R5973AD datasheet(21/38 Pages) STMICROELECTRONICS

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R5973AD

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.

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, 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 has to 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

 is the expected system efficiency, D is the duty cycle and IO 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  = 1). The maximum and minimum duty cycles are:

Equation 18

Figure 12. Application schematic

IRMS





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

–



-------



DMAX

VOUT VF

VINMIN VSW

–

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