PM6670STR datasheet(42/53 Pages) STMICROELECTRONICS

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Application information

PM6670S

42/53

It must be taken into account that in some MLCC the capacitance decreases when the

operating voltage is near the rated voltage. In Table 14 some MLCC suitable for most of

applications are listed.

8.1.3

Output capacitor selection

Using tantalum or electrolytic capacitors, the selection is made referring to ESR and voltage

rating rather than by a specific capacitance value.

The output capacitor has to satisfy the output voltage ripple requirements. At a given

switching frequency, small inductor values are useful to reduce the size of the choke but

increase the inductor current ripple. Thus, to reduce the output voltage ripple a low ESR

capacitor is required.

To reduce jitter noise between different switching regulators in the system, it is preferable to

work with an output voltage ripple greater than 25 mV.

As far as it concerning the load transient requirements, the Equivalent Series Resistance

(ESR) of the output capacitor must satisfy the following relationship:

Equation 36

where VRIPPLE is the maximum tolerable ripple voltage.

In addition, the ESR must be high enough high to meet stability requirements. The output

capacitor zero must be lower than the switching frequency:

Equation 37

Table 14.

Evaluated MLCC for input filtering

Manufacturer

Series

Capacitance (µF) Rated voltage (V)

Maximum Irms

@100 kHz (A)

TAIYO YUDEN UMK325BJ106KM-T

TAIYO YUDEN

GMK316F106ZL-T

2.2

TAIYO YUDEN

GMK325F106ZH-T

2.2

TAIYO YUDEN

GMK325BJ106KN

2.5

TDK

C3225X5R1E106M

MAX

RIPPLE

ESR

∆

≤

out

ESR

⋅

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