10 ESD Protection

Refer to the information below for general ESD protection guidelines related to touch applications.

  • Protect the touch electrodes mechanically.
  • Add protective circuitry to the touch circuitry.
  • (Re)Tune touch with protective circuitry.

In general, touch sensors are covered by a front panel (also known as an overlay). These overlays inherently protect touch sensors from ESD. The typical materials used for front panels are dielectric, for example:

  • Glass
  • Acrylic
  • Plastic (LEXAN, PMMA, ABS, …)
  • Wood, etc.

Care should be taken while designing the front panel (material and thickness) such that:

  • Dielectric breakdown does not happen for the targeted ESD specification.
  • The overall mechanical structure may not be designed with a uniform material. In such cases, care should be taken that the gap between the machinal structure is kept away from the touch circuit. This can be achieved by extending the panel material by ~50 mm on all four sides of the touch sensor PCB. The simple target is to avoid ESD pulses that may hit touch electrodes or feeding lines directly through the material gap.
  • Make sure that the front panel does not have any conductive material. These conductive materials can accumulate the charge and create a strong electric field that travels a long path and hits the circuit.

Even though the possibility of ESD striking the touch electrode is very small (if mechanically taken care of), we always recommend protective options in the electrical circuit to take care of the unlikely event.

  • Keep a resistor between the MCU and the touch sensor. A higher-value resistor is better for ESD. A typical series resistor of 1 kΩ, in an 0603 package can be used as additional protection with a proper front panel.
  • Make sure that the resistor has the appropriate power rating. In general, larger packages come with the ability to sustain higher power. We recommend a package of 0603 or larger, which is better suited than 0402.
    • These targets enable the resistor to sustain the ESD energy and prevent exploding.
    • It also prevents arcing between the resistor leads.
  • We also recommend providing an option on the PCB to connect a low-capacitance (< 1 pF) TVS diode. This optional TVS diode can be used to provide additional protection if the resistor and front panel are not helping. This can be placed on the sense lines closer to the sensor electrode. Please be aware that TVS diodes are the last line of defense – not the first option to choose. Good ESD preventive design makes TVS diodes obsolete, reducing BOM costs in the process. TVS diodes will act as noise rectifiers and will reduce the noise sustainability of the design.