AN576 Datasheet, PDF (1/6 Pages) STMicroelectronics – PCB LAYOUT OPTIMISATION

AN576
APPLICATION NOTE
PCB LAYOUT OPTIMISATION
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
Track A
I
DISTURBANCE
SOURCE
V_CL
Track B
DEVICE
TO BE
P
PROTECTED
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
the track A the TRANSIL P clamps the surge at a maximum voltage V_CL and thus protects the sensitive
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.
May 2004
REV. 2A - 3588
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