AN4630 datasheet(11/33 Pages) STMICROELECTRONICS

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AN4630

Components dimensioning

For RF decoupling purposes select a capacitor value such that for the frequency to be

decoupled is close to or just above the series resonant frequency (SRF) of the capacitor. At

SRF the parasitic impedance resonates with the device capacitance to form a series tuned

circuit and the impedance presented by the capacitor is the effective series resistance

(ESR).

For DC blocking or coupling applications at RF, typically a capacitor with low insertion loss

and a good quality factor is required. Since a capacitor’s quality factor is inversely

proportional to its ESR, select a capacitor with a low ESR and ensure that the SRF of the

capacitor is greater than the frequency of operation. If the working frequency is above the

SRF of the capacitor, it appears inductive.

All the capacitors of the BlueNRG application board used for the matching network and for

the crystals have to be COG.

2.2

Inductors

An inductor is a passive electrical component used to store energy in its magnetic field.

Any conductor has inductance. An inductor is typically made of wire or other conductor

wound into a coil, to increase the magnetic field.

Inductors differ from each other for construction techniques and materials used to

manufacture. A lot of different types of inductors exist (air core inductor, ferromagnetic core

inductor, and variable inductor), but this document will focus on the inductors useful for RF

only. Usually in RF the air core inductors are used. The term air core describes an inductor

that does not use a magnetic core made of ferromagnetic material, but coil wound on

plastic, ceramic, or another nonmagnetic form. They are lower inductance than

ferromagnetic core coils but are used at high frequencies because they are free from

energy losses called core losses.

Usually, the real circuit of an inductor is composed of a series resistance and a parallel

capacitor. The parallel capacitor is considered to be the inter-winding capacitance that

exists the turns of the inductor. If the inductor is placed over a ground plane then, this

capacitance also includes the capacitance that exists between the inductor and the ground

plane. The series resistor can be considered as the resistance of the inductor winding.

In term of circuit performance, as already mentioned for the capacitors, the self-resonant

frequency and the quality factor are the main inductor parameters, especially for the circuit

where the losses need to be minimized. At the self-resonant frequency, the inductor

impedance is at maximum. For frequency above the self-resonance the inductor behavior

change and it appears capacitive.

In general wire wound inductors have a higher quality factor than a multilayer equivalent.

They also reflect and radiate more energy that can give rise to higher emission levels,

especially in term of self-coupling. Inductive coupling can give rise to undesired circuit

operation: to minimize coupling mount the inductors in sensitive circuit areas at 90 degrees

to one another.

In the BlueNRG application board two different inductor types are used:

DC-DC converter coil: the nominal value is 10 uH for the BlueNRG and 4.7 uH for the

BlueNRG-MS. The DCR has to be less than 1 ohm; the rated current has to be higher

than 100 mA.

RF matching and filtering coil: in this case the best solutions are the high Q coils, but

a good compromise between application cost versus RF performances is to choose an

inductor with a medium Q.

DocID027262 Rev 1

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