A5974AD datasheet(24/49 Pages) STMICROELECTRONICS

24 / 49 page

Application information

A5974AD

24/49

Doc ID 018762 Rev 1

Application information

8.1

Component selection

●

Input capacitor

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

maximum RMS input current.

As 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, which 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 input current. The maximum RMS input current (flowing

through the input capacitor) is:

Equation 17

where

η is the expected system efficiency, D is the duty cycle and I

O is the output DC

current. This function reaches its maximum value at D = 0.5 and the equivalent RMS current

is equal to IO divided by 2 (considering η = 1). 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 max.

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

●

Electrolytic capacitors:

These are widely used due to their low price 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.

●

Ceramic capacitors:

If available for the required value and voltage rating, these capacitors usually have a higher

RMS current rating for a given physical dimension (due to very low ESR).

The drawback is the considerably high cost.

RMS

⋅

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

–

-------

⋅

MAX

OUT

INMIN

–

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

MIN

OUT

INMAX

–

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

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