AN3029 datasheet(17/42 Pages) STMICROELECTRONICS

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AN3029

Recommendations

6.1

Printed circuit board

For technical reasons, it is best to use a multi-layer PCB with a separate layer dedicated to

the VSS and another layer to the VDD supply. This results in a good decoupling, as well as a

good shielding effect. For many applications, economical requirements prohibit the use of

this type of board. In this case, the most important feature is to ensure a good structure for

the VSS and power supply.

6.2

Component position

A preliminary layout of the PCB must separate the different circuits according to their

electromagnetic interference (EMI) contribution. This reduces cross-coupling on the PCB,

for instance, noisy, high-current circuits, low voltage circuits, and digital components.

6.3

Ground and power supply (VSS, VDD)

The VSS should be distributed individually to every block (noisy, low level sensitive, and

digital) with a single point for gathering all ground returns. Loops must be avoided or have a

minimum surface. The power supply should be implemented close to the ground line to

minimize the surface of the supply loop. This is due to the fact that the supply loop acts as

an antenna, and is therefore the main emitter and receiver of EMI. All component-free

surfaces of the PCB must be filled with additional grounding to create a kind of shield

(especially when using single-layer PCBs).

6.4

Decoupling

The standard decoupler for the external power is a 1 µF pool capacitor. Supplementary 100

nF capacitors must be placed as close as possible to the VSS/VDD pins of the

microcontroller to reduce the area of the current loop.

As a general rule, decoupling all sensitive or noisy signals improves electromagnetic

compatibility (EMC) performances.

There are two types of decouplers:

•

Capacitors close to components. Inductive characteristics, which apply to all capacitors

beyond a certain frequency, must be taken into account. If possible, parallel capacitors

with decreasing values (0.1, 0.01,... µF) should be used.

•

Inductors. Although often ignored, ferrite beads, for example, are excellent inductors

due to their good dissipation of EMI energy and there is no loss of DC voltage (which is

not the case when simple resistors are used).

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