Designing Next-Generation TFT-LCDs for Mobile Telephones
As engineers explore the complexities of mobile-telephone system design, many are finding that transflective displays – now the dominant solution for color telephones – are not as attractive as a new generation of brighter, more-colorful transmissive displays.
by Bill Lindblad
BECAUSE design flexibility and the latest technology are essential in selling thin-film-transistor liquid-crystal-display (TFT-LCD) module solutions in the mobile- telephone market, the successful solutions provider must offer both. A short design and product-development cycle and a wide array of integration options should also be included. A large percentage of mobile-telephone developers desire a very simple module design with limited add-on features, but others want a fully integrated solution.
Addressing all of these market needs is not an easy task for a display-module designer. To do it effectively, a designer must understand the latest advances in display technology as well as all of the electrical, mechanical, and optical parameters of the developer's telephone design. These include the baseband processor, camera, graphics media processor, electrical interface, and mechanical dimensions. As designers explore these various components of mobile-telephone system design, many are revising long-held opinions about which types of TFT-LCDs are best suited for color telephones.
International DisplayWorks, Inc.
Fig. 1: Moving major mobile-telephone functions, such as the camera and graphics-engine ICs, into the upper section of clamshell telephones has increased the interface wire count to 50 or more connections between the upper and lower sections of the clamshell.
Electronic Circuitry
Module designers must be knowledgeable about the many interfaces and their variations and be capable of offering any of them in their product solution.
One issue that arises immediately when an engineer begins to design the circuitry for a TFT-LCD product is that there are currentlymore than 10 different TFT-LCD-module inter-faces that are widely used in the mobile-telephone industry. Each customer's TFT-LCD module has its own specific interface requirements. Some base their interface designs on a system architecture using an RGB interface that connects to a RAM-less TFT driver, others use a CPU interface that connects to a RAM-based TFT controller/ driver.
In next-generation products, many mobile-telephone developers will incorporate baseband ICs and TFT driver ICs that support the new Mobile Pixel Link (MPL). This new interface is very similar to the low-voltage differential-signaling (LVDS) standards that have been used in flat-panel products for the past 10 years.
The development of MPL has been driven by recent trends in the mobile-telephone industry to move major functions, such as the camera and graphics-engine ICs, into the upper section of clamshell telephones. This move increases the interface wire count to 50 or more connections between the upper and lower sections of the clamshell (Fig. 1). It has also increased the level of electromagnetic-interference (EMI) noise and the cost of the elbow flex circuit.
The MPL-interface specification addresses two of these issues by reducing EMI noise and the number of interconnects between the tele-phone's major media components – the camera, graphics processor, and speaker.
Until recently, the cost of the MPL approach was prohibitive because a designer had to incorporate a stand-alone LVDS transceiver at each end of the link. But now that these transceivers can be incorporated into next-generation base-band and TFT driver ICs, it is possible to incorporate the advantages of an MPL at reasonable cost.
Improving Color Reproduction
As every provider of display solutions knows, color gamut, contrast, and luminance are the most critical performance attributes of a TFT-LCD. These are the key attributes that make the color reproduction and overall appearance of one TFT-LCD product superior to another.
With the recent improvements made in wide-viewing-angle films, vertical-alignment LC cells, in-plane switching, and improved aperture ratios, the viewing angle and contrast ratio of small-format TFT-LCDs are not major improvement targets in next-generation TFT-LCD-module designs. In addition, the color gamut found in today's standard white light-emitting diodes (LEDs) is already optimal for good color reproduction. So, there is really no need to commit R&D resources to improving this aspect of LED performance since current yellow-phosphor-based white LEDs can easily produce an NTSC ratio of greater than 80%, i.e., the area of the display's color gamut on the 1931 CIE Chromaticity Diagram is 80% of the standard gamut established for color television by the National Television Standard Committee (NTSC) (Fig. 2). When using white RGB-based LEDs, it is even possible to exceed 100% of the NTSC color gamut (Fig. 3). The NTSC standard was used in the previous example for purposes of visualization only because there is currently no standardized color-space target for small TFT-LCD modules.
So, the industry's current focus is on performance improvements related to the color reproduction of a TFT LC cell and to the luminance output of the LEDs and backlight system. Unless the luminance of the LED-backlight system is increased, it would be very difficult to improve the color gamut of a TFT-LCD product because of the direct relationship between color gamut and luminance in a TFT-LCD optical system.
Fig. 2: Current white LEDs based on yellow phosphor can deliver a color gamut that is greater than 80% of the standard color gamut established for color television by the National Television Standard Committee (NTSC). This 1931 CIE Chromaticity Diagram shows the full NTSC color gamut.
Improvements in color gamut are obtained by narrowing the bandwidth of the color filters, which causes the color coordinates of the points representing the primary colors to move further out to the edge of the color space. Unfortunately, this improvement in color gamut comes at the expense of light transmission through these narrower filters.
As most TFT-LCD-module designers already know, each 5% improvement made to the color gamut reduces the luminance of the display surface 6–8%. Fortunately, as we shall see, there have been significant improvements in LED luminous output over the last year, so an increase in the color gamut of small-format TFT-LCDs is now possible.
In addition to using brighter LEDs, it is possible to make two basic adjustments to the color filters: (1) increase the thickness of the filter and (2) ensure that the chromaticity of the color filter is optimized to the spectrum of the backlight. Both of these techniques are being implemented in next-generation mobile-telephone displays to improve the color saturation of the display product. It is important for a designer to specify the color filter and backlight luminance correctly, so that the customer receives the luminance and power consumption that is required.
Increasing Luminance
In the last few years, there has been much debate regarding the need for transflective TFT-LCDs in mobile telephones. Some telephone designers insist that transflective TFT-LCDs are necessary if users view their products in direct sunlight, while others state that consumers will buy the mobile telephone based solely on how the display looks in the store, so it really does not matter if the display can be viewed in direct sunlight.
Then there are those who argue that a telephone which incorporates a transflective TFT-LCD will consume less power because the TFT-LCD can be viewed without having the backlight turned on. In a telephone with a color TFT-LCD, the backlight is always on regardless of whether the display is transflective or transmissive. This is mainly due to fact that even a telephone with a transflective display is most useful visually for merely reading the time or date displayed on it.
In addition, telephone applications are typically designed to automatically turn the backlight on as soon as any key is pressed on the phone, which largely defeats the purpose of using a transflective TFT-LCD to save power.
Beyond these questionable advantages, there are many actual disadvantages in using a trans-flective TFT-LCD. Among these are a 10% increase in the cost of the TFT-LCD panel and reduced transmission and color saturation.
A recent change in direction by a number of large mobile-telephone manufacturers has bypassed all the issues surrounding a transflective TFT-LCD solution and focused on increasing the overall luminance of a trans-missive solution (Fig. 4). These manufacturers believe that increasing the luminance to a level approaching 500 cd/m2 will allow the display to perform very well in 95% of the ambient environments in which their products are used.
To obtain a luminance of 500 cd/m2 from a transmissive TFT-LCD, the backlight surface luminance must be greater than 5000 cd/m2. A year ago, it was not possible to achieve such a high luminance in a small TFT-LCD backlight system without employing an expensive microlens light-guide technology or using a large number of LEDs. Today, when an economical single white LED approaches a luminous intensity of 1500 mcd, it is possible to achieve in excess of 5000 cd/m2 on a non-microlens backlight surface using only four white LEDs.
Fig. 3: A 20% increase in color gamut is readily apparent, but designers of mobile telephones will have to decide whether it is worthwhile for their applications.
This new level of luminance provides a display designer with a number of options. One option is to greatly improve the color gamut of a given product by increasing the thickness of the color filter and operating the backlight at the 5000-cd/m2 level. Another option is not to modify the color filter but to utilize all of the luminance to make the product sunlight readable. A third option is to integrate a control that monitors ambient light and dims the backlight in low ambients, thus conserving power.
This third option makes it possible to maintain the brightness of last year's product while reducing the power consumption of the back-light by 120 mW, or 40%. This power savings equates to approximately 12 extra minutes of talk time and 10 extra hours of standby time, assuming that the backlight is operating 10% of the time.
Module Size
Most mobile-telephone developers believe that the size of the TFT-LCD module in a new product is always a major concern, one that module designers must address before it be-comes an issue. Three of the major mechanical dimensions critical to the module design are (1) the overall thickness of the module and (2) the distance (and tolerance) from the edge of the active pixel area to the outer edge of the module.
The thickness of a TFT-LCD module is determined by the components in the stack. These include the TFT LC cell, plastic frame, light guide, polarizers, LEDs, and brightness-enhancement films. Since most of the components that make up this stack are already as thin as they can possibly be, the focus is on the design of the light guide and plastic frame. A key determinant of the light-guide thickness is that the height of the LED must always be the same as or less than the thickness of the light guide. If this rule is not met, there will be a significant loss of light because of the geometrical mismatch at the coupling point.
One consequence of this rule is that it be-comes difficult to maintain an adequate lm/W backlight efficiency for a light-guide thickness of less than 0.8 mm. When a light guide is this thin, there is a large coupling loss at the LED–light-guide interface because of the mismatch between the 0.8-mm-tall LED package and the less-than-0.8-mm light-guide inlet.
Another critical mechanical dimension is the relationship of the active pixel area to the outer edge of the TFT-LCD module. This might seem easy to resolve, but the tolerances associated with these dimensions is one of the most critical and difficult mechanical-engineering factors in the entire design. The problem arises when a ±0.150-mm tolerance between these two components is required.
A tolerance this tight is important to a mobile-telephone designer for two reasons. First, in order to keep the telephone small, it is important to position the lens opening in the telephone housing as tightly as possible over the active area of the TFT-LCD panel. If the tolerance becomes greater than ±0.150 mm, non-symmetrical alignment between the active area of the TFT-LCD and the lens opening becomes visible to the end user.
The only way to meet this tight tolerance for a mechanically aligned TFT-LCD–to– frame process is to limit the TFT LC cell manufacturer to a ±0.1-mm tolerance and the plastic-frame molder to a ±0.05-mm tolerance. Even with these tight tolerances, the mechanical-alignment process used in production has to be virtually flawless.
Integrated Features
Designing additional functions into the display module has become more common in many new and next-generation mobile-telephone designs. This new direction is being driven primarily by space constraints caused by the addition of camera, MP3, and video-related functions. In addition, many telephone designers are also integrating the graphics media/acceleration processor, LED constant-current charge-pump circuitry, camera-flash LED, and speaker into the module design. Many of these components are being placed on the front and back of the TFT-LCD module, and some are attached to the flexible circuit and/or printed-circuit board (PCB) that is normally used as the interconnect to the panel system.
Outlook
Designers of mobile products are currently improving display quality by increasing resolution, contrast ratio, viewing angle, and color depth. In addition, designers are adding numerous integrated features to many displays. Next-generation products will focus on improved color gamut, increased luminance, and a reduced number of connections between the upper and lower sections of a clamshell telephone.
As the mobile telephone continues to evolve, it is critical that display design engineers understand what the market requires for next-generation products and to stay proficient in these technologies. •
Fig. 4: The trend in the luminance of small displays is sharply upward, thanks to the development of cheaper, more-efficient LED backlights.