Nov 11 2013
Vertical Alignment (VA) is a display technology in which the liquid crystals naturally align vertically to the glass substrates (homeotropic alignment). When no voltage is applied, the polarized light passes through the cell without a change in polarization and the light is completely blocked by the second polarizer set at 90o to the first, creating a perfectly black state. When voltage is applied, the LC molecules rotate to a horizontal position allowing light to pass through and create a white display image.
Compared to the traditional Twisted Nematic (TN) displays, VA displays have deeper black back-ground colour, much higher contrast ratio, much wider viewing angle and better image quality at extreme temperatures. However, the VA display technologies have stronger requirements for the display drivers than the TN displays, mainly in terms of higher LCD supply voltage (VLCD) and/or higher frame frequency (FF). The requirements depend on the specific VA technology developed by each LCD manufacturer as well as on the multiplex rates (backplane drive configuration) used in application. See Figure 1:
Figure 1: VA displays requirements
Note that the VA displays in mass production are only available with multiplex rates up to 1:64. The major LCD manufacturing companies are working to develop the VA technology for higher multiplex rates.
NXP expanded its LCD drivers’ portfolio in order to meet the strictest VA requirements.
From one side, all the new Chip-On-Glass (COG) and packaged LCD drivers have been specifically designed to drive the VA displays. The maximum value of the VLCD voltage has been increased to 9.0V in the drivers with multiplex drive mode up to 1:8, to 12V in the drivers with multiplex drive mode up to 1:9 (PCA8538UG) and to 16V in the drivers with multiplex drive mode up to 1:18 (PCA8539DUG). Similarly, the frame frequency has been designed programmable in a wider range typically from 60Hz up to 300Hz or even up to 360Hz as in the drivers with multiplex rate up to 1:18.
From the other side, the existing LCD drivers have been upgraded with new versions, which are pin-to-pin compatible with their respective predecessors, but delivering higher VLCD and/or higher frame frequency. In such a way, the customers can easily replace the existing NXP LCD drivers with the new ones. This allows the usage of the better performing VA displays with no or minimal hardware and software changes.