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TPS54218: Selecting ESR value when calculating fz

Narek
Prodigy 235 points
Part Number: TPS54218

One of the steps in designing the compensation circuit described in the datasheet is to calculate the zero frequency, fz, formed by Cout and its associated ESR.  Since the ESR of a ceramic capacitor is highly dependent upon the frequency, which value along the ESR curve should be selected?  For example, if we take the frequency point with the lowest ESR value along the ESR vs. Frequency curve and we look 1 to 2 orders of magnitude in frequency on either side, the ESR can increase by a factor of 5 to 10, which makes a huge difference in fz.

  • 0
    TI__Guru**** 191445 points

    Narek,

    That procedure is using a simplified model of the capacitor consisting of an ideal capacitor in series with an ideal resistor. For that model the impedance decreases linearly with frequency until f = 1 / 2*pi*R*C, where C is the capacitance and R is the ESR.  In a real capacitor as you  noted, after the resonant frequency the capacitor becomes inductive and the impedance increases again.  We use that minimum value of impedance in our calculations, or sometimes just estimate it at 3 mohm.