AN1330

Application information

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5

Application information

5.1

Component selection

●

Input capacitor

The input capacitor must be able to withstand 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

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

Equation 18

–

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.

–

Tantalum capacitors: Good, small tantalum capacitors with very low ESR are

becoming more available. However, they can occasionally burn if subjected to very

high current during charge. Therefore, it is better to avoid this type of capacitor for

the input filter of the device. They can, however, be subjected to high surge current

when connected to the power supply.

●

Output capacitor

The output capacitor is very important to meet the output voltage ripple requirement.

Using a small inductor value is useful to reduce the size of the choke but it increases the

current ripple. So, to reduce the output voltage ripple, a low ESR capacitor is required.

I

RMS

I

O

D

2D

2

•

η

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

–

D

2

η

-------

+

•

=

D

MAX

V

OUT

V

F

+

V

INMIN

V

SW

–

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

=

D

MIN

V

OUT

V

F

+

V

INMAX

V

SW

–

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

=

and