No one likes the recession, but Hollywood may be an exception. why? Even if the answer is not so obvious, it is very simple. When the economy declines, the box office income will rise because consumers seek a more "economic" way of entertainment. It’s much cheaper to go out to the movies with the whole family than to go to a restaurant for dinner and then watch Broadway shows at the theatre.
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Perhaps unexpectedly, another form of symbiotic and reciprocal entertainment, which is currently having a positive impact on the sales of flat-panel LCD TVs that use LEDs for backlighting during the current economic downturn. Many consumers are not going to the cinema, but are happy to rent or download the recently released movie DVD and watch it comfortably in the living room. However, the difference this time is that they want to look a bit more, using the latest LED high-definition (HD) TVs. According to a recent survey by iSuppli, in the first quarter of 2009, the delivery of flat-panel TVs in the US and Canada increased to 7.8 million units, an increase of 17.3% from 6.6 million units in the same period in 2008. In addition, the survey also pointed out that even during the current economic downturn, consumers will continue to buy flat-panel TVs. The only caveat is that as disposable incomes fall, American consumers are being attracted to smaller TV sets. As a result, 32-inch and smaller TV sets have encountered supply limitations.
White LEDs for Backlighting To understand the driving forces behind the popularity of flat-panel LCD TVs with white LED backlighting, it is important to understand some of the following facts: compared to other forms of lighting, such as cold cathode fluorescent lamps (CCFL) ), why should white LEDs be used.
Large flat panel LCD TVs have traditionally used CCFL backlighting. However, these CCFL backlighting televisions have various shortcomings such as fast moving image blurring and inaccurate color reproduction. For the current generation of LCD TVs, it is impossible to get true black, and there is room for improvement in the dynamic range of all colors. For example, most LCD TVs offer contrast ratios of 450 to 650 cd/m2. The main problem with these HDTVs is that CCFL backlighting cannot be completely turned off or local dimming of CCFL backlighting is not possible.
In contrast, with high-brightness (HB) LED backlighting, an array of LEDs can be divided into many separate lighting groups. Each group typically consists of 8 to 10 30 to 50 mA LEDs that can be independently dimmed by a single pin for local dimming. This design provides an order of magnitude (>4 000 cd/m2) that is almost an order of magnitude higher than the CCFL design. In addition, by adjusting the brightness of the backlighting LED group with a wide dimming ratio, more midtone colors can be reproduced, thereby providing a more vivid picture.
In addition, the ability to partially turn off the LEDs can reduce motion blur. By completely turning off the LEDs between frames, image blur associated with fast moving objects is virtually eliminated. In solving this fast moving image blur problem, the very fast response speed of the LED is crucial, and CCFL backlighting LCD TVs often encounter this problem.
Another driving force behind the removal of CCFLs is the desire to eliminate toxic mercury vapors in this type of illumination while improving efficiency and performance. In addition, governments in many countries have helped push this trend, and these countries have imposed new “green†measures to eliminate toxic materials, including mercury found in CCFLs. Clearly, this has helped drive LED backlighting to play an important role in the design of new LCD HDTVs.
It is estimated that from the traditional fluorescent lamp to LED lighting, the total energy demand in the world may be reduced by up to 10%, which shows that LED is more environmentally friendly. Clearly, as people strive to accelerate the reduction of total carbon dioxide emissions from energy generation, this 10% reduction in global lighting energy demand is a major contributor to this transition.
So, what factors support such a huge LED backlighting growth potential? First, the current generation of LEDs is more than twice as efficient as fluorescent lamps, greatly reducing the power required to provide the required light output (measured in lumens). As LEDs continue to evolve, their luminous efficiency will continue to increase and is expected to double in the next few years. Secondly, LEDs have a lifetime of more than 100,000 hours. In contrast, fluorescent lamps have a lifespan of up to 10,000 hours and a life difference of up to 10 times.
How to drive LED
Driving the LED requires careful design considerations, as overdrive can create heat dissipation problems that can significantly reduce light output and shorten the useful life of the LED. This has led many analog IC manufacturers to design specialized LED driver ICs to drive LEDs correctly. In order to understand the challenges of designing and manufacturing these LED driver ICs, it is necessary to understand what is needed for white LED illumination. The white LED must be driven by a constant current source so that the white spot does not move, that is, the brightness must be uniform. In addition, since the white LED is a diode, its internal forward voltage (Vf) drop must be overcome. This Vf varies with the white LED current rating and will change with temperature. The Vf of a typical 20 mA white LED varies between 2.5V and 3.9V over the entire operating temperature range. Most applications use more than one white LED, and it is also possible to configure these LEDs in parallel, in series or in series (eg, parallel strings of series LEDs). This means that a white LED driver IC must be able to supply sufficient current and voltage for a particular configuration of LEDs, while using a conversion topology that meets both the input voltage range and the required output voltage and current requirements.
LED dimming considerations Traditionally, LED dimming has been performed with a DC signal or with a filtered PWM signal by adjusting the forward current flowing through the LED. Reducing the LED current adjusts the LED light output intensity, however, the change in forward current changes the color of the LED because the chromaticity of the LED changes as the current changes. Many applications, including LCD HDTV backlighting, are unlikely to allow any shift in LED color. In these applications, a wide dimming range is required because ambient light changes are different and the human eye is sensitive to small changes in light intensity. Controlling LED light intensity by using a PWM signal allows the LED to dim without changing color.
“True Color PWMTM Dimming†dims the LED with a PWM signal. It essentially turns the LED on and off at full PWM current at the PWM frequency. The human eye has a limit of 50 to 60 frames per second. By increasing the PWM frequency (for example, 80 to 100 Hz), the eye can be made to feel that the pulse source is continuously turned on. In addition, by modulating the duty cycle (the length of the "on time"), the intensity of the LED can be controlled. With this dimming method, the color of the LED remains the same because the LED current value is either zero or a constant value. Many LCD HDTV designers require a dimming ratio of more than 3000:1 to suit a variety of ambient lighting conditions.
An innovative LED driver for LCD HDTVs A new product from Linear Technology solves the design problems associated with driving white LEDs when white LEDs are used for backlighting large flat panel TVs. The LT3754 is an innovative LED driver IC for HDTVs with 26-inch or larger flat panels. This boost mode LED driver has 16 separate channels, each capable of driving up to 15 LEDs with 50mA and Vf of approximately 3.2V. Therefore, each LT3754 can drive up to 240 50mA white LEDs. As a result, a 26-inch LCD HDTV requires only one LT3754 to provide the necessary backlighting. All 16 channels are controlled by a single PWM input that achieves a PWM dimming ratio of up to 3000:1.
The LT3754 uses a small inductor or a slim ceramic output capacitor. The only other components required are an input capacitor, MOSFET, and a current setting resistor, as shown in Figure 1. Each channel follows a programmable main current to allow 10 to 50 mA of LED current per string. These channels can also be connected in parallel to provide greater LED current. The output voltage adapts to changes in the LED Vf to provide optimum efficiency, and open LED faults do not affect the operation of the connected LED string. The LT3754 is available in a compact 32-pin, 5mm x 5mm QFN package.
Figure 1 38W LED Driver
The LT3754 has 92% peak efficiency at 400kHz switching frequency, eliminating the need for any external heat sink. In addition, due to the switching frequency of up to 1MHz, the size of the required inductors and capacitors on the PCB is greatly reduced, thus allowing the use of very thin TV housings. This is a key factor, as most consumers want to install HDTVs on the walls of the living room, which requires a lightweight and slim form factor for aesthetic reasons.
The LT3754's PWM dimming capability is up to 3000:1. Figure 2 shows this 3000:1 PWM dimming waveform and an LED current square wave waveform. Even at a turn-on time of only 3.3 μs, the 20 mA LED current is turned on and off in synchronization with the 100 Hz PWM signal. A higher PWM dimming ratio can be achieved with a lower PWM frequency, but 100Hz guarantees no visible flicker.
Figure 2: 3000:1 PWM dimming waveform for the Figure 1 circuit
The full LED brightness of all 16 channels is set by a single external resistor. Each channel is programmed to have the same amount of LED current, set between 10 and 50 mA. The LED current accuracy between the channels is within ±2%. True color PWM dimming uses a reduced duty cycle to provide accurate dimming without any shift in the color of the emitted light. The fixed frequency, current mode control circuit operates stably over a wide range of input and output voltages and currents. The external power switch and exposed thermal pad in the 32-pin QFN package provide sufficient power and thermal management to handle the power and heat generated by the 16 channels at 50mA.
Conclusion Clearly, LED has become the mainstream choice for backlighting of large flat panel LCD displays in HDTVs. However, system designers still need LED driver ICs that meet their specific design performance requirements, otherwise they may end up in extreme dilemma. Therefore, the LED driver IC must be able to provide sufficient current and voltage for many different types of LED configurations with a conversion topology that satisfies both the input voltage range and the desired output voltage and current requirements. Therefore, LED driver ICs need to have the following features to meet the needs of designers.
â— Wide input voltage range
â— Wide output voltage range
â— High efficiency conversion
â— Strictly regulated LED current matching
â— Low noise, constant frequency operation
â— High PWM dimming ratio
â— Small and compact footprint, requiring minimal external components
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