ST1S14 datasheet(22/46 Pages) STMICROELECTRONICS

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Application information

ST1S14

22/46

DocID17977 Rev 2

Application information

7.1

Component selection

7.1.1

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 has

to 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 17

where

ηis the expected system efficiency, D is the duty cycle, and IO is the output DC

current. Considering

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

equivalent RMS current is equal to IO divided by 2. The maximum and minimum duty cycles

are:

Equation 18

and

Equation 19

Where VF is the freewheeling diode forward voltage and VSW the voltage drop across the

internal PDMOS. Considering the range DMIN to DMAX, it is possible to determine the

maximum IRMS going through the input capacitor. Capacitors that may be considered are:

Electrolytic capacitors:

These are widely used due to their low cost and their availability in a wide range of

RMS current ratings.

The only drawback is that, considering ripple current rating requirements, they are

physically larger than other capacitors.

RMS

⋅

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

–

-------

⋅

MAX

OUT

INMIN

–

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

MIN

OUT

INMAX

–

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

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