Original Post by Vignesh Sanmugam, Dec. 5, 2008

The trend of the twenty-first century is that everything is going flat: the world, the economy, the stock market and, now, even displays —flat panel displays (FPDs) are everywhere.  LCD TVs and LCD Monitors have dominated the consumer market with their aggressive pricing and extensive marketing.  And let’s not forget about consumer favorites such as iPods and other MP3 players, mobile phones, PDAs and kids toys. As the prices for TFTs plummet, displays will continue finding their way into almost every possible application (including the toys for your children’s Happy Meals). The technological advancements within the Flat Panel Displays are making designers’ dreams a reality. The choice of designing with a display has thus become inevitable, but the dilemma is “Which display do I use?”It is always that first step which is a challenge, but with displays it is one the easiest decisions. From the CEO to Marketing, everyone wants a Flat Panel Display in their product, which is no longer a luxury but a necessity to be competitive. It is assumed for this discussion that the overwhelming consensus is to move forward with a TFT Display over Color Passive LCD or Organic Light Emitting displays. As one steers through the requirements for a TFT Display, a broader outline consists of mechanical specifications, electrical specifications, optical specifications, environmental specifications and functional specifications.

Deciding the diagonal of the display establishes the foundation for the design process. This is based on the application and the overall mechanical considerations of the final product. Flat Panel Displays are generally categorized as Mobile/PDA Displays (1.0” to 3.5”), Small Displays (3.8” – 7.0”), Medium Size Displays (8.4” to 12.1”), Monitor Displays (15.0” to 19.0”), Large Displays (20.1” to 32”) and Signage Displays (32”+).  The displays that are developed for the industrial market place are considered to have a minimum 3 to 5 years life cycle and the specifics are subject to each display manufacturer’s focus and road map. The LCDs that are developed for the mobile and laptop market tend to be volatile from a lifecycle perspective and this is where your distributor’s knowledge base is of great value in making your selection process.

The size of the display and the resolution of the screen are somewhat interrelated. As for the resolutions, there are limitations to what is available with the size of the screen. The size of the screen and the Aspect Ratio (the ratio of width to height) signifies the maximum available pixel options for the selected display. Wide format displays with mostly a 16:9 aspect ratio were initially used in AV applications and are now commonly sought after for many industrial applications too. TFT displays are either based on a-Si (amorphous silicon) or p-Si (polysilicon), with the p-Si having the ability to accommodate a higher pixel density within a given display size. In addition the p-Si technology has many additional advantages such as the driver IC integrated into the substrate, high-aperture ratio and increased electron mobility. There are now many displays with nonstandard resolutions that could be of interest depending upon the application. The viewing angle of the display is another design consideration that is based on the application and the angle of user interface. One of the challenges with the viewing cone of the display is the issue of color shifts that could significantly impact the selection of a particular display or manufacturer. Most TFT manufacturers now have displays that have wide and symmetrical viewing cones. 

The brightness and contrast of the displays are the next vital part of the selection process. The display brightness is measured in “nits”, which is a unit of illuminative brightness equal to one candela per square meter that is measured perpendicular to the rays of the source. Contrast Ratio is a measure of the brightness of white to the darkness of black that the display is capable of producing. If this ratio is high, the image tends to be sharper and vibrant. The application and the environment under which the display is to be utilized / viewed determines the target levels of the brightness and contrast required for the LCD that is being selected for the given purpose. Note that determining the optimal combination of brightness and contrast can pose a challenge as these are perceived values based on each individual. A general rule of thumb with respect to brightness and contrast is that decreased brightness can be compensated by increasing luminance contrast.

One of the design considerations in determining the brightness of the display is the backlight system of the display. Until recent time, most displays incorporated CCFL (Cold Cathode Florescent Lamp) back lights. The CCFL backlight system contributed to the major part of the power consumption of a display. In addition, these CCFL backlights required a high voltage kick-off for the start and this not only increased the need for an additional component (and inverter), but also could pose interference issues for the smooth functionality of the Flat Panel Display. The increased demand for low-power solutions, instantaneous kick-off during extreme cold weather conditions, EMI/noise issues and thermal management challenges, more and more displays are now manufactured with LED backlights. Thus, the selection of the backlight system also targets any and all power requirements of the display/application. The whiteness of the white LEDs can deteriorate over a period of time, which can impact the color performance of a display, but LED backlights hold far more benefits over CCFL backlights, especially as LED costing continues to decline.

The critical component of designing displays is to ensure that the display being selected for the application can be appropriately driven by the electronics that is available to the designer. The display interfaces differ from panel to panel and manufacturer to manufacturer. In the digital world, displays with resolutions of VGA and above fall into two main categories – CMOS/TTL, which is a parallel interface or LVDS which is a serial interface.  Whereas sub-VGA panels can vary between built-in capabilities, directly accepting TFT signals or the need for specific Timing Controllers (TCON) to provide the required signals to drive the display. Driving these displays can be accomplished in many ways depending upon the application and or available resources.

The panels can be driven (i) Directly by Microcontrollers / FPGA (ii) Single Board Computers or (iii) AD Controller Board. The current day processors, controllers and/or FPGAs have the necessary built-in capabilities to drive a TFT display. This largely depends upon the display if it is a 6 or 8 bit/color LCD. Another option is for the display to be driven by an SBC which has built-in TTL or LVDS capability. It is critical that the timings are appropriate to ensure effective functions of the Display. In an event the Single Board Computer selected does not have a TTL output for the Display but only LVDS and the display is a TTL one , you can use a low-cost conversion board. The off the shelf option is to use a standard Display Controller Board that is capable of accepting the required input such as analog RGB or S Video or Composite Video or DVI and drive the appropriate CMOS/TTL or LVDS display.

Thus, the critical design considerations for a TFT Display are: its size, resolution, brightness, contrast ratio, viewing angle, back light system, power and interface.

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