Electronic Design

On-cell Touch Screen Panel Slims Down Mobile Displays

Multitouch interfaces are found on most tablets and smartphones with real buttons and switches disappearing from these devices. Simplifying the multilayer touch interface has been the quest of many vendors in this market. Single layer multitouch technology is available from companies such as Cypress Semiconductor (see Single Layer Multitouch Cuts Cost In Half) and IDT (see Multitouch Using Single Layer Sensor). These technologies are designed to be applied on top of conventional displays and normally combined as part of the construction process.

Hydis Technologies, a has created a compact, multitouch display (Fig. 1) that employs a single layer sensor that is applied directly to the display. This allows Hydis to deliver a touch display as one compornent rather than combining a touch sensor and an existing display. The on-cell Touch Screen Panel (TSP) approach also simplifies construction of multitouch displays and reduces system cost.

74032_fig1sm-Hydis-on-cell-touch

Figure 1. Hydis shrinks the conventional LCD touchscreen (left) with a significantly more compact stack (right).

The on-cell approach allows multitouch functionality to be embedded on top of a thin-film transistor (TFT) rather than as a separate component above the display. In addition to simplifying construction and reducing costs, the architecture also delivers better optical qualities because there are fewer layers. Hydis eliminates the air gap, shield layer, two ITO film layers and reduces the mask layers from four to one.

The reduction in the number of layers also reduces parallax errors providing a superior touch interface. Fewer layers also means backlight intensity can be reduced while providing the same brightness level. The usual multitouch display has an 88% transparency while the Hydis display is 93%. This helps reduce lighting power requirements and extends battery life.

Hydis' on-cell technology can be applied to displays up to 22-in at this point. Most tablet and smartphone multitouch displays are limited to 12-in with a 3mm to 5mm side bezel. The on-cell approach does not need a side bezel. The multitouch support hanles any number of touches although typical implementations are likely to handle 10 touches as do most high end multitouch displays.

Hydis Technologies is a subsidiary of of E Ink Holdings, creator of ePaper used in most e-readers. Hydis also invented Fringe Field Switching (FFS) LCD technology that is used on many currently available smartphones and tablets. The on-cell technology can be used with these displays.

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