Challenges of display integrated with touch(3)

Additionally, the requirements of smartphone OEMs continue to escalate, putting more burdens on the TSC. Advanced UI features such as water rejection, 1 mm passive stylus support, proximity detection, glove / nail touch, and the need to operate with ever higher charger noise push the requirements on the TSC even further in terms of signal-to-noise and signal processing for on-stack, let alone for display-integrated panels.

The industrial design attractiveness of thin display modules is also not the exclusive territory of the in-cell display industry. On-stack vendors continue to innovate so that their stacks rival in-cell displays in thickness by reusing the phone's cover glass as the substrate for the sensor. This "sensor-on-lens" approach is then coupled in many cases with a true single-layer sensor pattern such as Cypress' Single-Layer Independent Multitouch (SLIM) sensor, which avoids jumper bridges between X and Y traces even when only using a single layer of ITO material. There have also been evolutions in non-ITO (e.g. metal mesh) transparent conductors to improve touch performance and reduce sensor cost, as well as in the strength and optical properties of both cover and sensor substrate glass.

This dual industry trend of on-stack vs. display integrated touch eventually drives innovation and lower cost for both designs. Cypress works with both the discrete sensor vendors as well as display integrated touch vendors. Unlike some our touch IC competitors that bet the farm on either technology via acquisitions or technology partnerships, we are not biased towards either.

The latest firmware of Cypress Gen4 TSCs, currently in production, offer the features needed to support in-cell touch and advanced on-stack architectures, such as sensor-on-lens, with both a traditional two-layer sensor and the true single-layer SLIM sensor. This flexibility is a result of the underlying programmable SOC (PSOC) architecture of Cypress touch screen controllers. For instance we can create, or process, the required synchronisation signals between DDI and TSC without any external glue logic or the need for a custom part, due to the presence of programmable configurable logic blocks in the chip hardware.

To illustrate this, figure 2 shows the touch accuracy results obtained on a hybrid in-cell 4.51" 16:9 LCD panel (with shared VCOM/TX layer) using a standard Cypress TSC. The panel has a HD720 (1280x720) resolution. Figure 2 shows the histogram of touch accuracy results across the full panel. Performance is very similar to what can be achieved with traditional LCDs with on-stack sensors.

Figure 2: Accuracy and linearity results on a hybrid in-cell panel.

Display integrated touch is real from this year with actual handsets in production. The industry now takes a realistic approach by re-using mainstream projected capacitive touch technology. In this article we tried to objectively point out some of the opportunities and challenges that still exist with this technology, as well as the response of the discrete sensor vendors. Cypress supports both industry factions in their quest for the same goal of ever thinner touch modules with improved touch performance at the lowest cost point.

About the author
As strategic marketing director in Cypress' touchscreen controller BU, Bart DeCanne actively keeps track of industry trends in the touchscreen area to drive Cypress' new product development. Bart has 18+ years of electronics industry experience, of which the last 14 years have been at tier-1 mixed-signal semiconductor suppliers Texas Instruments, Silicon Labs, and Maxim, in various marketing and product line management positions. He has been involved with touchscreen controller ICs since 2008. Bart holds a MSEE from University of Ghent, Belgium and a MBA from UT-Austin.

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