4.5 ESD and Transients

Handling ESD is usually quite simple: make sure that the user cannot touch the sensitive parts of the system. This is, in most cases, taken care of by the equipment enclosure and only I/O pins leaving the system need special attention. However, ESD discharges may induce currents in nearby paths, causing incorrect values of the signals on these.

Keep in mind that both ESD pulses and other types of transients are very high frequency phenomena, and that stray capacitance and inductance have a very important influence of their behavior. A transient on one line may also affect the behavior of other signals nearby.

The important thing is to make sure that the most efficient path to ground is one that does not affect the system. If, for instance, the most efficient path to ground for an ESD pulse is along the I/O line, to the microcontroller pin, through the ESD protection diode, and then to ground, a logic high input may be read as low. If the system software cannot be made to handle this (and that is usually the case), the system requires some kind of hardware that will create a more controlled path to ground.

The RF filters listed above will, of course, also work on ESD and transients, and may, in some cases, be sufficient. But reducing a 4kV spike to a 4V spike requires a very strong filter. It can by done by large series resistors, but that is not always an option. Large series resistors on input lines will increase the impedance of the ground path described above. This will reduce the amount of noise that reaches the microcontroller pin. The disadvantage of this is that the system also gets high impedance for low frequency and DC signals, and this is therefore not useful for I/O pins that are also used as outputs.

Then over-voltage protectors are a better solution. There are many types of these, most of them acting as very fast zener diodes. They will have very high impedance to ground as long as the I/O line voltage is within the specified limits, but will switch to a very low impedance value when the voltage is too high. A transient is then very effectively shorted to ground.