ST AN576 Application note

AN576
APPLICATION NOTE
PCB LAYOUT OPTIMISATION
INTRODUCTION
Protection requirements are becoming more and more well known and are often defined by rules or stan­dards. 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 B
Track A
I
DEVICE
DISTURBANCE
SOURCE
INFLUENCE OF THE PROTECTION LOCATION
The circuit presented in Figure 1 shows the classical approach for the protection location. Here the pro­tection 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 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.
V
CL
P
TO BE
PROTECTED
and thus protects the sensitive
CL
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Figure 2. Recommended protection location
Track B
P
Track A
V
CL
DEVICE
TO BE
PROTECTED
I
DISTURBANCE
SOURCE
In this case the current due to the clamping phase of P remains located in the disturbance area and the track B is not affected.
To summarize, it is recommended that the protection device is located as close as possible the distur­bance source. For example, all the lines coming into the board ought to be protected close to the connec­tor.
INFLUENCE OF THE PCB LAYOUT ON THE ESD PROTECTION
These days, printed circuit boards are often auto-routed by computer aided design and the track lengths are not optimized.
Figure 3. Non-optimized LAYOUT for ESD
A
P
U
DISTURBANCE
SOURCE
B
DEVICE
TO BE
PROTECTED
Figure 3 shows the classical non-optimized layout. When a surge occurs the protection device P acts and there is a clamping voltage V
across it. Due to the fast rise time of the ESD overvoltage there is a high
CL
di/dt between the points A and B. This di/dt generates, in the parasitic inductances located between A and P and between B and P, overvoltages up to several hundred volts. So the applied voltage V across the device to be protected is the sum of the clamping voltage and the voltage across the parasitic inductance: thus the sensitive module is not protected.
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In the case of Figure 4, the design topology is based on a 4 point circuit. When a surge occurs the Transil clamps at Vcl and due to the design the di/dt effects remain on the left hand side of P. Therefore the volt­age V seen by the sensitive device is roughly equal to V
Figure 4. Optimized layout for ESD
I
DISTURBANCE
SOURCE
N.B.: The surface mount family SOD6 and SOD15 are particularly suited to this kind of application.
P
V
CL
CL
.
DEVICE
TO BE
U
PROTECTED
Figure 5. Printer circuit board protection against ESD with case
PRINTED BOARD
MODULE CASE
These days most inputs are protected against ESD (though not always effectively) and so the voltage be­tween the lines and ground never exceeds dangerous values.
However, this does not prevent the total electrical potential from increasing, possibly resulting in sparks between one point of the board and the module case. To avoid this problem we recommend a bidirectional Transil (BZW04P37B) between the printed circuit board ground and the metallic parts of the case.
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Figure 6. Distributed protection
DISTURBANCE
SOURCE
SENSITIVE PARTS
INPUT / OUTPUT
PRINTED BOARD
DISTRIBUTED PROTECTION
The printed circuit board shown in Figure 5 represents a general case. In this board the input/output lines are protected close to the connector and overvoltages are cancelled close to the disturbance sources. The other lines to be protected are the power supply wires which carry 3 kinds of disturbances:
– The overvoltages resulting from mains perturbations.
– The surges coming from the other boards supplied by these lines.
– The disturbances generated on the board by the normal operation of the resident module, for example
the di/dt due to the fast switching of a buffer.
To suppress these surges we suggest a powerful Transil (1.5 KE for example) close to the power supply input on the board, and some lower power devices (e.g. BZW04) distributed around the board area.
CONCLUSION
Due to the parasitic inductance of PCB tracks, a protection device chosen purely according to disturbance standards does not assure immunity from surges. Carefully designed PCB layout plus correct device se­lection from the STMicroelectronics range is essential to guarantee adequate protection.
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REVISION HISTORY
Table 1. Revision History
Date Revision Description of Changes
March-1993 1 First Issue
3-May-2004 2 Stylesheet update. No content change.
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Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
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