1/6

May 2004

AN576

APPLICATION NOTE

PCB LAYOUT OPTIMISATION

REV. 2A - 3588

INTRODUCTION

Protection requirements are becoming more and more well known and are often defined by rules or stan-

dards. To satisfy these requirements, there is, in the majority of cases, a standard solution or a dedicated

product.

However, knowledge of the disturbances and the use of suitable protection devices are not sufficient in

themselves to solve the problem. In many applications, the correct design of the PCB layout is essential

for success.

Figure 1. Classical protection circuit

INFLUENCE OF THE PROTECTION LOCATION

The circuit presented in Figure 1 shows the classical approach for the protection location. Here the pro-

tection device (see Note 1) is located close to the module to be protected. When a disturbance occurs on

part.

During this clamping action there is a current through P and also in the track A. This phenomenon induces

a voltage on track B, where it is close to A. To avoid this undesirable parasitic overvoltage on track B, the

circuit of Figure 2 is recommended.

Note: 1. TRANSIL devices are used as examples throughout this document, but the same arguments are valid for TRISILS.

Track B

Track A

I

V

CL

P

DEVICE

TO BE

PROTECTED

DISTURBANCE

SOURCE