Guide To Design Touch Sensor

Introduction

The "Capacitive Touch Sensor Design" document provides detailed information on how to design buttons, sliders, wheels, and surface/proximity touch sensors. The guidelines provided in this article must be used in addition to the guidelines provided in the "Capacitive Touch Sensor Design" document.

LEDs and PWM

Details on how touch performance is influenced by LEDs and PWM signal (irrespective of its usage with LEDs) and recommended solutions for such problems are provided here.

Cross-Talk

PWM can change a pin state from low to high during touch measurement. This change in state can inject or remove charge to a nearby electrode or touch sense trace during the ‘share’ stage of touch acquisition. This is observed as noise in the touch signal. Isolating the PWM signal from the sensor and sense lines fixes this issue.

  • It is better to provide an edge-to-edge gap of more than two times the touch signal trace width between touch signal lines and PWM lines.
  • Avoid running the touch sense trace and PWM trace in parallel.

VDD Implications

PWM can cause noise in the Vdd pin. Adding a decoupling capacitor (typically 100 nF) on the power line can help to reduce the noise level. Multiple decoupling capacitors can also be used to remove various frequency noise.

Open-Drain Driver

An open-drain driver has an inherent difference in reference coupling between the ‘driven’ and ‘floating’ states. When an open drain driver changes its state, it causes a delta on the touch signal (even without PWM). If LEDs or open-drain circuits are close to the touch sensor or sensor traces (less than 4 mm away) they must be bypassed with a capacitor that has a typical value of 1 nF. An additional capacitor needs to be added closer to the LED to make sure that a sudden surge in current does not create noise in the power supply. The typical value of these capacitors is 100 nF.

ESD protection

General Guidelines

  • 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 material in nature, 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.

Note: Touch parameters will have to be re-tuned if a resistor value or any aspect of the touch sensing line is changed.

Electrical Fast Transients (EFT)

EFT noise is injected in the power lines and therefore EFT is not a touch-specific issue. EFT can cause the MCU behave erratically and therefore influence touch. Refer to the "EMI, EMC, EFT, and ESD Circuit Design Consideration for 32-bit Microcontrollers Application Note" document for detailed information on preventing EFT failures.

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