AN2689 datasheet(39/42 Pages) STMICROELECTRONICS

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AN2689

PCB layout recommendations

Doc ID 14310 Rev 3

39/42

5.1.3

Parasitic coupling due to the loop-effect

In the presence of electromagnetic fields, caused by relay arcing or antenna emissions for

example, a loop coupling effect may occur as shown in Figure 60 (red dashed line)

Figure 60.

Loop effect representation

This coupling is dependent on the loop size and the source distance (see Figure 61).

Figure 61.

Electromagnetic field applied to a loop

The loop effect voltage U is given by

Where H is the magnetic field, E the electric field and

 the wavelength.

U is in Volts, L, h and

 are in meters and E in Volt/meter

The lower the L and h values, the smaller would be the voltage U. That means the smaller

would be the parasitic coupling. So the solution is to minimize the track loop between the

protection device and the module as shown in Figure 62.

Figure 62.

Loop optimization

Module to protect

Vnom

Vmax

Transil

Hazard

source

E,H

()

[] ()

[]

LhE

Big loop

Small loop

Module to protect

Vnom

Vmax

Transil

Module to protect

Vnom

Vmax

Transil

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