Touchscreen Touch IC ESD/EOS Protection Solutions and TVS Selection

Touch technology facilitates human-machine interaction through touchscreens or other touch-sensitive surfaces. As technology advances, touchscreens have become integral to modern electronic devices, including mobile phones, in-car systems, and IoT interfaces. 

The earliest touchscreens were resistive, consisting of two layers of conductive material. When a user touched the screen, the two layers made contact, altering the resistance and enabling position detection. Resistive screens offered advantages such as lower cost and simpler technology but supported only single-point touch, and continuous gestures often led to signal interruptions. Capacitive touchscreens, including self-capacitive and mutual-capacitive types, are now predominant. Most contemporary capacitive touchscreens are mutual-capacitive, allowing for more flexible designs like narrow bezels, borderless displays, and foldable screens, making them the preferred choice for consumer devices. 

The primary advantage of touchscreens is their intuitive operation. Users can interact directly with icons and buttons, making them accessible and easy to use, even for those unfamiliar with the device. Touch panels also integrate display and input into a single piece of hardware (Fig. 1), enabling device miniaturization and simplification. Central to the touchscreen is an IC chip that processes calculations, with a layer of transparent electrodes arranged in specific patterns on the circuit board. The top surface is covered by an insulating glass or plastic layer. When a finger approaches the surface, the capacitance between multiple electrodes changes simultaneously. By measuring the ratio of these currents, the exact touch position can be identified (Fig. 2).

Despite their widespread use in various portable products, touchscreens are susceptible to ESD/EOS noise when connected to wired or wireless chargers and other loads. Plugging and unplugging these connections can introduce noise that couples into the sensor through the device's physical layer, affecting touch accuracy and stability. For instance, in mobile phones, inadequate edge layout, poor antenna design, or unprotected interface devices are significant sources of ESD/EOS. Touch processors, manufactured using advanced deep nanometer CMOS technology, can accommodate more transistors in the same chip area, enhancing computational performance. However, this increased density leads to reduced ESD tolerance (device-level HBM typically requires a minimum of 2kV). Each I/O of the processor includes built-in ESD circuits, with the maximum allowable current and admittance of these I/O ESD cells being proportional to their size. Due to I/O capacitance and chip size constraints, these cells cannot be freely designed, making touch processors increasingly sensitive to ESD as technology advances. When exposed to external ESD, the clamping voltage of the I/O ESD cell rises with the ESD current. If the ESD energy exceeds the IC's maximum allowable voltage, it can cause malfunctions or crashes. In severe cases (factory ESD test standards: contact discharge ±8kV / air discharge ±15kV), the ESD protection circuit within the IC may break down, allowing energy to damage internal circuits and cause repeated failures. 

Conversely, TVS protection devices, typically manufactured with process technology in the hundreds of nanometers range, offer superior ESD protection compared to integrated IC ESD solutions. Their ESD discharge capability scales with chip size, and their tolerance significantly surpasses that of internal ESD protection circuits. Therefore, incorporating external TVS devices is recommended for system-level ESD protection. This approach ensures that ESD current is first diverted through the external TVS, greatly reducing the risk of IC damage (Figs. 3 and 4).

As the demand for longer standby times in mobile phones, tablets, and smart wearables increases, and as the desire for larger battery capacities grows, internal space constraints make the small-package TVS advantageous. AMAZING Microelectronic Corp. recommends 0201 (0.6mm × 0.3mm) packages to meet PCB space requirements and internal ESD test standards. As indicated in Table 1, the AZ5A23-01F and AZ5B85-01B are recommended for communication interfaces such as SPI/IIC, both meeting ESD Level 4 test requirements. Additionally, AMAZING Microelectronic Corp. collaborates with Touch IC manufacturers to validate reference design circuits before mass production, providing optimal technical support to meet ESD/EOS design requirements.

Table 1: Touch FPC Interface ESD/EOS Protection Solutions

In the coming years, capacitive touchscreens will remain the dominant technology in smartphones, with infrared, optical, and acoustic sensors following closely behind. Regardless of the technology used for human-machine interaction, interface chips must address ESD/EOS issues early in the design phase to minimize defect rates during production and end-use, thus avoiding repair costs and protecting brand reputation.