ST1S31 datasheet(16/35 Pages) STMICROELECTRONICS

16 / 35 page

Application information

ST1S31

16/35

DocID022998 Rev 4

Application information

5.1

Input capacitor selection

The capacitor connected to the input must be capable of supporting the maximum input

operating voltage and the maximum RMS input current required by the device. The input

capacitor is subject to a pulsed current, the RMS value of which is dissipated over its ESR,

affecting the overall system efficiency.

So the input capacitor must have an RMS current rating higher than the maximum RMS

input current and an ESR value compliant with the expected efficiency.

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

Equation 15

where Io is the maximum DC output current, D is the duty cycle, and

is the efficiency.

Considering

= 1, this function has a maximum at D = 0.5 and is equal to Io/2.

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

Equation 16

where ESR is the equivalent series resistance of the capacitor.

Given the physical dimension, ceramic capacitors can well meet the requirements of the

input filter sustaining a higher input RMS current than electrolytic / tantalum types. In this

case the equation of CIN as a function of the target peak-to-peak voltage ripple (VPP) can be

written as follows:

Equation 17

neglecting the small ESR of ceramic capacitors.

Considering

= 1, this function has its maximum in D = 0.5, therefore, given the maximum

peak-to-peak input voltage (VPP_MAX), the minimum input capacitor (CIN_MIN) value is:

Equation 18

Typically, CIN is dimensioned to keep the maximum peak-to-peak voltage ripple in the order

of 1% of VINMAX.

IRMS





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

–



-------



VPP

CIN FSW



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



----

–



 D D



----

–





ESR IO



CIN

VPP FSW



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



----

–



 D D



----

–





CIN_MIN

2VPP_MAX FSW



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

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