A6984 datasheet(35/47 Pages) STMICROELECTRONICS

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A6984

Design of the power components

5.1

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 a 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:

Equation 25

Where IO is the maximum DC output current, D is the duty cycles, is the efficiency. This

function has a maximum at D = 0.5 and, considering

= 1, it is equal to I

O/2.

In a specific application the range of possible 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:

Equation 26

and

Equation 27

Where

V

HIGH_SIDE and VLOW_SIDE are the voltage drops across the embedded switches.

The peak-to-peak voltage across the input filter can be calculated as:

Equation 28

In case of negligible ESR (MLCC capacitor) the equation of CIN as a function of the target

VPP can be written as follows:

Equation 29

RMS





---------------

–



-------



MAX

OUT

V

LOW_SIDE

INMIN

V

LOW_SIDE

V

HIGH_SIDE

–

--------------------------------------------------------------------------------------------------

MIN

OUT

V

LOW_SIDE

INMAX

V

LOW_SIDE

V

HIGH_SIDE

–

----------------------------------------------------------------------------------------------------



-----------------------



----

–









----

–





ESR I



-------------------------



----

–









----

–





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