Application information

AN1882

16/30

Doc ID 10209 Rev 4

5

Application information

5.1

External components selection

5.1.1

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, neglecting the ripple across the inductor.

The RMS input current (flowing through the input capacitor) is:

Equation 10

Where

η is the expected system efficiency, D is the duty cycle and Io the output DC current.

Supposing

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

The maximum and minimum duty cycles are:

Equation 11

Equation 12

Depending on the output voltage value the worst case can be with the maximum or

minimum input battery voltage. Usually the best choice for the input capacitor is the MLCC

(multilayer ceramic capacitor) thanks to its very small size and very low ESR.

Table 4

provides a list of some MLCC manufacturers.

5.1.2

Output capacitor

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

small inductors values reduce size and cost of the application but increase the current

ripple. This ripple, multiplied by the ESR of the output capacitor, is the output voltage ripple.

Tantalum and ceramic capacitors are usually good for this purpose. Ceramic capacitors

have the lowest ESR for a given size, so for very compact applications they are the best

choice. POSCAP capacitors from Sanyo are also a good choice for the output filter. Below is

a list of some capacitor manufacturers.

Table 4.

Recommended input capacitors

Manufacturer

Series

Cap value (µF)

Rated voltage (V)

ESR@600 kHz (m.)

Panasonic

ECJ

10 to 22

6.3

10

TAIYO YUDEN

JMK

10 to 22

6.3

10

I

RMS

I

O

D

2D

2

⋅

η

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

–

D

2

η

2

-------

+

⋅

=

D

MAX

V

O

V

INMIN

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

=

D

MIN

V

O

V

INMAX

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

=