Generate consistent LED brightness in RGB displays (Fig.)

Light-emitting diodes (LEDs) have been widely used in a variety of terminal devices, from car headlights, traffic lights, text displays, billboards and large-screen video displays, to general and building lighting and LCD backlighting applications, the rapid adoption of LEDs has made the most common devices need to be redesigned. As the efficiency and brightness of LEDs increase and the cost decreases, LEDs may replace traditional lighting technologies in consumer applications. By comparing some technologies used in large screen displays with LED-based LCD backlight, this paper describes how to solve some design challenges when using LED.

Stadiums or billboards use many display panels and thousands of LEDs. In each display array, the brightness of each LED (also known as pixel) varies greatly, sometimes as much as 15%-20% between the brightest and darkest LEDs. Although this problem is common to all LED applications, it is particularly prominent in some high-quality display systems that require pixel consistency. To compensate for this difference, manufacturers usually use two methods: one is to purchase matched or filtered LEDs from suppliers; The second is to use a high-quality LED driver with "point correction" function.

LED suppliers offer matched LEDs at a certain extra charge. They then test and combine these RGB (red, green, blue) light-emitting diodes with LEDs that produce similar brightness at the specified current. Although this method provides the required brightness consistency for low-end lighting systems with minimal design effort, each pixel decays or decreases brightness at different rates over time, making this a temporary solution. In other words, within the next one to two years, the brightness of each pixel will no longer be consistent. In addition, when a defective panel needs to be replaced, the brightness of the newly replaced panel will be visually different from other panels.

High-end display systems require high brightness matching, so it is not enough to use LED matching alone. To achieve the consistency of pixel and panel brightness over the entire lifetime of the display unit, manufacturers commonly use LED drives with point correction capabilities. Point correction is a method of controlling pixel brightness by adjusting the current flowing into each LED in the array. With point correction, the processor controls all current flowing into the LED panel, while the LED driver adjusts the current supplied to each LED and produces consistent brightness. Therefore, no lookup tables are needed, and no complex multiplier task for LED is required for each refresh cycle. The processor can use the saved resources to perform other tasks. To achieve point correction, manufacturers measure the brightness of each LED by taking photos. The darkest LED in the system is designated as the base LED, and all other pixels are compared with it. For this correction, the current supplied to each pixel is multiplied by a decimal (or fraction) proportional to the LED light intensity. In TI TLC5940, the point correction values for each LED can vary significantly during each refresh cycle and can be stored in the integrated EEPROM. This "two-point correction" method provides flexibility to update the entire panel brightness with changes in external lighting conditions, and provides long-term and non-volatile point correction information to ensure panel brightness consistency. Brightness indicators change over time, data in EEPROM can be recalibrated, and data in EEPROM can be rewritten if a panel fails and requires replacement. Here is a specific example to illustrate this approach.

For simplicity, only 16 LEDs of one color in a complete display system consisting of multiple panels and thousands of LED pixels are considered. The brightness index of the green pixel in the video panel may require that the green LED of the pixel have a brightness of 80mcd. The selected LED (Osram LP E675) is divided into four groups according to brightness: 45-56 mcd, 56-71 mcd, 71-90 MCD and 90-112 mcd. The brightness of each group was measured at a current of 50 mA. Select the brightness group and ensure that each LED has a brightness of at least 80mcd. For chips like TLC5940, a resistor can be used to set the current of each IC so that each IC can drive 16 LEDs. The resistance value must be high enough to enable the darkest LED to produce a brightness of 80 mcd. Therefore, according to the data of LP E675, the chip must have 43 mA drive current to produce 80 MCD brightness. By measuring the full current (43mA) brightness of the LED at the installation location, the LED brightness histogram shown in Fig. 1 can be obtained. The x-axis is the LED current in mA and the y-axis is the LED brightness in mcd. As shown in Figure 1, the brightness difference between each LED in the measured panel can be as high as +10% without point correction. Such a large difference in brightness is unacceptable in high-end monitors. The histogram gives the corresponding data for each LED to be adjusted or point corrected to produce uniform brightness. For example, when full brightness is programmed, the IC must correct the brightness of LED 1 from 83mcd to 80mcd. TLC5940 has 6-bit point correction (64-step) steps, corresponding to 1.56% full-range resolution per step.

Figure 1LED brightness and forward current histogram before point correction

Figure 2LED brightness and forward current histogram after point correction

The point correction values for each LED can be calculated using the following formula.

DCproduction is the point correction required for production, Lbaseline is the desired brightness level, and Linitial is the brightness measured on the current.

The calculated point correction values are rounded to the nearest decimal, then the original brightness is multiplied by a new point correction factor to obtain the updated LED brightness value.

After calculating and storing the point correction data for each LED, the LED driver can be programmed to its current so that the LED driver can automatically adjust the current supplied to each LED, resulting in a histogram as shown in Figure 2. If the point correction data is programmed into the EEPROM of the TLC5940 chip, the point correction data can be loaded each time the panel is turned on and will remain until it is recalibrated under the panel.

For indoor or outdoor industrial video monitors, such as billboards and large-screen monitors, static adjustment (i.e., remain unchanged after calibration until manually adjusted) is not enough. This panel adjustment is part of the monitor's daily maintenance routine. Emerging market applications pose a greater challenge. As this technology enters consumer electronics and homes, how can you control and adjust the changes of LED over time?

Although this development is still in its infancy, some monitors now use this technology. Sony's 40-inch Qualia 005 panel and Samsung's 46-Inch LNR460D panel have all introduced LCD TVs with LED backlights. Contrary to popular belief, the diodes in these two TV monitors are not "white", but instead produce "adjustable" white light by controlling and mixing RGB LEDs. Compared with traditional bulbs, LED backlighting has many advantages: higher power efficiency, less motion picture dragging, wider color spectrum (in some cases greater than 105% NTSC), longer lifetime, adjustable color temperature, and so on, and its picture quality is very high. Although TV engineers face the same challenges as traditional panel manufacturers in terms of brightness changes, they must also focus on temperature changes, as TV backlight applications are sensitive to changes in LED brightness with temperature. In addition, the TV can only achieve its display quality if its backlight property is adjusted to meet the different ambient lighting conditions in each consumer's living room. In addition to the characteristics of consumer applications, there is a need for dynamic brightness adjustment.

To create this dynamic adjustment loop, several internal sensors to measure temperature and brightness changes of the LED and external sensors to measure changes in environmental conditions are needed. In its most basic form, the control loop collects data with these sensors and inputs the data into the processor, which then evaluates the data and provides intelligent correction to LED driver chips such as TLC5940. In addition, the processor generates updated point correction data by combining the original factory calibration point correction values with new dynamic data.

Using the previous example, if the ambient brightness meter measures ambient lighting conditions that require only 70% full brightness or 56 mcd, the processor will calculate a new 44.8 ambient light point correction value. If the LED brightness decreases by 10% due to temperature rise, the processor calculates a temperature point correction of 71.1. Combining all three point correction values to produce new point correction data can compensate for these three brightness changes.

From above, the expected brightness of 56mcd can be obtained by using the combined point correction value of 48. Note that the starting current in this calculation is set to 90% of the starting production current because the temperature causes a decrease in brightness.

Only LED drives that provide and combine dynamic and static point correction methods can provide backlight solutions for specific viewing conditions of consumers. In the prototype TVs provided by Sony and Samsung, LEDs reduce the resources needed to control a single LED in series. To design full dynamic control of the backlight display unit, a single LED needs to be controlled. As a result, LED manufacturers are currently developing advanced technologies for more flexible array configurations.

Smart backlight for TV sets is the next innovative technology that will be applied to the home, which will greatly improve the picture quality of TV sets and improve people's visual experience in use.