AN2115 datasheet(19/33 Pages) STMICROELECTRONICS

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AN2115

Application information

Doc ID 11165 Rev 5

19/33

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 minimum 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. The list below

provides some capacitor manufacturers.

6.1.3

Inductor

The inductor value establishes the ripple current flowing through the output capacitor. The

ripple current is usually fixed at 20% - 40% of the output current, an approximation of which

is obtained with the following formula:

Equation 13

For example, with Vout = 3.3 V, Vin = 4.2 V (Li-Ion battery fully charged), FSW = 1.4 MHz, IO

= 600 mA and

ΔI = 200 mA, the inductor value is about 3.3 µH. The peak current through the

inductor is given by:

Equation 14

Note that if the inductor value decreases, the peak current (which must be lower than the

current limit of the device) increases. This peak current must be lower than the saturation

current of the choke.

This is particularly important when using ferrite cores because they can saturate severely.

The inductance value decreases abruptly when the saturation threshold is exceeded, thus

causing an abrupt increase in the current flowing through it.

The inductor should be selected with system stability taken into consideration (see the

paragraph regarding slope compensation).

Parasitic resistance should also be considered when selecting the inductor, as too high a

value can decrease efficiency. In the following table some inductor manufacturers are listed.

Table 4.

Recommended output capacitors

Manufacturer

Series

Cap value (µF)

Rated voltage (V)

ESR (m

Ω)

Panasonic

ECJ

10 to 47

6.3

Panasonic

EEF

22 to 47

6.3

60 to 90

Taiyo Yuden

JMK

10 to 47

6.3

Sanyo POSCAP

TPA

47 to 100

6.3

80 to 100

Murata

GRM

10 to 22

6.3

5 to 10

out

–

()

ΔI

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

•

ΔI

-----

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