Техническая Спецификация для STMicroelectronics Evaluation board for A5970AD EVALA5970AD EVALA5970AD

DocID14661 Rev 5

21/42

42



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:

Where 



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

 is the output DC 

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

O

 divided by 2 (considering 

 = 1). The maximum and minimum duty cycles are:

and

Where V

 is the freewheeling diode forward voltage and V

 the voltage drop across the 

internal PDMOS. Considering the range D

MIN

 to D

MAX

, it is possible to determine the max. 

IRMS going through the input capacitor. 

I

RMS

I

O

D

2 D

2





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

–

D

2



2

-------

+



=

D

MAX

V

OUT

V

F

+

V

INMIN

V

SW

–

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

=

D

MIN

V

OUT

V

F

+

V

INMAX

V

SW

–

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

=