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Transcript
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/