am looking for a formula or rule to select this \$\endgroup\$
    – Nakh0d4
    Oct 20 '20 at 18:37
  • \$\begingroup\$ Said formula will be dependent on all above physical quantities, as well as ESR and ESL of your capacitor. \$\endgroup\$
    – winny
    Oct 20 '20 at 19:07
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From section 1.1

As a general rule of thumb, keeping the peak to peak ripple amplitude below 75 mV keeps the rms currents in the bulk capacitors within acceptable limits.

Emphasis mine

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