R6986TR datasheet(49/62 Pages) STMICROELECTRONICS

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DocID026996 Rev 2

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R6986

Application notes

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

Equation 44

Where IOUT 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:

Equation 49

Typically CIN is dimensioned to keep the maximum peak-peak voltage across the input filter

in the order of 5% VIN_MAX.

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



----



CINMIN

IOUT

4VPPMAX fSW



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

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