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LM3481 Input Cap selection in Webench

Mark Guastaferro
Genius 11645 points
Other Parts Discussed in Thread: LM3481, TPS40210

How does Webench determine the value of the input cap for the LM3481 in a boost converter configuration?.

I noticed that as the switching frequency is reduced,  the input capacitance gets larger. At low freqeuncies, it is quite large. Data sheet refers to supporting the RMS input current, but only mentions nominal minimum values of Cin.

  • 0
    TI__Intellectual 805 points
    Hi Mark,

    The selection routine that Webench uses for the input capacitor is based off the definition of current through a capacitor I = C*dv/dt -> C = I*dt/dv. We expand on this by taking efficiency and input voltage ripple into account also. We can see that you are correct in your observation that the input capacitance is inversely proportional to the frequency. The TPS40210 is very similar to the LM3481 and it has a great section in the data sheet about selecting Cin that also shows this relationship.
    Regards
    -Jason