Capacitive Touch with Displays
August 13, 2012, Display Search Emerging Display Technology Conference, Santa Clara, CA—Kurth Reynolds from Synaptics talked about various sensor types and the on-going industrial design to improve the user experience. The whole package now needs system-level knowledge and tools to address the complex issues.
Sensor types are proliferating as sensors and displays become more highly integrated. The discrete capacitive sensor uses PET or glass. Newer versions put the sensor on the lens and eliminate one glass layer. On-cell technologies are emerging for LCD and AMOLED displays. The discrete sensor on glass designs use either PET or glass for the sensor. The basic manufacturing is to apply apply a pattern on the surface and attach to a flex circuit.
Sensor on lens construction places the sensor array on the substrate and doesn’t require a separate glass for the sensors. For LCDs, the sensor is under the color filter glass. The challenges are in making jumpers for crossovers and lowered optical performance. So far, most manufacturers are having yield issues due to the 3-step manufacturing process.
In-cell sensors have the advantage that you have to only pattern one layer on the filter glass. The cell is defined by the materials stack and various stack-ups allow for good vertical alignment and in-plane switching. The sensor is applied on existing glass layers, so no additional processing is needed except for the ITO patterns. These technologies provide very good brightness or the OEM can opt for lower power. Of course the elimination of the separate sensor glass makes the display and sensor assembly(s) thinner and lighter, as well as less expensive. The downside is that the construction reduces the signal-noise ratio. In cell arrays are now being used in some of the latest smart phones.
For the next-generation sensor arrays, the touch and display drivers will be integrated together to reduce power, noise, and latency, while improving reliability and costs. The foreseeable chip architecture will include a 16-bit CPU, capacitive sensor array, diode driver, timing control, and power management as well as other supportive functions. The touch latency feedback loop will be reduced from about 70 ms to less than 20 since the screen and touch sensor are synchronized in the single drive chip. The interface functions can be integrated with the host and OS through an overlay memory, or the system can be set up to operate the display without the host.
