PM6675AS datasheet(38/48 Pages) STMICROELECTRONICS

38 / 48 page

Application information

PM6675AS

38/48

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 concerns the load transient requirements, the Equivalent Series Resistance

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

Equation 37

where VRIPPLE is the maximum tolerable ripple voltage.

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

capacitor zero must be lower than the switching frequency:

Equation 38

If ceramic capacitors are used, the output voltage ripple due to inductor current ripple is

negligible; then the inductance could be smaller, reducing the size of the choke. In this case

it is important that the output capacitor can adsorb the inductor energy without generating

an over-voltage condition when the system changes from a full load to a no load condition.

The minimum output capacitance can be chosen by the following equation:

Equation 39

where Vf is the output capacitor voltage after the load transient and Vi is the output capacitor

voltage before the load transient.

MAX

RIPPLE

ESR

∆

≤

out

ESR

⋅

MAX

LOAD

min

OUT

−

⋅

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