AN2115 datasheet(18/33 Pages) STMICROELECTRONICS

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Application information

AN2115

18/33

Doc ID 11165 Rev 5

Application information

6.1

External component selection

6.1.1

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.

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 I

o is the output DC

current. Assuming

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

equivalent RMS current is equal to Io/2. The maximum and minimum duty cycles are:

Equation 11

Equation 12

Depending on the output voltage value, the worst case could occur when the input battery is

nearly discharged. Usually the best choice for the input capacitor is the MLCC (multi layer

ceramic capacitor) thanks to its very small size and very low ESR. Table 3 provides a list of

some MLCC manufacturers.

6.1.2

Output capacitor

The output capacitor is very important for satisfying the output voltage ripple requirement.

Very small inductor values reduce the size and cost of the application but increase the

current ripple.

Table 3.

Recommended input capacitors

Manufacturer

Series

Cap value (µF)

Rated voltage (V)

ESR@1.4 MHz

Ω)

Panasonic

ECJ

10 to 22

6.3

Taiyo Yuden

JMK

10 to 22

6.3

Murata

GRM

10 to 22

6.3

5 to 10

RMS

•

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

–

-------

•

MAX

inMIN

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

MIN

inMAX

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

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