Large-size LED backlight applications are accelerating, and chip manufacturers are responding with multiple solutions

2010 is an important year for the development of large-size LED backlights. The advantages of LED backlights, such as ultra-thinness, energy conservation and environmental protection, and high color quality, have become a strong driving force for the development of backlights. According to the forecast data of research institutions, in 2010, the global sales volume of LED LCD TVs will reach 32 million units, accounting for about 19% of all LCD TVs; in 2011, the shipment ratio of large-size panels using LED backlights will reach 56%, and the shipment volume of traditional backlight panels will decline to 44%; by 2015, the shipment penetration rate of LED panels will reach 78%. In the Chinese flat panel market, LED will completely surpass CCFL ordinary LCD TVs in the next two years.

Zheng Zhaoxiong, senior director of global sales and marketing of semiconductor power and portable products at OnSemi, said that traditional LCD TVs use cold cathode fluorescent lamps (CCFL) backlights, which generally boost the PFC voltage to more than 1,000 V or even 2,000 V AC high voltage after 24 V conversion (traditional power supply) or without 24 V conversion (LIPS power supply) to drive CCFL tubes in strip or U shapes; LED-backlit LCD TVs use several LED light strips, mainly using edge-light or backlight solutions for backlighting, so an important requirement for power management is to provide high energy efficiency; in addition, taking TVs as an example, since LED-backlit LCD TVs are sold in ultra-thin appearance, their power management solutions must have a low height to match the slim design, while also taking into account heat dissipation, reducing electromagnetic interference, matching brightness of different channels, dimming , and fault diagnosis and protection.

“As mentioned above, these conditions require semiconductor devices and solutions to provide high energy efficiency, use smaller (especially low-height) components to match slimmer designs, while taking into account improved heat dissipation, reduced electromagnetic interference, high brightness matching (to provide backlight with consistent brightness), dimming, diagnostics and protection, etc.” Zheng Zhaoxiong added.

Currently, ON Semiconductor can provide both edge-lit and direct-lit LED backlight solutions. Taking the edge-lit solution as an example, ON Semiconductor provides a cost-effective multi-channel linear LED side-lit solution, which uses the NCP1397 primary-side controller and 500 V high-voltage MOSFET NDF05N50 in the half-bridge conversion stage, and uses the 6-channel linear LED driver CAT4026. The target energy efficiency of CAT4026 is higher than 90%, and the typical energy efficiency is 94%. It provides an excellent current matching accuracy of <±2% and a 400 Hz PWM dimming range of <1%, and uses an external power bipolar junction transistor to provide heat dissipation to better handle heat dissipation issues.

In addition to linear side-lit solutions based on NCP1397, NDF05N50 and CAT4026, ON Semiconductor also provides direct-down buck drive solutions based on NCP1397, NDF05N50 and NCP1034. The NCP1034 drives a pair of external N-channel MOSFETs with a programmable switching frequency of up to 500 kHz, which can flexibly adjust the operation of the integrated circuit (IC) to meet system-level requirements. In addition, the output voltage can be precisely regulated using the internal 1.25 V reference voltage for low-voltage applications. The device's protection features to prevent electrical faults, such as undervoltage lockout and hiccup current limiting, provide the required system-level safety in the event of a fault event. Advantages include driving high-efficiency field-effect transistors (FETs) and providing fixed frequencies for one or more devices.

It is worth mentioning that ON Semiconductor provides a complete solution for LED-backlit LCD TVs. In addition to edge-lit or direct-lit backlight solutions, it also includes power factor correction (PFC), standby switching power supplies and related rectifiers, operational amplifiers, MOSFETs and discrete devices, providing customers with a one-stop solution option.

Tony Armstrong, director of product marketing for Linear Technology's power products, believes that traditional LCD HDTVs use CCFL lamps as backlights, and the contrast ratio they can provide is only between 450cd/m2 and 650cd/m2. The main problem is that "CCFL backlights cannot be completely turned off or locally dimmed." In other words, the current generation of LCD HDTVs cannot achieve true "black" and the dynamic range of all colors is low.

In contrast, when using HB LED backlighting, an array of LEDs (up to 1,600 for a 46" display) that can be locally dimmed or turned off in backlighting "groups" provides contrast ratios that are almost an order of magnitude higher than CCFL designs (>4,000cd/m2). In addition, by adjusting the brightness of the backlighting LED groups, more mid-tones can be reproduced, making the image more lifelike.

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“Another benefit is the ability to completely shut off the LEDs in localized locations, reducing motion blur. By completely shutting off the LEDs between frames, image blur caused by fast moving objects is virtually eliminated, so a fast response rate of the LEDs is critical,” said Tony.

He reiterated the importance of driver ICs to the reliability and life of LED products. In addition to making the solution more compact and flat, driver ICs also operate at high switching frequencies. In inductor-based DC/DC switching topologies , these high switching frequencies can reduce the value, size and cost of inductors and output capacitors. In addition, in some cases, driver ICs can also achieve on-chip integration of Schottky diodes and boost diodes, thereby reducing the number of external components. This in turn reduces design complexity and reduces the size and cost of the solution.

For LED lighting applications >100W, Tony said that the choice of LED driver circuit depends largely on the input voltage range, the cumulative voltage drop of the LED string itself, and the current required to drive the LED. This leads to a variety of different feasible LED driver topologies, such as: buck, boost, buck-boost, and SEPIC. Among them, the standard buck converter is the simplest and easiest to implement, followed by boost and buck-boost converters, and the SEPIC converter is the most difficult to implement because it uses complex magnetic design principles and requires designers to have a high degree of switch-mode power supply design expertise. "No matter what the topology is, Linear has targeted an optimized LED driver solution."

Liu Xuechao, business development manager of high-performance analog products at TI Semiconductor Division, emphasized that the advantages of LED TV in terms of energy saving, environmental protection, long life, wide color gamut, and easy ultra-thinness are unquestionable. However, at present, the cost of LED as a semiconductor light-emitting diode is still relatively high. Compared with TVs using CCFL, its cost is 20% to 50% higher than that of traditional LCD TVs, especially direct-type backlight LEDs, which cost much more than traditional LCD TVs. On the other hand, whether using direct-type or side-light technology, all manufacturers hope to have dynamic PWM dimming, and they also need to solve the bottlenecks of efficiency and cost. At the same time, the current LED backlight panel solutions are different, and how to make LED drivers suitable for different panels and improve compatibility is also a question worth thinking about.

Taking TI's 8-channel LED driver controller TLC5960 with integrated intelligent thermal controller as an example, Liu Xuechao believes that "4 sets of margin voltage monitoring feedback (HVM) and FET controller" and "robustness, comprehensive system diagnostic functions, LED/FET open circuit and FET short circuit protection" are the biggest advantages of this product. In addition, TI also has a solution for LED TV motherboard power supply and backlight power supply 150W LIPS, whose topology includes UCC28051 (TM PFC), UCC25600 (LLC resonant half bridge), UCC28610 (standby and system power supply), TPS40210 (DC/DC Boost conversion) and TLC5960 (8ch LED constant current drive), which can fully meet the standby and working power consumption requirements.

Fairchild also recently announced an integrated power supply and backlighting solution FSFR1700L developed specifically for LED TV. According to test results, compared with the 180W power required by ordinary CCFL solutions, the power of the new solution can be reduced to less than 110W. The new design has features such as high-performance power factor correction (PFC), high-efficiency DC/DC, current balance and system protection. It is said to be "one of the first all-in-one (from AC/DC to LED backlighting) solutions provided by semiconductor manufacturers on the market today, and is an ideal choice for 32 to 55-inch LCD TVs."