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TPS56121 and TPS56221 input capacitor selection equation in datasheet

K. Sasaki14
Guru 10200 points
Other Parts Discussed in Thread: TPS56121, TPS56221

The datasheet of TPS56121 and TPS56221 has the input capacitor calculation equation on page 17. It is equation (11). However the equation is not same between TPS56121 and TPS56221. Is Either equation of TPS56121 or TPS56221 wrong? If it is so, please let me know the correct equation.

Best Regards,

Kohei Sasaki

  • 0
    TI__Guru**** 191445 points
    Those are both approximations for the required Cin. It is all based on the simple equation I = C dV/dt, rearranged as
    C = I dt/dV. TPS56221 has a simple approximation that assumes all the ripple current is supplied by the input capacitor during the on time of the high side FET. In that case, Cin = (Iout * D) / (Vripple * Fsw), remember the dt term can be represented by D / Fsw, where D is the duty cycle (Vout/Vin). So substituting Vout/Vin for D gives Eq.11 for TPS56221.

    TPS56121 uses a more detailed model. It assumes that the input supplies the average current ( Iout *D) and that the input capacitor supplies only the additional current (Iout - Iout *D). Now the new equation is Cin = (Iout * (1-D) * D)/ (Vripple * Fsw). Substituting Vout/Vin for D, you get the equation from the TPS56121 datasheet.

    In real life, some of the ripple current is supplied by the input voltage source, but I use the TPS56121 equation personally.