L6986 datasheet(49/68 Pages) STMICROELECTRONICS

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L6986

Application notes

6.6

Design of the power components

6.6.1

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 depends 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:

Equation 44

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 Io/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 45

Equation 46

Where

VHIGH_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 47

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

VPP can be written as follows:

Equation 48

Considering

this function has its maximum in D = 0.5:

IRMS

IOUT



----

–



 D



----



DMAX

VOUT VLOWSIDE

VINMIN VLOWSIDE VHIGHSIDE

–

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

DMIN

VOUT VLOWSIDE

VINMAX VLOWSIDE VHIGHSIDE

–

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

VPP

IOUT

CIN fSW



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



----

–



 D



----



ESR

IOUT IL





CIN

IOUT

VPP fSW



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



----

–



 D



----



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