Application information

ST1CC40

22/37

DocID18279 Rev 5

7.1.3

Input capacitor

The input capacitor must be able to support the maximum input operating voltage and the

maximum RMS input current.

Since step-down converters draw current from the input in pulses, the input current is

squared and the height of each pulse is equal to the output current. The input capacitor

must absorb all this switching current, whose RMS value can be up to the load current

divided by two (worst case, with duty cycle of 50%). For this reason, the quality of these

capacitors must be very high to minimize the power dissipation generated by the internal

ESR, thereby improving system reliability and efficiency. The critical parameter is usually the

RMS current rating, which must be higher than the RMS current flowing through the

capacitor. The maximum RMS input current (flowing through the input capacitor) is:

Equation 24

where

current. Considering

 = 1, this function reaches its maximum value at D = 0.5 and the

are:

Equation 25

and

Equation 26

Table 6. Inductor selection

Manufacturer

Series

Inductor value (µH)

Saturation current (A)

Würth Elektronik

WE-HCI 7040

1 to 4.7

20 to 7

WE-HCI 7050

4.9 to 10

20 to 4.0

Coilcraft

XPL 7030

2.2 to 10

29 to 7.2

D

2D

2





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

–

D

2



2

-------

+



=

+

–

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

=

+

–

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

=