Application information

ST1S40

14/30

DocID17928 Rev 6

6

Application information

6.1

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 14

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

is the efficiency.

Considering

, 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 15

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

written as follows:

Equation 16

neglecting the small ESR of ceramic capacitors.

Considering

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

Equation 17

I

RMS

I

O

D

2D

2





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

–

D

2



2

-------

+



=

V

PP

I

O

C

IN

F

SW



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

1

D



----

–





D

D



----

1D

–





+



ESR I

O



+



=

C

IN

I

O

V

PP

F

SW



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

1

D



----

–





D

D



----

1D

–





+





=

C

IN_MIN

I

O

2V

PP_MAX

F

SW



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

=