VOUT=0.85∙ 1+R1R2

(28)

CR1 capacitor is sometimes useful to increase the small signal phase margin (please refer to the section Closing

the loop)

8.4

Design of the power components

8.4.1

Input capacitor selection

The input capacitor voltage rating must be higher than the maximum input operating voltage of the application.

During the switching activity a pulsed current flows into the input capacitor and so, its RMS current capability must

be selected accordingly with the application conditions. Internal losses of the input filter depend on the ESR value,

so usually low ESR capacitors (like multilayer ceramic capacitors) have higher RMS current capability. On the

other hand, given the RMS current value, lower ESR input filter has lower losses and so contributes to higher

conversion efficiency.

The maximum RMS input current flowing through the capacitor can be calculated as:

IRMS=IOUT∙ 1−Dη ∙Dη

(29)

duty cycles has to be considered in order to find out the maximum RMS input current. The maximum and

minimum duty cycles can be calculated as:

DMAX= VOUT+∆VLOWSIDE

VINmin+∆VLOWSIDE−∆VHIGHSIDE

(30)

Dmin= VOUT+∆VLOWSIDE

VINMAX+∆VLOWSIDE−∆VHIGHSIDE

(31)

voltage across the input filter can be calculated as:

VPP= IOUT

CIN∙FSW∙ 1−Dη ∙Dη+ESR∙ IOUT+∆IL

(32)

follows:

CIN= IOUT

VPP∙FSW∙ 1−Dη ∙Dη

(33)

Considering η = 1 this function has its maximum in D = 0.5:

L6982

Design of the power components

DS13683 - Rev 1

page 26/48