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NXP’s LCD Drivers: Accurate, Reliable Solutions for VA Displays in Automotive, Medical and Consumer Apps Driving electronics are used to supply the necessary signals to display information on an LCD. To do so, dedicated ICs are employed, placed as close as possible to the display or in the case of smaller displays, somewhere on a PCB or integrated directly with the LCD in one unit. Vertical alignment (VA) LCD displays have become very popular in automotive instrument clusters, climate controls and car radios as well as in home appliances, medical and other devices requiring a clear alpha-numeric display. VA is a display technology in which the liquid crystals naturally align vertically to the glass substrate between crossed polarizers (see Figure 1). When no voltage is applied, the liquid crystals remain perpendicular to the substrate creating a black state as light is completely blocked by the second polarizer set placed at 90 degrees to the first. When voltage is applied, the LC molecules rotate to a horizontal position allowing light to pass through and create a white display image. Another type of nematic liquid crystal, called twisted nematics (TN), is naturally twisted. Applying an electric current to these liquid crystals will untwist them to varying degrees, depending on the current's voltages, to cause a change in the passage of light. Figure 1: Liquid crystal alignment in a TN cell and a VA cell Compared to traditional TN displays, VA displays provide a deeper black color, a much higher contrast ratio between the characters and the background, a wider viewing angle and better image quality, even at extreme temperatures. On the other hand, VA displays typically require a higher LCD supply voltage (VLCD) and a frame frequency (ffr) that needs to be two to three times higher than for standard TN cells, which require a typical frame frequency of only 64 Hz. VA display technology is particularly well-suited for applications where the display needs to be readable in sunlight, or mounted on a black background (e.g, in instrument clusters in the car Figure 2), or located to one side of the viewer (e.g., in the center stack of a car, where it needs to be viewable at a wide angle). Figure 2: Conventional vs. VA technology Two drive methods can be used for multi-segment LCD alphanumeric character displays. In the static, or direct drive method, each pixel is individually wired to a driver. While this is the simplest driving method, as the number of pixels increases the wiring becomes very complex. An alternative method is called the multiplex drive method in which each segment control line can be connected to as many segments as there are "backplanes" or segment commons of the LCD glass. This method "multiplexes" each of the segment control lines so as to minimize the number of interconnects, which enhances device reliability. As an example, in a three MUX drive each segment line drives three segments. A more sophisticated IC example is the NXP PCA8538, a 1:9 MUX rate part that generates drive signals for a static or multiplexed LCD containing up to nine backplanes, 102 segments, and up to 918 elements (which can consist of up to 114 7-segment or up to 57 14-segment alphanumeric characters). In addition, up to four chips can be cascaded to drive larger displays with an internally generated or externally supplied VLCD. The LCD driver can be a big contributor to cost with a large portion of this cost originating from the package. Typically, manufacturers implement LCDs by connecting a cased LCD driver IC (SMD) with the physical display via a PCB. A newer, alternative design is called Chip-on-Glass (COG) technology in which the LCD driver mounts directly on the display glass, reducing the number of tracks and layers on the PCB, cutting board size and complexity and trimming system cost. COG does require tight production and design coordination with LCD module manufacturers and because of that NXP, the pioneers of COG LCD technology, has worked closely with leading manufacturers to simplify PCB layouts and improve the upgradeability, flexibility and reliability of their LCD displays. Additional cost-saving features on NXP’s LCD drivers include the integration of peripheral functions such as an on-chip charge pump and an on-chip temperature sensor. With a charge pump you have the possibility to regulate the VLCD without the requirement for external additional circuitry, so VLCD can be set to the optimum value for a specific display; this may also allow designers to choose less expensive displays. With a charge pump you can also generate adequately high VLCD even in systems with otherwise only 3.3V (or less) supply. Similarly, the on-chip temperature sensor allows temperature compensation (regulated VLCD over temp) for optimum optical performance at all times. NXP offers a wide portfolio of parts specifically designed to drive VA LCD displays, including new COG as well as packaged LCD drivers. The company has upgraded its existing LCD drivers with a new series of parts. The maximum value of the VLCD voltage has been increased to 9.0V in drivers with a multiplex drive mode of up to 1:8, 12V in drivers up to 1:9 and 16V in drivers with multiplex drive mode up to 1:18. The frame frequency has been designed to be programmable over a wider range, typically from 60 Hz up to 300 Hz or even up to 360 Hz for 1:18 multiplex rate drivers. Since the new parts are pin compatible with those they replace designers can easily upgrade their existing NXP LCD drivers, resulting in better performing VA displays with either no or minimal hardware and software changes. For more information: http://www.nxp.com/products/interface_and_connectivity/lcd_drivers/