System-level method to optimize LED backlight design

LCD TV system designers are faced with many choices when developing the next generation of LED-based flat-panel TVs. They must choose whether to use a direct-lit or edge-lit LED backlight structure, but also from a variety of dimming technology, power management methods and thermal protection LEDs are quickly becoming the mainstream backlight for large displays, but designers also need to solve thermal management , power management, brightness adjustment, and other issues to design a performance-optimized system. Xiaoping Jin and Arkadiy Peker are for analysis.

In the LCD TV industry, LEDs are fast becoming the choice of backlight technology, and it is also constantly entering the field of larger screens. LEDs are small in size, low in power consumption, and good in display performance, so there is basically no doubt that it will soon become the mainstream backlight for all LCD products. However, system designers must know how to optimize the LED backlight design so that they can win in the market, and the competition in this market will be fierce.

LCD TV system designers are faced with many choices when developing the next generation of LED-based flat-panel TVs. They must choose whether to use a direct-lit or edge-lit LED backlight structure, but also from a variety of dimming techniques, power management methods and thermal protection technology. They should focus on how to achieve accurate and stable color chart and how to better synchronize the LED backlight control and video processor to achieve Dl, D2 and D3 backlight addressing technology. Also, designers must choose the best LED backlight setting graphical user interface (GUI).

As TV panels become larger and more complex, the decisions become more important. As the TV panel is fully turned to 1080p resolution and the display area is increasing, the role of the TV backlight unit (BLU) in image display quality is also more important.

There are several key factors that must be considered when designing the BLU for next-generation LEDs for large LCD TVs. For example, the illumination between each side and from the center to the corner must be consistent, and a balance between achieving high dynamic range contrast and deeper blackness. The gamut is also important and must meet or exceed the NTSC requirements. Video quality must meet all the requirements of Motion Picture Response Time (MPRT), and lighting efficiency should be high in order to reduce energy consumption.

In order to achieve satisfactory image quality, TV panel manufacturers have some options, including side-light (DO), or scan dimming (D1) or local or regional dimming (D2) for efficiency. The best image quality can be achieved by using RGB LED BLU to achieve D3, ie independent local or regional dimming for the brightness of the red, green and blue LEDs.

In addition to image quality, there are many cost or implementation issues associated with each of the aforementioned BLU designs. Both the edge-lit and the direct-lit LEDBLU have different requirements, which in turn affects system-level energy efficiency and has an impact on the design of the power system architecture.

Designers must choose the best LED string driver module. Of course, the choice of lighting control system is also important. It can accurately control the LED current over a wide range of input voltages and operating temperatures, while still providing enough Thermal Protection. Similarly, in the BLU implementation of RGB-LED, accurate color management should also be provided; the synchronization of BLU and system video processor is to achieve BLU addressing mechanism (including DO, Dl, D2 and D3 brightness adjustment scheme).

Typically, LED-based BLUs always contain multiple white or red, green, and blue LED strings. The BLU also integrates lighting control and a controller with a video processing interface for brightness adjustment. In addition, the BLU will also have a ambient light sensor (for W-LED BLU) or an RGB light sensor (for RGB-LED BLU) with external diagnostics.

Power module integrated on the BLU with one or more power supply units (PSUs), external heating management and an interface to the BLU lighting control and management module. The final component is the LED driver and the corresponding protection mechanism. For BLU power systems, the basic requirements for design are low cost, small size, and high efficiency, all of which are affected by the implementation of the brightness adjustment function. For example, an edge-lit BLU with DO brightness adjustment would require a large number of PSUs with small size, while a direct-down type would accept a smaller number and larger size with a D2 local or regional dimming BLU. Higher PSU.

To understand the impact of the brightness adjustment function on the PSU design, it is important to understand how the LED works. The LED is a current-driven device whose output brightness is directly proportional to the input current, and the current is output through a constant DC power supply to maintain a stable luminance. The constant current source remains independent of the LED forward voltage (VF), even if the temperature changes.

Figure 1 is an RGB-LED BLU in which the RGB-LED array is driven by a constant voltage source (PS) that is controlled by a separate current source. For W-LED strings, it is ok to replace each colored LED with a white LED.

The RGB-LED BLU shown in Figure 1 presents a tradeoff in power efficiency. The PSU must be able to support LED arrays or LED strings (maximum VF total). Because there will be differences in VF between LEDs of different colors and even LEDs of the same color, this will cause excessive energy consumption. These differences are usually caused by changes in the LED manufacturing process. Take Philips Lumileds' red-light Luxeon LED for example. Its typical VF voltage drop is 2.95V, while the typical VF voltage drop for green LEDs is 3.99V.

Therefore, the maximum voltage of the PSU should be 39.9V, which will cause excessive energy consumption for the red LED array. The consumption is Pd=(39.9V-29.5V)×0.35A= 3.64W.

To increase energy efficiency, system designers can design a separate DC/DC converter for each R/G/B array and convert it into a voltage by adjusting the voltage across the current sense resistor of the LED string. A constant current source. This method is relatively expensive, but it has higher energy efficiency, and can also be used in W-LED solutions, just replace the color LEDs with white LEDs.

Thermal management is an important issue to consider in design. The LED BLU must be able to manage the thermal parameters generated by the LED. As the temperature rises, the presence of this parameter reduces the amount of luminescence. If the PWM power cycle increases faster than the LED's VF, the LED backlight system will enter a state of heat loss. The illumination control and management module must measure the temperature of the LED. If this temperature exceeds a certain preset value, the duty cycle of all RGB PWM signals should be reduced to avoid LED thermal runaway, thereby avoiding LED BLU and display damage.

Color management is also important. For W-LEDBLU, only one display brightness sensor is needed to maintain overall brightness, but the RGB-LEDBLU system requires a more complex RGB sensor and a closed color ring to manage presets or user-defined whites. point. The best image quality is achieved by applying advanced brightness adjustment techniques such as Dl scan dimming, D2 local or regional dimming, and D3 RGB based area or local dimming (Figure 2).

In order for the LED BLU to be synchronized with the video processor on a per frame basis, the information of each LED BLU illumination area must be updated synchronously as each data is displayed on the panel. If the data displayed on these areas and the panel are not accurately synchronized, the image quality will decrease. The most ideal synchronization also requires high-speed connections, such as the use of the SPI bus. Furthermore, special communication protocols may be required for D2 local or regional brightness adjustment, as well as for each of the D3 RGB based local or regional brightness adjustments.

In addition, a high-speed interface is used to synchronize the VSYNC data during clock signal and panel data synchronization. The SPI bus can be turned "on" or "off" to control each area in Dl scan brightness adjustment mode.

Microsemi also proposed a method of brightness adjustment called adaptive local brightness adjustment, which allows the backlight circuit to illuminate the image area it is responsible for when it needs illumination, while darkening the dark areas. This new solution can make the black area darker, while significantly improving color, contrast, motion clarity and grayscale, while saving power by avoiding unnecessary backlighting.

This backlight control scheme has been demonstrated in Microsemi's LX23204 backlight controller, which supports long, high voltage LED strings (up to 300V) under edge-lit backlight conditions. The company also includes this technology in the LX24132 (32-port) LED backlight controller and the LX23108L (8-port) LED driver, which provides a flexible, integrated solution for use in direct-lit or edge-lit backlight applications for both flat panels. White LEDs in LCD TVs can also be used in RGB LED solutions.

When designing LED backlight panels, LCD display designers have a range of options. The choice of backlighting and technology has a significant impact on cost and imaging quality, especially on large screen sizes. Proper lighting control and management mechanisms optimize energy efficiency and image quality while reducing overall system cost.

The best image quality is achieved by D3 local or regional RGB independent dimming technology using a direct-lit RGB LED BLU. However, with the latest Super Contrast dimming technology, excellent image quality and dynamic contrast can be achieved in the ultra-thin side-lit BLU design, while also having a very deep black level.