Do I need a TFT display?
The first thing to consider is whether you need a TFT. First, remember: color adds cost. You can save a lot of money by utilizing monochrome graphic LCD modules for your design. They are inexpensive, relatively easy to connect and control, and readily available. Monochrome displays require less hardware overhead and less complex software. It is easier to manage one color per pixel than three colors per pixel (red, green, and blue). In addition, "monochrome" does not necessarily mean just black and white. Depending on the backlight and LCD material, so-called monochrome LCDs come in a variety of colors, including black and white, yellow, green, blue, and variations in between.
There are also color passive LCD modules. Depending on the design guidelines, color passive panels may have a variety of advantages over TFT panels in areas such as outdoor brightness and overall color brightness. When deciding between a TFT panel and a color passive panel, don't forget about refresh rate and viewing angle; TFTs can offer viewing angles close to 180°, while passive color modules, while improving, typically only offer 70° to 80° viewing angles.
The LCD panel consists of a layer of LCD material and one or more layers of polarizing material, which is made of plastic, glass, or other materials (Figure 1). When you align two layers of polarizing material with each other, light can pass through. However, when one layer is polarized at 90° to the other, light is blocked. the LCD fluid acts as a dynamic polarizer. When you apply a voltage to an LCD cell, the cell rotates 90°, blocking light or letting it pass through, depending on the orientation of the polarizers underneath the LCD layer.
In passive LCD technology, these cells are the equivalent of capacitors. Once charged, they release their respective voltages and slowly turn back to their original positions. For this reason, passive panels are unable to change the state of these cells quickly and are not suitable for fast motion graphics. To overcome this disadvantage, TFT LCDs use transistors to drive the LCD cells. Although this method is more complex and expensive, it achieves faster control of the LCD units. As a result, TFTs are able to display full motion video and graphics, whereas passive LCD modules begin to blur when the image moves faster than 8 to 15 frames per second (depending on the manufacturer).
Don't let these factors leave passive color panels out in the cold. Explore options when looking for the perfect display. For example, your application might require an outdoor handheld system that needs to be readable in sunlight. You might find that both a small TFT display and a color passive LCD work well for this application, but the TFT display might have a better viewing angle and the passive panel's colors look better. Often, selecting a monitor is as much about marketing needs as it is about engineering specifications, and both are equally important. Displays can have a direct impact on the appearance of a product, and you will find that they are often very subjective. When choosing a display, whether passive or active, make sure that the vendor or distributor you work with will show you (and, if necessary, the marketing decision makers and management decision makers in your organization) the display at work. Today, most monitor vendors have demonstration systems that allow you to watch the monitor play back recorded pictures or video, perhaps with inputs for VCRs or DVD players.
Sizing a TFT monitor
The first thing to consider when choosing a TFT monitor is size. With a passive LCD module, you measure the length and width of the display, whereas you measure a TFT panel like you would a CRT monitor: the diagonal length. Thus, a monochrome module 122X92 mm in size might approximate a 5.7-inch (about 145 mm) TFT. When designing the structure of your product, consider whether you plan to update your monochrome product to a color display at a later date. 3.9 inches (for PDAs and handheld systems), 5.7 inches, 8.4 inches, 10.4 inches, 15 inches, 17 inches, 19 inches, 21 inches, and so on. Other less common sizes are 4.5, 5, 6, and 7 inches.
Size depends on the application and annual requirements. For example, the cell phone market is highly variable. If your application requires only a few hundred or a few thousand displays per year, you need to make sure that your display vendor doesn't declare your display of choice end-of-life when a new cell phone design makes it obsolete. This can happen when you're in the middle of a design cycle.
This size-variation scenario can also have a number of benefits. 10.4-inch displays, once reserved for laptops, are now the most efficient point in the price/size ratio in the display space. That's because there are a lot of applications for that size of LCD. In addition, manufacturers have high yields when cutting glass of this size, and there is a user base for LCDs of this size. A system that includes a 10.4-inch display may be cheaper than one with an 8.4-inch display. Whichever way you go, get feedback from your intended display supplier.
Resolution for TFT displays
Along with choosing the size of your TFT display, you'll need to choose the resolution. As the visible area of a display decreases, so does the user's ability to distinguish individual pixels. As a result, it's rare to see displays smaller than 6 inches with VGA or higher resolutions, and most smaller displays offer QVGA or lower resolutions. Similarly, 15-inch panels typically offer at least XGA resolution.
You'll also find specialized panels with special resolutions, such as 480×234 or 320×96 pixels. These panels are specialized for video, automotive, and rack-area applications. For example, if your project requires video playback, you may find such panels more useful than TFT monitors with QVGA or VGA resolutions. It should also be noted that the screen aspect ratio is just as important as the size and resolution. Most TFT panels have a 3:4 screen aspect ratio, but you can also find dedicated TFT panels with 16:9 screen aspect ratios, or with that ratio "almost" 3:4 or 16:9.
Backlighting TFT displays
Now it's time to consider the backlighting, sidelighting, and frontlighting used by TFT panels. In this article, we use the term "backlight" to represent all three, because even though you install them in different ways, their basic purpose is the same.
The most common TFT display backlight is a CCFL (cold cathode fluorescent lamp), which is similar to the fluorescent tubes commonly used in offices and homes. They are small, relatively easy to use, good sources of white light for TFT displays, and can usually be replaced in the field. CCFLs are also often used in color passive LCD panels and monochrome passive LCD panels.
Depending on size and nits rating, TFT displays use one or more CCFL tubes, although these tubes have some drawbacks. For example, depending on size, a CCFL tube requires 300 to 750V, 2 to 7mA AC to operate. This requirement means that if you want to use CCFLs, you must add your own or an off-the-shelf CCFL inverter module to your design. Such modules offer everything from simple power conversion to soft-start, brightness reduction, and temperature compensation, and they range in volume price from a few dollars to about $25.
Because CCFLs are fluorescent tubes, they also have temperature limitations. Low temperatures cause CCFLs to flicker and produce less light, while high temperatures shorten tube life. Temperature extremes often make it difficult to design TFT displays for outdoor and automotive applications. Because TFT displays have a narrower temperature range than semiconductors and other electronic parts, designers sometimes use heaters or cooling fans as a precautionary device to help overcome temperature-related design hurdles.
Also, while CCFLs have a lifespan of a few thousand hours, they can burn out or dim over time. They generate some heat, which you have to take into account, and, because they're made of glass, you might jar or drop them. Manufacturers are increasingly utilizing replaceable CCFLs for TFT displays, and you may want to consider this type of product, depending on how long you expect your product to last, and whether you want to replace the entire TFT display at a later date, rather than just the CCFL. If you have designed your product to last a few years before the user upgrades, if the product is obsolete, if the cost of shipping additional CCFL tubes is too high, or if the labor costs associated with replacing them are too high, it may be just as economical to replace the entire TFT display.
LED backlighting is an option for 5.7-inch and smaller TFT displays. While monochrome LCDs have been using TFT panels for many years, LED technology has recently enabled the white light needed to illuminate these panels. LEDs offer several advantages over CCFLs, such as stability over the full operating temperature range, mechanical durability, and simple power requirements. However, like CCFLs, LEDs generate heat and it is sometimes difficult to arrange them to illuminate TFTs without creating hot spots due to the small surface area of LEDs. White LEDs also have a rated lifetime. Over time, some white LEDs burn out the adulterating materials used by manufacturers to make them white. While this defect does not cause the LEDs to fail, it does cause color variations in the output of the LEDs and therefore the output of the TFT panel.
This color shift may or may not be an issue in the overall design, depending on the application, but design engineers should consider it and discuss it with TFT panel suppliers. White LEDs are also a newer technology and are currently expensive. Because of these limitations, it's not common to see TFT panels larger than 5.7 inches with LED backlighting.
Some designers who have worked with monochrome LCDs may be familiar with EL (field-emitting) panels. Although inexpensive, FTL panels emit a soft green to yellow to blue light, depending on the FTL material and the applied voltage and frequency. Because FEL panels produce light in a limited range of colors, they are not suitable for TFT displays. Also, FTL panels have a shorter lifetime than LEDs or CCFLs, and their light intensity diminishes over time. They also require high frequency and high voltage, similar to those metrics of the CCFLs that drive them.
Driving TFT displays
There are many options for driving TFT displays. Some passive panels use simple controllers, others may hang off the system bus in your product design. However, the signals and data needed to drive a TFT require that a dedicated TFT controller must be used, along with a system processor and a video subsystem that can provide the bandwidth to drive the TFT video. Many chips, chipsets, boards, and SOC (System-on-Chip) options, ranging from simple converters to complex video and graphics systems, are available to handle this task. For the connection between these controllers and the TFT itself, TFT manufacturers have adopted two near-standard interfaces. The first is a parallel interface, often referred to as TTL (transistor-to-transistor logic) or LVTTL (low-voltage transistor-to-transistor logic), which sends the TFT data and clock signals over discrete connecting lines (Figure 2). The second is a differential serial interface, aptly named LVDS (low-voltage differential signaling), and several variations are available, including dual LVDS and TMDS (minimally transmitted differential signals) (Figure 3).
The data and clock signals transmitted via LVDS are time-domain multiplexed to several differential serial pairs, which are then decoded by TFT. For LVDS schemes, it is important to note that LVDS pairs do not correspond to a specific data channel or type. There are generally no red, green, or blue pairs. The scheme spreads the multiplexing of data bits across multiple pairs for high efficiency (Figures 4 and 5).
When looking at interface types, you'll also see specifications for the number of interface bits, with common examples being 12, 18, 24, 36, and 48 bits. These numbers encompass the entire interface and are sometimes misleading. For example, TFTs use 4 to 8 bits of data per pixel per color. 24 bit TFTs have 8 bits for red, 8 bits for blue, and 8 bits for green for each pixel on the display. When there are 256 variations of each color, a 24-bit TFT will provide 16.7 million colors (256 x 256 x 256 = 16.78 million).
To ease the clocking requirements on the data sent to the display, higher-resolution displays may also use dual interfaces to support odd and even pixels. 48-bit TFTs are an example of this, and actually have two 24-bit interfaces for odd and even pixels. Each interface is then further divided into 8 bits per pixel per color, which gives the TFT a color depth of 16.7 million colors. Note that a higher number of bits does not necessarily make the display better, it just makes it different. It's never a bad practice to discuss your needs internally and with your prospective TFT supplier. Table 1 lists several very popular combinations and corresponding color depths.
Depending on the TFT integration, you'll also find other interfaces such as DVI (Digital Visual Interface), analog RGB, and NTSC/PAL video. TFTs with these types of inputs have additional drive electronics and control electronics placed inside the display module and are designed for specialized applications or for easy integration into a design.
In some cases, you may not want to use a TTL or LVDS interface to a TFT, for reasons that may include available time, budget, the need for a simple design, engineering resources, or data in another format. In these cases, there are a variety of TFT controller cards available from third party companies. These cards include electronic components that format data from inputs such as DVI, analog RGB, NTSC/PAL video, etc., and convert them into the TTL or LVDS signals required by the TFT. In addition, they provide functions such as brightness, contrast, color balance, image shaping, backlight control, brightness reduction, and power management and sequencing for the TFT.
Using a third-party control card can save you time, though it can also add cost. These tradeoffs should be considered, which can help. For example, think about why a certain design needs an RGB output from a PC video card, single board computer, or other video electronics in order to drive TFTs. Maybe you need a standard RGB output to drive a CRT or projection system, or maybe you need an RGB output on a TTL or LVDS interface for ease of implementation or to drive longer cables. In either case, as long as you're not using a TTL or LVDS interface, you're adding extra cost, and that's because you have to use a separate controller, converter board, or system. Integrated displays are the exception to this rule, though.
Integrated displays are TFTs designed for specialized applications (Figure 6). Such displays include additional electronics in the TFT module to minimize the number of external components required for the TFT to operate. These electronics may include a CCFL (Cold Cathode Fluorescent Lamp) inverter, a DC/DC converter, and a TFT controller that supports different input signals; DVI, NTSC, PAL, analog RGB, and digital RGB are just a few of the other inputs you may find. Some integrated panels even support multiple input types, such as analog RGB and NTSC/PAL.
Harmonizing
Finally, think about how the display, power supply, and controls work together. Figures 6, 7, 8, and 9 give some typical applications using TFT and other related things you need to consider.
Figure 6 shows an integrated TFT with an NTSC video input. a typical use might be for an in-car video entertainment system, where you can connect a DVD player, VCR, and satellite receiver to the display. In this application, space, cost, and tolerance for a wide temperature range are important design criteria. Designers of such applications might prefer a complete package because they are concerned about how and where to implement the driver and inverter boards.
Figure 7 shows a system similar to that of Figure 6, but without integrated TFTs. In this example, an off-the-shelf control board converts composite video and analog RGB video inputs to TTL or LVDS signals, which are then used by the non-integrated TFTs. A "button board" connects to the control board to provide dry-contact buttons that adjust the controller's various on-screen display parameters, such as brightness, alignment, contrast, and color balance.
Figure 8 depicts a single-board computer. Many single-board computers support both direct-attached TFTs and standard RGB computer video output. This example uses the single board computer's TFT connector (either TTL or LVDS) to connect to a TFT panel. Some single board computers have additional inputs, this example includes an optional DVI connection. The CCFL inverter, although a separate module, is also connected to this single board computer, which supplies power to the inverter and either an analog signal or a PWM based signal to dim the CCFL tubes. The single board computer powers the entire system with a set of dual power supplies that also power any IDE drivers connected to the single board computer.
This last example depicts a fully embedded system (Figure 9). In this case, the TFT controller can be a separate chip or included in the SOC, FPGA or system microcontroller. It can use main memory or a separate frame buffer memory to store graphics and text. The controller can drive the TFTs directly or require additional driver buffer logic circuitry. As with the single-board computer example, the CCFL inverter connects to the embedded system, allowing power sequencing, as well as CCFL dimming via analog or PWM dimming control, if the system uses a CCFL inverter that supports dimming.
Considering some of the details
While the highlights of selecting and using a TFT display have been discussed earlier, there are a few more subtleties that are worth mentioning, and which may be important to your application and the TFT selection process. First, contrast ratio is the ratio between the blackest blacks and whitest whites that a TFT can display. The higher the ratio, the less light and dark colors will mix, and the brighter and crisper the image will be. Higher contrast ratios, however, increase costs.
The second detail is bad pixels. Even very good TFTs can have bad or missing pixels. While this concept may sound odd, every TFT manufacturer should have its own quality control process in place. There are hundreds of thousands of pixels in a display, and each manufacturer should determine how many failed pixels they feel are acceptable, starting with zero failed pixels. For example, a 10.4-inch SVGA (800 x 600 pixels) monitor with 480,000 pixels might allow a maximum of nine bad pixels at a rate of less than 0.002%. Further, manufacturers may specify that these bad pixels cannot be in close proximity to each other in order to prevent visible "holes". For very high design requirements, TFT manufacturers may do "box-sorting" (grading or sorting displays to minimize defects) to achieve high quality, so don't rule out a particular manufacturer unless you've already asked them.
Viewing angles are a possible problem, especially in oddly shaped TFTs or small TFTs. Viewing angles tell the designer from which point the display was originally designed to be viewed (Figure 10). The common viewpoints are 12, 3, 6, and 9 o'clock. Imagine a centerline perpendicular to the display surface, such as the center of a clock. Viewing angles dictate that the best viewing point is a point slightly above, below, to the left, or to the right of that center point. This doesn't mean you can't view it from other angles, it just means that it is the best viewing point for that TFT design. Imagine a person sitting in front of a laptop, and that person is usually sitting slightly higher than the device, looking down at the screen. For this reason, laptops have a 12-point viewing angle. a PDA may have a 12-point or 6-point viewing angle because you may be holding it and looking at the screen from above or below
On the other hand, a display for a rack-mounted device, an in-car GPS system, or a medical device may need a 3-point or 9-point viewing angle for optimal performance.
If you're working on an application designed for portable systems or limited power supplies, you may want to consider power supplies. TFTs consume more power than passive LCDs and may require power sequencing. In addition, the power consumption of a TFT is dynamic because it uses more or less power depending on the image being displayed. Most TFT datasheets detail maximum power consumption or power consumption based on various tests, often in mosaic mode, designed to provide an average power rating. This information is not intended to discourage portable system designers from using TFTs; it's just another thing to consider in the selection process.
Some displays, usually small TFTs, require additional chips such as timing ICs, grayscale generators, and DC/DC converter circuits. In space-constrained applications such as PDAs and other handhelds, it makes sense to save space by removing the extra ICs from the TFT's module and combining them with other circuits or moving them to an ASIC or FPGA. Doing so allows for smaller, more compact TFT panels. This fact should be kept in mind when evaluating small TFTs. You may want to ask your display vendor if the TFT includes all the components or if you will need additional ICs beyond the usual TFT interface requirements.
Temperature and pressure are topics of concern in outdoor applications, avionics, or applications for extreme environments. TFTs and CCFL tubes can fail in whole or in part, or perform poorly, in heat, cold, or at pressures that deviate from normal. One thing that is important in applications involving extreme environments is to consult with the TFT manufacturer. While some panels are designed for a wide range of temperatures, such as those in automotive or handheld applications, very few datasheets give information about pressure extremes, such as suitability for unpressurized avionics applications. However, should you find the perfect display and it doesn't meet these needs, you can overcome the temperature and pressure issues by using heaters, coolers and pressurized enclosures.
Cables are an often overlooked TFT component. While TFT manufacturers have tried to establish standards for connectors, until recently there was no formal agreement or specification. Some TFT cables and connectors have high minimum order requirements because they are difficult to manufacture or require expensive equipment to populate these tiny, multi-pin connectors. A third-party cable manufacturer or electronics distributor can sometimes be helpful.
One notable source of standardization is the SPWG (Standards Panel Working Group, www.spwg.org), a collaborative group of companies that aims to introduce formal electromechanical standards for TFTs. The SPWG doesn't dictate optical or performance characteristics, leaving those to the individual TFT manufacturers to deal with, but rather it supports all TFT manufacturers to The SPWG does not specify optical or performance characteristics, leaving these to the individual TFT manufacturers, and instead supports the efforts of all TFT manufacturers to reduce costs and maintain portability in their designs.
Most electronics distributors work closely with "third-party enhancement" companies to provide and coordinate accessories, cable kits, and enhancements for the TFTs they sell. Many distributors also have dedicated departments or business units that work with customers, TFT manufacturers, and third-party organizations to provide the best possible service when selecting and using TFTs.
Why shell out money for TFTs when you can buy a complete LCD monitor for less and eliminate the various parts you would otherwise need? For one thing, LCD monitor manufacturers either make their own TFTs or buy them in bulk, which reduces costs. Since LCD monitor suppliers are constantly redesigning their products to reduce costs and add features, there is no guarantee that the LCD monitor you are using now will come with the same parts in the future. Also, consider the time and labor involved in removing the TFT from the monitor in good condition and reusing it. Finally, if you do have a problem or issue, there is little or no way to do quality control, get an RMA (return material authorization) for the defective panel, or replace the material.
While TFTs are not the ultimate display technology, they are popular, readily available, cost-effective, and easy to select and use in a design, making them a good choice.