Challenges and key solutions in LED backlighting and lighting

Due to the rapid development in backlight and lighting applications, LED technology, which is sold as energy-saving, environmentally friendly and long-life, is returning to the ranks of hot technologies. Although the industry still needs to overcome some technical challenges to improve the existing technology, major semiconductor manufacturers have already launched solutions with their own characteristics.

Facing many challenges

For many engineers who are new to LED technology, how to reduce the space occupied by the system and maximize the brightness are the first problems they encounter when choosing an LED solution supplier. However, since LED technology is still in its infancy, suppliers still need to overcome many challenges.

Shone Tran, EZ-Color product marketing manager at Cypress, believes that for all high-power LED designs, device binning, temperature, color accuracy, and signal communication may be the four main challenges that semiconductor solution suppliers are facing. However, there are still some subtle differences in different market segments. For example, color accuracy, temperature, and signal communication are the most urgent issues to be solved in LED backlight, while in automotive lighting, temperature and color accuracy control technology are more important. As for general lighting, both aspects need further improvement.

Tran's view on the different market segment requirements coincides with the opinion of Jiang Jialiang, marketing manager of the power management department of ON Semiconductor Asia Pacific. The difference is that the latter believes that cost is the most important factor in LED backlighting and general lighting. As for automotive lighting, he pointed out: "Existing solutions must continue to improve in reliability (such as whether they can withstand harsh ambient temperatures)."

Jiang Jialiang said that if LED technology wants to make more progress in the field of general lighting, it must consider how to more conveniently replace existing traditional light sources such as halogen lamps and spotlights. "For example, assemble LEDs and conversion circuits into small lamps." Timon Rupp, LED Marketing Director of Osram Opto Semiconductors, holds the same view. Osram Opto Semiconductors is currently focusing on the automotive and general lighting markets. Its close relationship with the lighting giant Osram enables it to understand terminal needs more deeply than other suppliers. "LED technology is trying to compete with traditional light sources in the field of general lighting." Rupp said, "The main challenge faced by semiconductor suppliers is how to make breakthroughs in system integration, such as providing so-called 'turnkey' or 'plug and play' solutions that make it easier for consumers to switch from traditional light sources to this emerging technology."

Compared with other respondents, Wu Zhimin, marketing manager of power management products in Asia Pacific at National Semiconductor, has more specific opinions. "In the backlight solution of driving multiple LED light strings, current matching between light strings is the main challenge. Both LCD TV and automotive applications have high requirements for contrast." Wu Zhimin said. In addition, he also pointed out that since LEDs are widely used in general lighting, suppliers must also be able to provide a variety of drivers with a full range of input voltages.

"White LED drivers must be able to provide sufficient current and voltage for a specific LED configuration and use a conversion topology that meets the input voltage range and the required output voltage and current requirements." said Tony Armstrong, product marketing manager for Linear's power product group. He pointed out that an excellent LED driver must have some or all of the following features: wide output voltage range; high-efficiency conversion; tightly regulated LED current matching; low-noise, constant-frequency operation; independent current and dimming control; compact size, small board area, and very few external components.

Main solutions

EZ-Color is a flash-based programmable LED driver launched by Cypress last summer specifically for high-brightness (HB) LED intelligent lighting systems. It can control up to 16 HB-LEDs (CY8CLED16), which is 4 to 5 more than similar competing products. In order to speed up the development progress of designers as much as possible, the company also provides two design tools called PSoC Express and PSoC Designer. They are respectively for entry-level users who have no previous embedded design experience and embedded design experts. The evaluation kit (CY3261A-RGB) provided at the same time also provides an interactive graphical interface that can be used through CIE charts or directly control a single HB-LED for fast color mixing. EZ-Color also has CY8CLED04 and CY8CLED08, which can control 4 and 8 LEDs respectively. It is reported that based on the EZ-Color HB-LED driver, the company is also developing an optical feedback solution, in which a color sensor will be used for closed-loop color accuracy compensation.

For high-end mobile phone flash applications, OSRAM Opto Semiconductors has developed the CERAMOS and OSLUX product lines. The company also provides products for automotive headlights (OSTAR Headlamp) and general lighting (OSTAR Lighting). In particular, the OSTAR Headlamp, which has passed automotive safety certification and has begun to provide samples, should be introduced. It does not use a lens, but through an improved converter and improvements in Thinfilm technology, it has achieved a peak brightness and power consumption of 620lm and 12W at an operating current of 700mA. The typical brightness of up to 500lm is also twice that of the previous generation, which is brighter than any LED currently available. The compact package allows it to be connected with small optical devices, thus saving space and giving designers more room to play. The company will start mass production of this product at the end of 2008.

National Semiconductor recently released a DC/DC synchronous buck constant current regulator model LM3433, which can provide stable negative current for high-power HB-LEDs in projectors and rear-projection TVs. Since multiple LED anodes are allowed to be directly connected to the ground point, the circuit's heat dissipation capacity is greatly improved. In addition, the PWM dimming control frequency of more than 40kHz also facilitates the implementation of technologies such as digital color temperature control, interference removal, field sequence lighting, and brightness control. Due to the fixed on-time, the device does not need to add output capacitors. The company will also soon launch a 3-channel constant current LED driver LM3431 with an integrated boost controller for automotive and LCD TV backlight applications. It can provide high-precision constant current for 3 LED strings (each string can have up to 10 LEDs). The maximum output current is 150mA, and it can drive 6 LED strings after appropriate expansion. In order to reduce voltage drop and improve efficiency, LM3431 also uses a cathode feedback voltage regulation control circuit. Using PWM dimming, the maximum dimming frequency is 25kHz, and the contrast ratio is 100:1. After further reducing the dimming frequency, the contrast ratio can exceed 1,000:1.

LED applications are diverse, and suppliers need to provide a variety of circuit topologies to achieve the best solution cost and performance in specific applications. Based on this idea, ON Semiconductor has adopted methods such as increasing the switching frequency, adopting low-loss topologies, and charge pumps in its LED drivers. For example, the NCP1216, which was launched using its experience in charger solutions, can be easily used to build solutions with or without safety isolation functions. As a PWM controller, it can be used with an external MOSFET for certain backlight applications from 10 watts to tens of watts. In addition, the NXP101X series with a built-in MOSFET is very suitable for smaller applications such as spotlighting due to its compact package. The low-voltage DC-DC switching regulator NCP3065 can be used for simple buck or boost configurations of LED lamps. In addition, if PFC and power conversion are required at the same time, the single-stage PFC controller NCP1651 is also a good choice.

Linear is also a major supplier in the LED driver market. LT3595 is the company's latest high-performance LED driver for large-size TFT-LCD backlight applications. 16 independent channels can drive up to 160 (16×10) 50mA LEDs. Dimming of each channel can be achieved through a separate PWM pin, with a dimming range of 5,000:1 and a circuit matching degree of 8% between the 16 channels. LT3595 also has functions such as open circuit detection and thermal protection. Since all power switches, Schottky diodes and compensation components are integrated, the converter circuit board area and component cost are greatly reduced.

LTC3212 is another new product launched by Linear specifically for RGB LEDs in portable display and lighting applications. It can drive 3 LEDs (input voltage 2.7V~5.5V) with a current of up to 25mA. Since only 3 small capacitors and 1~3 programming resistors are needed to form a tiny and complete RGB or white light LED current controller and power supply solution, it is particularly suitable for small battery-powered devices. Its multi-mode charging pump can perform low-noise, constant-frequency operation to minimize ripple. Designers can use 3 separate resistors to set the LED current source to control the dimming operation and brightness of the LED. When all 3 RGB LEDs are set to ON, the white light mode will optimize the current ratio of the red, green and blue LEDs to achieve the best white light.