AN4630 datasheet(10/33 Pages) STMICROELECTRONICS

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Components dimensioning

AN4630

Components dimensioning

The choice of the external components is essential for the correct application functionality.

In the next paragraph, the description of the main components, their functionality and how

to choose them is described.

2.1

Capacitors

A capacitor is a passive electrical component used to store energy in an electrical field. The

forms of practical capacitors vary widely, but all contain at least two electrical conductors

separated by a dielectric.

Capacitors differ from each other in construction techniques and materials used to

manufacture. A lot of different types of capacitors exist (double-layer, polyester,

polypropylene and so on), but this document focuses on the surface mount versions of

ceramics only. The other types of capacitors are not indicated for characteristic or cost for

the application targeted in this document.

A capacitor, as a practical device, exhibits not only capacitance but also resistance and

inductance. A simplified schematic for the equivalent circuit is shown in Figure 7:

"Capacitor equivalent circuit" .

Figure 7: Capacitor equivalent circuit

Typically for the capacitors the ESR (equivalent series resistance) and the ESL (equivalent

series inductance) are defined. The term ESR combines all losses, both series and parallel,

in a capacitor at a given frequency so that the equivalent circuit is reduced to a simple R-C

series connection. Same considerations for the ESL that is the equivalent series inductor

comprised of three components: pad layout, capacitor height and power plane spreading

inductance.

The main differences between ceramic dielectric types are the temperature coefficient of

capacitance and the dielectric loss. COG and NP0 (negative-positive-zero, i.e ± 0)

dielectrics have the lowest losses and are used for filtering, matching and so on.

For RF parts it is generally recommended that multilayer (or monolithic) ceramic capacitors

with a COG dielectric material, which is a highly stable class I dielectric offering a linear

temperature coefficient, low loss and stable electrical properties over time, voltage and

frequency.

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