In their ongoing quest for the thinnest and least expensive
touch-enabled display, display vendors seem to consider on-cell as an
intermediary step. On-cell integrates the touch layers inside the
display but keeps separate layers for touch “receive” (RX) and
“transmit” (TX) functions. Both TX and RX sensor layers are using the
color filter (CF) glass substrate instead of a separate glass or PET
substrate placed on top of the LCD module. As soon as one of the layers
moves below the CF glass, the stackup is called in-cell. For in-cell
stackups, there are two possibilities: “one-sided” in-cell if both TX
and RX move below CF glass, and “two-sided” or “hybrid” in-cell if only
one layer moves below the CF glass. (In practice, the layer below the CF
glass will be the TX layer since the RX layer is always closer to the
finger touching the screen.)
If at least one of the touch layers
is below the CF glass, the ITO or metal mesh transparent conductors that
form the touch pattern can be shared with a display driver connection.
This reduces the number of discrete layers, resulting in a thinner and
less expensive module. While there are other options, the industry seems
to be settling on hybrid in-cell panels, where 1 ITO layer implements
both VCOM and TX conductors, for traditional “vertical alignment” (VA)
LCDs. VA LCDs are typically used in smartphones, while tablets use
IPS-type LCDs to achieve a wider viewing angle. The VCOM is the
reference voltage for all TFT LCDs. The display driver interface (DDI)
IC will drive the R, G, or B video amplitude for a pixel to the source
of the pixel TFT when its gate is open, and a storage capacitor is
formed between the TFT’s drain and this VCOM reference potential. The
stored voltage creates an electric field across the liquid crystal
material that will then act as a valve to pass the brightness level from
the LCD backlight.
In a regular LCD, this VCOM layer is a
non-patterned ITO layer that extends across the whole active area of the
LCD. The ITO layer can be reused for the TX sensor by patterning the
layer into different stripes that represent the rows that will be driven
for the touch function. The corresponding RX column sensors are still
implemented on a dedicated layer of ITO (or another type of transparent
conductor) as shown in Figure 1.
Having the touch area limited to the display size is OK for some user interfaces. However, there are many user interfaces that require an active touch area beyond the display area. This is typically fix function buttons or an additional area surrounding the display for gestures. Having the touch sensor elements as part of the display will restrict the type of user interface that can be used in a product.
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