ESD protection devices

A very effective method to protect input/outputs lines from ESD discharges is to place special-purpose protection devices on the vulnerable traces. For touch sensing devices, they must have low capacitance. Table 4 ESD protection devices lists the devices recommended for touch sensing microcontrollers.

When permanent damage occurs, the firmware is inefficient but, irreversible damage is not always the only consideration of an ESD stress.

The maximum risk due to an ESD stress is degradation. However, in some less negative cases (such as temporary degradation or loss of function) when a simple system reset is needed, a self-recoverable application can be implemented by using the “watchdog timer on”. The system can restart from a known state and resume normal operations. The final outcome is a robust application.

The debounce firmware method is used to filter some unwanted signals and therefore helps reduce ESD stress effects.

Refer to application note Guidelines for designing touch sensing applications with surface sensors (AN4312) for detailed information.

Among the techniques to protect electronic systems against ESD stress, the cheapest and simplest method is to add a small serial resistor of about 50 Ω. For greater robustness a diode or an ESD protection device can be added. The drawbacks of these choices are cost, leakage, and input capacitance.

In an environment where ESD can strike frequently, the most effective way is to use a combination of the techniques described in Protecting against ESD. For example, use simultaneously a spark gap (with a 10 kΩ serial resistor), a ground ring, and robust firmware.

In the touch sensing domain, most of the protection is provided by a dielectric overlay. However, an ESD stress can strike from anywhere so, additional measures are recommended.