AN2689 datasheet(33/42 Pages) STMICROELECTRONICS

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AN2689

PCB layout recommendations

Doc ID 14310 Rev 3

33/42

PCB layout recommendations

Once the protection device has been selected, the designer has to pay attention to the

device placement on the board. This is because the device placement has a big impact on

protection efficiency and on parasitic electromagnetic coupling.

5.1

Parasitic inductances of the Transil and the PCB tracks

5.1.1

Parasitic inductance from Transil wiring

Figure 45 provides an example of a protection schematic.

Figure 45.

Protection circuit schematic using a Transil

Figure 46. shows what happens in the circuit when an over voltage occurs on the sensitive

protected line. The over voltage surge is suppressed by the Transil. When the voltage

reaches the breakdown voltage of the Transil, a current I crosses the protection device and a

residual voltage V is applied at the module input line.

Figure 46.

Current and voltage at the Transil termination when a surge is applied

This is the simple representation of the Transil action but in fact things are more complicated

and the more realistic circuit is given in Figure 47.

Module to protect

Vnom

Vmax

Transil

Sensitive line

Module to protect

Vnom

Vmax

Transil

Sensitive line

AN2689 Datasheet(PDF) 33 Page - STMicroelectronics