AN576

2/6

Figure 2. Recommended protection location

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 distur-

bance source. For example, all the lines coming into the board ought to be protected close to the connec-

tor.

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

Figure 3 shows the classical non-optimized layout. When a surge occurs the protection device P acts and

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.

P

I

Track B

Track A

V

CL

DEVICE

TO BE

PROTECTED

DISTURBANCE

SOURCE

DISTURBANCE

SOURCE

A

P

U

B

DEVICE

TO BE

PROTECTED