LCD manufacturers have been fairly slow to introduce LED-backlit LCD panels into the high-performance and high-reliability markets, especially for larger display formats. However, there are solutions available today for these larger panels with edge-lit or direct-view LCD-backlight retrofits. This article will examine several LED backlighting options currently commercially available to today's engineers.
by Suzanne Thomas and Stephen Soos
ALTHOUGH THE SHIFT from cold-cathode fluorescent lamps (CCFLs) to solid-state lighting as the primary choice in liquid-crystal-display (LCD) backlighting is only in its early stages, the transition is gaining momentum and the shift seems inevitable. Due to their superior luminance and power efficiency, light-emitting diodes (LEDs) are apt to continue to grow in popularity at a rapid pace.
According to iSuppli Corp., revenues for LEDs are climbing rapidly, driven by strong demand for LEDs as backlight units (BLUs) in keypads and mobile displays. Use of LED BLUs for notebook computers finally seems to be ramping up. Additionally, LED backlights have become ubiquitous in LCDs targeted at the industrial marketplace; however, the majority of these to date are less than 6 in.on the diagonal.
For system designers considering larger display formats (6–24-in. diagonals) for industrial, marine, military/avionics, or other higher-performance and higher-reliability applications, there are a limited number of LED options available on the market today.
This article will discuss several LED-backlighting options commercially available to today's engineers, including the relative merits and challenges associated with each option. Considerations for choosing an LED driver will also be discussed.
CCFL vs. LED
To date, the traditional backlight for LCDs have been CCFLs. While there are many differences between CCFLs and LEDs, as shown in Table 1, there is also one very significant similarity. Although the levels of voltage and current differ between the two technologies, both require a constant-current driving method for optimum performance.
LEDs offer many possible formats for backlighting LCD panels, such as direct-view or edge-lit and white or RGB, and the advantages of using LEDs are numerous, including
• Solid-state devices offer high reliability and long lifetime (50,000+ hours).
• Lower operating DC voltage.
• Improved color gamut.
• Suitable for wide-temperature-range conditions, no warm-up period required.
• Compact, flexible, and mechanically robust.
• Mercury- and lead-free, RoHS compliant.
• Electrical-to-light efficiency exceeds that of CCFLs (when effectively coupled to the light guide).
Comparing LED technology to CCFL technology is akin to comparing transistors to vacuum tubes. LEDs provide a compact, solid-state solution that operates effectively at lower voltage levels and commensurately higher currents than CCFLs. A typical individual CCFL might specify a lamp current of 5–6 mArms with a lamp sustaining voltage of 500–750 Vrms, which implies a consumed power of 2.5–4.5 W per lamp. A single Luxeon III LED can be driven (with suitable thermal management) at 1.0–1.5 A. With typically 3.0 V of forward voltage drop, this implies 3.0–4.5 W of consumed power.
An LED provides a predominately fixed voltage drop (Vfwd) over a specified range of drive current levels (Ifwd). As shown in Fig. 1, this implies a negative resistance because as Ifwd increases, the impedance of the diode must decrease in order to hold Vfwd constant (per Ohm's Law,E=IR).
It is therefore necessary, as in the CCFL world, to provide current-limiting or current control when powering LEDs. Spacing and insulation concerns associated with CCFL backlight designs are practically eliminated because LEDsrequire significantly lower operating voltages and do not require an ignition voltage like CCFLs.
When considering the driver from a constant-current perspective, it is evident that voltage must still be developed to move the current, but once it reaches the level where the desired current is satisfied, it will then stabilize. In this case, the stabilization point will be the total forward drop of all the LEDs connected in series.
The number of LEDs that could be connected in series would only be limited by the maximum voltage that could be generated by the LED driver. It is this generated voltage that, in turn, drives the LEDs to the desired amount of current.
In the past, when designing an LCD into a system, designers often would oversimplify or underestimate the challenges associated with integrating the CCFL backlight inverter. This was problematic due to the high-voltage nature of the CCFL inverter, and the tendency of this device to be noisy, to interfere with other electronics, and, at times, even cause visual anomalies on the LCD.
Surprisingly, though, driving LEDs presents system designers with some of the same problems, plus other unique challenges. Driving LEDs is not as simple as some may believe.
In the simplest application of LEDs, users could drive up to three nominal +3 Vfwd LEDs in series by simply using the native system voltage, +12 Vdc. The remaining +3 V would have to be reserved for a current-limiting resistor. This approach, albeit an inefficient one, might be very practical in low-power LED applications, such as small-format LED-backlit displays. Dimming and other control functions could be added as required.
In order to reach standard-, high-luminance, and ultra-high-luminance requirements for mid–to–large-format LCD panels, larger numbers of LEDs will be required, which means higher power. A more sophisticated constant-current-source approach is needed to efficiently supply appreciable amounts of power to these LEDs. Today, designers can choose from numerous IC LED drivers or a complete, integrated LED driver board.
IC or Complete Integrated LED Driver?
While few suppliers offer integrated, full-function stand-alone LED drivers, there seems to be a vast number of IC or chip-style LED drivers by comparison. This might create the perception that IC or chip-style LED drivers must be the ideal choice, but that is not necessarily the case. There are several key LED driver features to consider when deciding whether to use an IC to design a custom driver or an off-the-shelf integrated LED driver board. Some of these issues are
• Spatial Constraints: Does the application have a printed-circuit board or motherboard where the LED driver circuitry can reside and is there room to locate the circuitry on that board or will a separate PC board be required anyway?
Fig. 1: High-luminance LED impedance vs. drive current.
• Development Costs: If there is a board on which the LED drive circuitry can reside, is there engineering expertise and resources in-house to design a circuit around chip-style LED driver(s)? Typically, these chips contain just the "smarts" or control functionality, and the rest of the circuit must be designed and verified. Although many IC-driver manufacturers provide schematics, the economics of the design program or other factors may make an organic implementation undesirable for many companies. For applications involving multiple parallel banks of series-connected LEDs, some of these circuits can have many components and be quite complex.
• Life-Cycle Support: Is there value in having a fully tested and field-proven stand-alone LED driver design as well as the engineering and technical support from a reliable supplier? Remember, LED drivers are still power supplies. Most system designers would not entertain the idea of designing their own system power supply and, ironically, LED drivers require probably more sophistication than the power supply that is running the entire system. Without having power-supply experience, it is possible to run into problems after having thousands of units fielded.
• Driver Input Voltage: The LED driver must be able to run from a DC voltage that is readily available in the system. Some companies are promoting drivers based on a buck circuit topology, where the supply voltage for the LED drivers must be greater than the minimum required to power the LED string (or the total forward voltage drop of all the LEDs connected in series). In many cases, voltages required are 48 Vdc or higher, which in most systems are just not available.
• Driver Forward-Voltage Capability: One weakness of the IC or chip-style LED driver is that it is limited in the voltage that it can develop, which causes limitations in the number of LEDs that can be powered from a single IC or chip-style driver. For mid–to–large-format LCD panels, larger numbers of LEDs will be required, which would then necessitate multiple ICs and result in a highly complex circuit design and implementation of several parallel banks.
• Interconnection of Multiple LEDs: As the size of the display increases, the number of LEDs that are required also increases. In order to maintain some semblance of control of the number of wires and interconnections, the driver(s) architecture must be capable of efficiently driving multiple parallel banks of series connected LEDs, or be capable of developing higher voltages for a single but larger group of series-connected LEDs.
• Overall Solution Requirements: A full-function LED driver board, as shown in Fig. 2, may provide the flexibility to be used for LED-backlight retrofits (edge or direct view), as well as for OEM-factory LED-backlit panels. Additionally, a full-function driver may also provide enhanced control features such as wide dimming range, NVIS control capability, ambient-light sensing, and luminance feedback, which are often required for LCD systems today. Additional components and circuitry would be required to support these functions using a chip-style driver in most cases.
Configuring LEDs for Backlighting
LED-based backlights are still too new to have any interconnection standardization. The number of LEDs used will depend upon the size of the electrical and optical performance of the LEDs themselves, the LCD panel performance, luminance required, and thermal-management issues.
Interestingly, we have observed a "chicken and egg" scenario relative to the way LCD OEMs and others configure LEDs and the overwhelming number of LED IC or chip-style LED drivers as described above.
Many factory LED-backlit panels have the LED backlight configured in multiple parallel banks of series-connected LEDs, and it appears that these configurations are often chosen in order to use the IC or chip-style LED drivers, where the total forward-voltage drop of all the LEDs connected in series must not exceed that which can be generated by the chip driver. Multiple channels or banks are valuable for their redundancy; if one bank goes out (the LED fails or the driver fails), the panel remains usable. However, if they are employed due to limitations of the LED driver circuitry, multiple channel drivers create unnessessary complexity.
Fig. 2: Complete, integrated LED driver boards can be used for LED retrofit backlights and for OEM LED-backlit panels.
It would seem to make more sense to design an LED backlight to reach specific luminance, optical, and thermal performance goals, and then choose an integrated LED driver which has the capability to drive it, rather than to allow an inherent weakness (i.e., limited voltage capability) of the chip-style LED driver to dictate the backlight design.
As an example, one supplier offers a 12.1-in. panel that utilizes 24 white LEDs connected in four banks of six each, as shown in Fig. 3.
The backlight requires 23 W of LED power (approximately 1 W per LED) to achieve 1000 cd/m2. Each LED drops by approximately +3 V for a combined drop of +18 V per bank. The connections to these banks are brought out such that four independent constant-current drive sources are needed to effectively drive the panel. Each bank would consume 6 W of power at 0.33 A of drive current into the LEDs.
Making a simple change in the way the LEDs are wired internally to either two banks of 12 or one bank of 24 in series would have several advantages:
• Driver circuitry could be scaled to the appropriate power level required, thereby reducing the number of banks required and the cost associated with multiple drivers.
• Driving all of the LEDs in series allows them to be inherently driven by the same magnitude of current.
In Fig. 4, 30 white LEDs were re-wired to a single series-connected bank. In its new configuration, the backlight required just 9 W to achieve 1000 cd/m2.
One of the advantages of a CCFL is that it provides a continuous length of light with the electrical terminations at the ends. When using LEDs in an edge-lit configuration, as shown in Fig. 4, wiring all of the LEDs in series from left to right or top to bottom allows for a simple two-wire interconnection. Depending upon how the LEDs need to be mounted within the display to obtain the best optical coupling to the light guide, the ability to string all of the LEDs in series with the electrical terminations at the end could provide a significant packaging advantage.
LED Solution Options: As previously mentioned, although limited, there are LED options commercially available to system designers today for larger display formats (6–24 in. on the diagonal).
LCD Panels with Stock LED Backlights
A number of LCD manufacturers now offer panels with LED backlights from the factory. The best case scenario for a system designer would be to find the right-sized panel that meets all of the performance specifications for the application and which comes with a factory LED backlight that is capable of meeting or exceeding the desired luminance for the application. This would likely be an excellent option from a cost standpoint.
Fig. 3: An example of multiple parallel banks of series-connected LEDs.
Fig. 4: An example of 30 white LEDs connected in series with a complete full-function driver.
Unfortunately, this is the exception. Few panels today are readily available for any larger-sized formats – the majority are 6 in. on the diagonal and smaller – in mass-production quantities. Standard luminance and high luminance (up to approximately 600 or 650 cd/m2) are possible to achieve with these panels, but ultra-high luminance (>800 cd/m2) is not.
LED drivers will still be required for the vast majority of these factory LED-backlit panels. As mentioned above, one challenge to LED-driver suppliers is the lack of standardization in LED-backlight configuration and LED connectivity, both inside and outside these panels. Inside the backlight, some similarities exist in the way panel manufacturers configure the LEDs, which tends to be in multiple banks of series-connected LEDs, but that is where the similarities end. Some manufacturers choose a common anode connection (Fig. 5), while others choose a common cathode configuration (Fig. 6). Other panels bring out a separate cathode and anode connection for each bank, as shown in Fig. 7.
Outside the backlight, some panels have a separate, dedicated connector for the LEDs, while other panels house the LED connections on the same connector as the display data, power, and other signals.
This lack of standardization seriously challenges the ability of system designers to address connectivity of LED drivers.
Over time, we will see increasing numbers of panels with stock LED backlights from the LCD-panel manufacturers. Certainly, as the LED device efficiencies continue to improve over time, greater and greater luminance levels will be met with the factory LED-backlit panels. Until then, suppliers are offering retrofits to existing LCD panels in various configurations as described in the next section.
Retrofitting Existing Display Backlights
The first option to be considered is retro-fitting the LCD backlight from the stock edge-lit CCFL to edge-lit high-brightness LEDs (white or RGB), as pictured in Fig. 8.
This approach is feasible and commonplace for panels ranging in size from 6.4 to 24 in. In fact, many manufacturers offer edge-lit panels with field-replaceable CCFLs, making a backlight retrofit fairly simple and minimally invasive to the panel itself. In most designs, the stock optics of the panels can remain unchanged.
As shown in Table 2, edge lighting with white LEDs vs. RGB LEDs presents both similar and dissimilar challenges. Both need effective diffusion and coupling to the display light guide, but RGB LEDs also require color sensors and color mixing, making the electrical-drive requirements quite complex. In addition, wiring of an RGB edge-lit LED might not be feasible for all panels given the limited space available in the former CCFL channel.
Although commercially available LEDs can achieve 70–90 lum/W, designers cannot underestimate how critical it is to efficiently couple the LEDs to the light guide in the panel. Using the brightest LEDs in the world is moot if you cannot effectively get the light where you need it. Suppliers have been able to overcome this challenge by offering complete LED solutions, as described below.
Some companies are offering generic LED strips or rails, in standard sizes such as 10.4 and 12.1 in. These rails are intended for use with a broad variety of panels from numerous manufacturers. Since it is critical to achieve the most efficient coupling to the light guide as possible, is becomes apparent that this approach has some inherent weaknesses. A generic rail might be a great fit for one or two panels, but might offer poor performance with many other panels. This approach also does not offer the thermal, diffusion, or spacer materials that might be necessary to the design.
Others companies are taking a semicustom approach, tailoring each LED rail (or rails) and the associated LED drivers to a specific panel or group of panels. The result is a highly efficient solution, offering a very significant improvement in power-in to light-out efficiency over the stock CCFL backlight (as described in Fig. 1). Due in part to the effective coupling of the LEDs to the light guide, these solutions typically can offer at least a 2:1 improvement over the stock CCFL backlight for panels up to 18 in. on the diagonal, which means that system designers can expect to double the luminance for the same amount of power consumed (or vice versa – they can achieve the same luminance while consuming half the power).
Fig. 5: Common-anode configuration.
Let us consider a specific example. We used a 12.1-in. SVGA TFT-LCD panel whose stock backlight consisted of two CCFL lamps, both oriented along the top edge of the panel. The luminance advertised by the LCD manufacturer for this configuration was 400 cd/m2 typical, when the CCFL lamps were driven to a nominal 5-mArms current per lamp. This luminance level was verified, and power con-sumption was measured at 6 W of CCFL power.
The CCFL backlight was then modified with a high-performance LED rail consisting of 31 series-connected white LEDs, plus a matched LED driver board. All of the panel's stock optics remained the same. The resultingperformance of the LED backlight is as follows:
• 400 cd/m2 with 2.9 W of LED power,
• 700 cd/m2 with 5.6 W of LED power,
• 1000 cd/m2 with 9.0 W of LED power.
Clearly, the LED-backlight performance exceeded that of the standard CCFL-equipped panel by more than 2 to 1. In fact, LED-backlight modifications on a range of panels (6.4–15 in. on the diagonal) from the same LCD manufacturer achieved similar performance improvements versus the stock CCFL backlights, as illustrated in Fig. 9. Comparable results have been found with panels from other LCD manufacturers as well.
For panels larger than 18 in. on the diagonal, LED-backlight retrofits can produce more modest improvements such as 1–1.5:1 improvement over the stock CCFL backlight.
Other advantages of the semicustom approach to edge-lit LED-backlight solutions include
• Support of standard-, high-, and ultra-high-luminance requirements, often using the identical LED rail(s) and driver(s).
• Allows for an open or short of one or more LEDs, without affecting the rest of the LEDs in the rail.
• Produces better luminance uniformity across the face of the LCD because the luminance variation across the length of the channel tends to be more consistent for an LED than for a CCFL.
• Withstands shock and vibration in an edge-lit configuration equal to or better than the CCFLs being replaced.
• Offers a quick design-to-market cycle because none of the display mechanicals or optics need to change, so little to no tooling cost or time is required.
Drawbacks to the edge-lit LED retrofit approach include
• High cost of high-brightness LEDs.
• Limited space available in the former CCFL channel offers limited design choices and flexibility.
• Stock CCFL backlights are discarded.
A second backlight retrofit option is modifying the LCD with a direct-view LED back-
light (white or RGB), or with an LED BLU, which consists of optical films, a light guide, and edge-lit LED assemblies. These options are quite similar to existing aftermarket high-brightness CCFL backlights that have historically been used for achieving ultra-high luminance (>800 cd/m2).
Fig. 7: Independent connections.
Fig. 8: Edge-lit LED solution with an LED driver.
Drawbacks to this approach include
• Panels must be completely disassembled in order to use this approach, making it a much more invasive modification option.
• Presents a greater optical design challenge due to the point-source light nature of the LEDs.
• Requires an increase in thickness in the Z dimension to assure uniformity of luminance across the face of the LCD panel.
• Many optical components of the stock LCD are discarded, including CCFLs, light guide, and films.
• A direct-view LED backlight will cost more than an edge-lit backlight, will have a longer design-to-market cycle, and may involve significant tooling costs and time.
• LCD-manufacturer warranty will be void.
If there is a compelling reason to use RGB LEDs, this approach might be advantageous, as the disassembly of the panel and the resultant increased depth to achieve luminance uniformity also makes space available for the complex wiring required. Otherwise, the direct-view backlight seems to be a brute force approach that is largely unnecessary because luminance and optical goals have been shown to be achievable employing an edge-lit approach.
Fig. 9: Comparison of stock CCFL and LED retrofit backlight luminous efficiencies.
Regardless of which backlight retrofit method is employed, thermal management is a significant challenge for high- or ultra-high-luminance applications, especially when the display product is located in a high-operating-temperature environment because the performance and life of LEDs degrade in high-temperature conditions. For some LEDsolution suppliers, good thermal performance is more a result of the configuration of the LEDs, proprietary drive techniques, and driver technology than the use of traditional thermal-management techniques such as heat-sinking and convection, although these methods could also be employed in extreme applications. A thermal-management problem does not have to be solved if it does not exist to begin with.
Although beyond the scope of this article, there are a number of additional LED design considerations that should not be overlooked, such as the challenges of color chromaticity and differential aging, as well as the use of various optical feedback and color-management methods, and the assessment of high-power low-density vs. low-power high-density approaches.
LCD manufacturers have been fairly slow to introduce LED-backlit LCD panels to address the needs of industrial, military, avionics, and other high-performance and high-reliability markets, especially in larger display formats.
Despite this, a number of suppliers today offer LED-backlight solutions for these larger panels with edge-lit or direct-view LCD-backlight retrofits, or designers can choose to design their own LED-backlight solution.
Although LEDs are becoming the backlight technology of choice, offering numerous benefits, there are still some significant challenges for system designers to overcome which include
• The cost is still higher than that for CCFLs. Designers must find a balance between the desire to meet optical and luminance goals and keeping the LED count as low as possible.
• Thermal management is a major challenge for high- or ultra-high-luminance applications, especially when the display product is in a high-operating-temperature environment because the performance and life of LEDs degrade in high-temperature conditions.
• Complexity and cost of system design can vary significantly depending on the choice of LEDs (white vs. RGB) and configuration (series connected LEDs vs. multiple parallel banks of LEDs).
• LED drivers are a necessity whose complexity should not be minimized. •