LCD LED backlight uniform brightness control

Michael Day, Tarek Saab (author of this article is TI Texas Instruments)

The wide application of light-emitting diodes ( LEDs ) in various terminal devices has already caused many people to fall through the glasses: from the existing technologies such as car headlights, traffic signs, digital display, advertising billboards, and large displays, to general building lighting. Other applications such as devices and LCD backlights, and the growing popularity of LED light sources have led to the redesign of the most popular products. As efficiency and brightness continue to increase and costs continue to fall, LEDs will eventually replace traditional light sources as the primary source of illumination for consumer applications. This article compares the LED backlight of a liquid crystal display with the light source technology of other large displays to illustrate the challenges that LED application design faces.

Image point correction technology solves the problem of LED brightness difference

Large billboards for stadiums or advertising often contain dozens of display panels and thousands of LEDs. In each display area, each LED (also known as a pixel) emits a large difference in brightness; usually the brightest and darkest LEDs can differ by as much as 15% to 20%, which makes the manufacturer must continue to solve Difficulties in quality and management. Although all LED applications encounter such problems, they are particularly serious in high-quality applications that require uniform brightness, such as dynamic video displays. Manufacturers mostly use two methods to compensate for the difference in brightness. One method is to buy LEDs with similar brightness from the supplier; the other is to use high-quality LED drivers with "dot correction". Component.

LEDs with similar brightness have many advantages and their price is usually higher. The supplier will first measure the brightness of the LED at a particular current and then combine the red, green and blue LEDs of similar brightness. This method provides uniform brightness for low-order illumination systems with minimal impact on the design; the disadvantage is that each LED has a different brightness loss rate, so the brightness becomes uneven over time. This flaw makes it only a short-term solution, otherwise after a year or two, the picture will become a piece. In addition, if the panel fails and needs to be replaced, the brightness of the new panel and the original panel will also vary greatly.

High-end display systems are more demanding for brightness matching, and LEDs with similar brightness alone cannot meet the requirements of such applications. Therefore, in order to maintain uniform brightness for the pixels and panels over the life of the display, manufacturers will use advanced LED Driver components with built-in pixel correction. Image point correction is a method of managing pixel highlights by adjusting individual LED currents: the processor only needs to control the total current delivered to the entire LED display area, and the LED driver component adjusts the individual LED currents to produce uniform brightness; The presentation processor can use its computational performance for other tasks without having to query the table for uniform brightness or complex multiplication for each LED in each update cycle. In order to provide the image point correction function, the manufacturer will measure the brightness of each LED through the screen, and then designate the darkest LED of the system as the reference LED, and then adjust the brightness of other pixels to be the same as the reference LED; the adjustment method is The current per pixel is reduced by a certain percentage based on the LED output brightness. Texas Instruments' TLC5940 or other similar components store the image correction values ​​for each LED in the chip's built-in EEPROM memory or dynamically modify it during each update cycle. This dual image point correction method is very flexible. It can update the brightness of the entire panel when the external lighting conditions change, and provide long-term preservation of image point correction information to ensure uniform brightness of the panel. Engineers can update EEPROM data at any time as long as the pixel brightness changes over time, or if the panel needs to be adjusted and replaced due to a fault. We will use the following examples to illustrate this approach.

Actual operation case

For the sake of simplicity, only 16 LEDs of the same color are considered here, which come from a large display system consisting of multiple sets of panels and thousands of LEDs. We assume that a green pixel green LED must emit 80 milli-candles to achieve the panel's brightness requirements for green pixels. The designer's choice of LED is Osram LP E675, its brightness at 50mA standard current can be divided into four levels of 45-56, 56-71, 71-90 and 90-112 candela; this means that the highest brightness is selected. A set of LEDs ensures that each brightness is at least 80 milli-candles. Components such as the TLC5940 can drive up to 16 LEDs, and each component requires only one resistor to set the maximum current value. When selecting this resistor, make sure that it sets enough current for the darkest LED to emit 80 milli-candles. The LP E675 component datasheet shows that it requires 43 mA of drive current to deliver 80 s. The designer can first measure the brightness of the LED at 43mA full current during installation and then produce a luminance histogram similar to Figure 1. The first row of data in the figure is the LED current value in mA, and the second row is the LED brightness value in milliwatts. It can be seen from the figure that the LED brightness is up to ±10% when the pixel correction function is not used, which is unacceptable for high-order displays. The driver components can be individually adjusted according to the data shown in the histogram, or "pixel-corrected" the current of each LED, so that they can finally produce uniform brightness; for example, it must adjust the brightness of LED1 at full current from 83 milli-candles to 80 millimeters. Candlelight. The TLC5940 provides 6-bit (64-order) pixel correction, with each step equivalent to 1.56% of full scale.

The following formula can be used to calculate the pixel correction value for each LED:

Among them, DCproduction is the image point correction value used in the manufacture of the product, Lbaseline is the required brightness, and Linitial is the measured brightness value at the maximum current.

Figure 1: LED brightness and forward current histogram before image point correction The calculated image point correction value is rounded to the nearest score value, and then multiplied by the original brightness to get the new LED brightness value:

After calculating and storing the image correction data of each LED, the LED drive components are set to the maximum current to individually adjust the current of each LED, as shown in the histogram of FIG. As long as the pixel correction data is stored in the built-in EEPROM memory of the TLC5940 chip, the system can take out these image correction data every time it is turned on until the next recalibration.

Figure 2: LED brightness and forward current histogram after image point correction
Dynamic brightness adjustment becomes an essential function of LED backlight TV

Indoor/outdoor industrial displays such as advertising billboards or large displays need only “static” adjustments (the calibration values ​​remain fixed until the engineer re-adjusts these calibration values); the panel brightness adjustment will be The system routine maintenance work is performed together. However, emerging market applications require more advanced adjustment methods, especially as the technology is gradually integrated into consumer products and family life, designers must control and adjust the phenomenon that LED brightness changes over time.

Although this transformation process is still in its infancy, the technology it needs is already there. Xinli (40吋 Qualia 005) and Samsung (46吋LNR460D) have introduced LCD TVs with LED backlights; but they are not white light LEDs as we imagined, but combine red, blue and green LEDs together. Control is performed to produce a white light source with adjustable brightness. LED backlights have many advantages over traditional light bulbs, such as higher power efficiency, less moving picture afterimage, wider chromatographic range (more than 105% NTSC in some cases), longer life, and adjustable color temperature; Its image quality is unmatched by traditional light sources. On the other hand, TV engineers using LED backlights will not only face the difference in brightness encountered by traditional panel manufacturers, but also encounter the challenges of temperature; this is because the brightness of LEDs changes with temperature. The application has a great impact. In addition, in order to provide the best picture quality, such TVs must adjust the various attributes of the backlight in response to the changing lighting conditions of the consumer's home. These technical considerations, coupled with the fact that television is a consumer application, make dynamic brightness adjustment an indispensable feature of LED backlights.

To provide this type of dynamic control loop, sensors must be installed inside the TV to measure LED temperature and brightness variations, and external sensors should be used to measure the brightness around the TV. The most basic control loop uses the sensor to collect the data, and then sends the measurement results to the processor, which evaluates the data and directs the LED driver components such as the TLC5940 to make adjustments. The processor generates new pixel correction values ​​based on the factory-set raw pixel correction data and the newly collected dynamic data.

In the previous example, if the ambient light source sensor found that the surrounding brightness is very low, it is enough for the LED to reach 70% or 56 s. of the maximum brightness. The processor will calculate a new "environmental light source" image point correction value. 44.8. At the same time, if the temperature rise causes the LED brightness output to decrease by 10%, the processor will calculate the "temperature" image point correction value of 71.1. By combining these three image point correction values, new image correction data can be calculated to compensate for the errors caused by the three brightness variations:

As shown above, setting the total pixel correction value to 48 produces the 56 gram brightness we need. This calculation process has set the initial current to 90% of the current used in the manufacturing process due to the reduced brightness of the LED due to the increase in temperature.

From the above analysis, only the advanced LED driver components that can provide and combine the static and dynamic image point correction methods can provide the optimal lighting solution for the consumer viewing environment.

Going to smart TV backlights

Both Synopsys and Samsung's prototypes use LEDs in series to reduce the number of LEDs needed to be controlled, but to truly provide full-dynamic backlight control, we still have to individually control the brightness of each LED. To achieve this goal, LED suppliers are developing new technologies to provide more flexible LED connections.

Smart TV backlights are another important innovation that is driving the transformation of home entertainment. The outstanding performance of the initial products proves that with the help of LED technology , the display quality will reach the unimaginable realm in the next few years, which will bring consumers a shocking visual feast.

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