After the liquid crystal display
(LCD) itself, the backlight is the most important element affecting quality and consistency of the displayed image. The LCD backlight can be one or more cold-cathode fluorescent lamps (CCFLs) mounted along the edges of the LCD or spaced uniformly over the back of the display, or it can be a string of light-emitting diodes (LEDs) arranged along the edges of the LCD or as a matrix over the back of the LCD assembly. The typical LCD
backlight has traditionally been CCFL.
Increasingly, however, LEDs are becoming the backlighting technology of choice in LCDs used in industrial, military, medical, marine, ATM, gas pump, and a wide variety of other applications. This article addresses the factors that need to be taken into consideration when driving LED backlights in order to achieve the benefits (such as higher brightness and longer life) while avoiding the pitfalls such as overheating the LEDs, or replacing the CCF lamps and inverter, without having to redesign everything.
LEDs are already commonly used to backlight LCDs used in a wide range of smaller displays, such as portables, handheld devices and notebook PCs. Today, the use of LEDs in LCD backlights is rapidly growing from the smaller LCD panels to applications across the entire spectrum of LCD sizes, such as industrial LCDs in the 5.7" to 23" size range. LEDs are being increasingly used in place of CCFLs for reasons including CCFLs' higher power consumption and, in some cases, mercury content.
Switching from CCFL to LED backlighting raises a number of issues which need to be addressed, such as input voltage, operating temperature, thermal management, and dimming. This article addresses selection criteria for drivers for LED-based LCD backlights, or backlighting units (BLUs), versus the familiar CCFL backlights, and what to be on the lookout for as the transition is made.
The new generation of high-bright LEDs (HBLEDs) provides higher brightness, as well as higher reliability, higher efficiency (lumens/watt), better dimming, longer life, and operation over a wider temperature range than do CCFLs. However, CCFLs are still the backlight of choice for a wide range of applications that do not require the advantages of LEDs or for those whose users do not need or want to make the capital investment of switching to LED BLUs at this time.
This can also be true of applications demanding absolutely high reliability, where vacuum-encapsulated inverters can be used to ensure reliable CCFL ignition, even in harsh environments where shock, vibration, humidity and extreme cold or heat are present. LED and CCFL backlights can and will co-exist for some time yet, and predictions that use of CCFLs may decline as rapidly as CRTs are probably somewhat overstated. Nonetheless, LCD backlighting is becoming a hot area of business opportunity for LED and LED BLU manufacturers and will gradually gain ground over CCFLs as the price/performance ratio of LEDs continues to improve.
LEDs create new challenges for the backlight driver, challenges that cannot always be completely met by the many single-chip IC solutions available on the market, despite wishes to the contrary. Getting optimum performance from LED BLUs often requires a full-function LED driver board, Figure 1.
Figure 1: LED driver boards are now available as standard product that can be used with virtually all major OEM LCD panels.
(Click on image to enlarge)
In such a board, the dc/dc converter portion supplies power while the rest of the on-board circuitry provides wide-range dimming, wide input voltage, and full brightness and enable control, while maintaining constant current over a wide input range to ensure brightness stability. It's more than a chip, and it's more than a power supply. It's an integrated, plug-and-play driver solution.
The LED Backlight
LEDs for LCD backlighting may be arranged along the edges of the LCD, or as a matrix over the back of the LCD assembly. The LEDs may be electrically connected in series or parallel; either configuration will provide uniform LCD lighting. LED strings arranged in parallel using a series resistor in each string provide string-to-string current balancing as well as lighting redundancy.
Unlike a CCFL, LED backlights don't require high ac voltages; therefore, they don't require an inverter. The basic LED driver is powered by 5 to 48 Vdc and uses a dc/dc boost converter to provide voltage to a constant-current driver that drives the LED string. For quality performance, a constant-current driver is required to compensate for LED voltage drops and changes with temperature, to ensure stable light output.
Selecting a LED Driver
LEDs are semiconductors with light-emitting junctions that are designed to use low-voltage, constant-current dc power to produce light; .since LEDs have polarity, the current only flows in one direction. Unlike fluorescent or discharge lamps, LEDs do not require an ignition voltage to start. However, too little current will result in little or no light, and too much current can damage the light-emitting junction of the LED diode.
For a given temperature, a small change in forward voltage produces a disproportionately large change in forward current. In addition, the forward voltage required to achieve a desired light output can vary with LED die size, LED die material, LED die lot variations, and temperature.
An IC-chip-based driver may seem to be a good choice for powering a LED backlight, but consider the voltage the device needs across it to accurately regulate current. Is there enough voltage across the device remaining to light the LED rail and provide proper current regulation? At what temperature are the LED voltages specified?
Let's say it is 25° C, or approximately 77° F (LED die wavelength characteristics are commonly reported at 25° C junction temperatures). If you were to take the LED string below 25° C (unlit) and then power the string, the LEDs may not light at all because the LED string voltage is greater.
It is important to select a LED driver that is designed to account for this voltage change and can light across the entire normal operating range of temperatures, with no time or expense devoted to designing a boost circuit. As with dc/ac inverters used for CCFL or EL backlights (or any other power supply), properly selecting a LED driver during the design phase of the display will help to avoid common pitfalls such as brightness instability.
Operating Temperature: The ambient temperature in which the LCD operates is a key consideration in the selection of the backlight driver. Although the performance of LED backlights is less sensitive to low temperatures, the high application temperatures have the most significant impact on LED function and reliability, compared all other variables. Recent advances in LED technology, packaging and materials have provided dramatic increases in LED brightness which, in turn, leads to increased LED temperature.
As LEDs heat up, the forward voltage drops and the current passing through the LED increases. The increased current generates additional heating of the junction. If nothing limits the current, the junction will fail due to the heat, a phenomenon referred to as thermal runaway, Figure 2.
Figure 2: Recent advances in LED technology have provided dramatic increases in brightness which, in turn, has led to HBLEDs running hotter.
By driving LED light sources with a regulated, constant-current driver, the light output variation and lifetime issues resulting from voltage variation and voltage changes can be eliminated. Therefore, constant-current drivers are generally recommended for powering LED light sources.
Thermal Management: The major challenge for HBLED backlights is to get the heat out of the LED device itself and then out of the display assembly. The key design point is to keep the LED junction temperature below its specified maximum junction temperature, to ensure reliability for the increasingly stringent demands of most LCD applications.
Removing the heat from LEDs is more difficult than removing heat from CCFLs because the heat is concentrated in the LED footprint which, at about 1/3 the size of a pencil eraser, is very small compared to external surface area of a CCFL. CCFL heat is simply radiated from the CCFL surface into the air. In contrast, heat removal from LEDs must be done by conducting the heat through the LED structure and through other materials to the metal display case, which will radiate the heat into the air.
Thus, LED cooling is more complex and the thermal design must include materials with good thermal conductivity, in proper physical contact with each other. Essentially, the PCB to which the LEDs are mounted, the rail to which the PCB is mounted, and the display case to which the rail is mounted must all operate efficiently to get the heat out. The efficiency with which the heat is removed (to stay below the specified maximum junction temperature of the LED) limits the amount of power with which LEDs can be driven and, consequently, limits brightness to some extent.
Some users are simply driving the LEDs as hard as they want to without regard for the junction temperature control and thus are running their backlights very hot. One customer reported that his LEDs had "turned brown", i.e., he fried them. Not a good thing! One solution for heat management is to use an LED rail which incorporates integral thermal management, Figure 3.
Figure 3: Special rails with efficient thermal management that gets the heat out of the LEDs are available as drop-in replacements for CCFL rails
(Click on image to enlarge)
These special rails use a design where the LEDs are put on a long, narrow PC board that fits into a metal channel or "rail" that is similar to the channel into which CCFLs are commonly fitted. The thermal management technology used inside the rails addresses the challenge of keeping the LEDs cool and preventing overheating. It is a technologically more efficient way to conduct heat from the LEDs and keep the junction temperature at or below specification, which is critical to preventing overheating and ensuring cool, high-brightness, long-life operation of the LED BLU.
Dimming: LCD applications requiring a wide range of brightness are constantly increasing. The driver must be capable of providing high brightness for daylight vision and low brightness for night vision. This again points to the need in many cases for a full-function driver board, not just a driver IC. Brightness control across this wide requirement range must be smooth and flicker-free.
Much higher dimming ratios can be achieved with LED backlights than with CCFLs because the basic switching time of an LED is measured in nanoseconds as compared to milliseconds for a CCFL. Although acceptable at one time, the limitations of analog dimming no longer meet most application requirements for LED BLUs. LED backlights can be analog dimmed, but this dimming scheme will not provide the high dimming ratios required by many of the more-demanding LCD applications, and also will produce varying color temperatures.
LED backlights are best dimmed using PWM (pulse width modulation) dimming, which provides significantly better dimming control where the duty cycle of the light source (in this case the LED) is modulated. In this type of dimming, the LED is pulsed ON and OFF at a fixed frequency, and the modulation of the duty cycle provides the variable brightness.
Mechanical Considerations: Since the driver board is proportionately similar to an inverter board, and the LED rail can fit down the same stock channel that the CCFL tube does, this is really not a major issue in terms of physically replacing a CCFL backlight and inverter with an LED driver board and rail. To put it another way, you don't have to re-invent the wheel here, which is always a nice thing!
The Driver Board
Figure 4 shows a constant-current chopper-driver schematic, which provides a dc current with 10% ripple to a LED string used to edge-light an LCD.
Figure 4: LED driver-board schematic diagram.
(Click on image to enlarge)
The pass-switching device is a P-channel FET which provides the current to the LED string and, in conjunction with the inductor, sense resistor, and boost voltage, establishes the chopping current and frequency.
The dc/dc boost stage is a closed-loop boost supply which provides sufficient voltage to drive current to the LED string with at least 2 V of headroom. The part of the diagram designated Section A shows a comparator and associated resistors which form a positive hysteresis circuit. It compares the voltage across the sense resistor to a known reference. Section B shows another comparator and associated resistors that buffer the Section A output to ensure proper hysteresis and provide drive to the pass device.
Section C supplies LED on/off and dimming control. The +ENABLE input turns the backlight on or off, and +PW pulse-width modulates the chopper driver on and off for dimming.
So, why a driver board and not an IC alone? The basic reasons are:
- The ability to handle wide input-voltage ranges (typically 8 to 24 V) and wide operating-temperature ranges in order to account for voltage changes and accurately regulate current to maintain brightness stability and reliability
- Separate brightness and enable controls
- The capability to provide on-board PWM dimming ratios of 1,000:1 and external PWM dimming to a ratio of 5,000:1 or, for higher-power HBLED arrays, as high as 20,000:1
- Design flexibility and ease of integration which permits use with existing OEM LED backlights or LED rails with integral thermal management
The driver board operates entirely on dc, performing dc/dc conversion onboard, with no ac involved. Therefore, it can take the input power of virtually any dc power source, whether regulated or unregulated voltage, from whatever power bus is available, and regulate the output power. This opens the door to applications beyond backlighting, such as general lighting or illumination, architectural lighting, automotive and aircraft lighting or anywhere an LED string or array can be used.
About the authors
Tom Novitsky is Vice President of Engineering at Endicott Research Group, Inc., Endicott, NY, and holds a BS degree in physics from Penn State University.
Bill Abbott is Director of Sales and Marketing at Endicott Research Group, Inc. and holds a BS Degree in Electrical Engineering from the Watson School of Engineering at Binghamton University.