Generating consistent LED high brightness in RGB display

Light-emitting diode (LED) has been widely used in a variety of terminal equipment, from car headlights, traffic lights, text display, billboards and large screen video display, and to the general architectural lighting and LCD backlight of the latest application, the rapid adoption of LED makes the most common equipment also need to re design. With the increase of the efficiency and brightness of LED, as well as the reduction of cost, LED may replace the traditional lighting technology in consumer applications. In this paper, by comparing the use of LED based LCD backlight large screen display technology used in some of the techniques described in the use of LED in the face of some of the design challenges.

The stadium or billboard displays a lot of display panels and thousands of LED. In each display array, the brightness of each LED (also known as pixels) will be very different, the brightest and darkest LED brightness difference between sometimes even up to 15% ~ 20%. Although this problem is a common problem for all LED applications, it is particularly prominent in some high quality display systems that require pixel consistency. In order to make up for this difference, manufacturers usually use two ways: first, from the supplier to buy a matching or filtered LED; the two is the use of "point correction" function of high-quality LED driver.

LED vendors offer a matching LED and charge a certain additional fee. They tested the RGB (red, green, blue) light emitting diodes together with a LED that produces a similar brightness on the specified current. This method can use minimal design work to low-end lighting system to provide brightness consistency required, but each pixel with time fading speed or brightness down speed is different, so this method is only a temporary solution. In other words, in the next one to two years, the brightness of each pixel will not be consistent. In addition, when you need to replace the defective panels, the new panel brightness will be visually different from other panels.

High end display system requires a high degree of brightness matching, so the use of LED matching method is not enough. In order to obtain the consistency of pixel and panel brightness in the whole life cycle of the display unit, the manufacturers generally adopt the advanced LED driver with point correction function. Point correction is a method for controlling the brightness of a pixel by adjusting the current of each LED in the array. Using the point correction function, the processor can control all the current flowing into the LED panel, while the LED driver can adjust the current supplied to each LED and generate the same brightness. As a result, it is no longer necessary to look up tables, and does not require LED to perform complex multiplication tasks in each refresh cycle, the processor can save the resources used to perform other tasks. In order to achieve the point correction, the manufacturer to measure the brightness of each LED. The darkest LED in the system is designated as the basic LED, while all the other pixels are compared with the. For this correction, the current supplied to each pixel is multiplied by a fraction (or fraction) proportional to the LED light intensity. As TI TLC5940, each LED point comes in each refresh cycle can be very different, and can be stored in the integrated EEPROM. This "double point correction method can provide for the whole panel brightness with the external lighting conditions change and update the flexibility, and can provide long-term and non-volatile point correction information, to ensure the consistency of the panel brightness. Brightness indicators will change over time, EEPROM data can be re adjusted, if the panel failure requires replacement, EEPROM data can also be rewritten. A concrete example is given to illustrate the method.

For simplicity, consider only consists of a plurality of panels and thousands of LED pixel full display 16 colors in the LED system. Brightness indicator panel in video pixel may require the pixel of the green LED with 80mcd. The selected LED (Osram LP E675) according to the brightness into four groups: 45 ~ 56mcd, ~ 71mcd, 71 ~ 90mcd and 90 ~ 112mcd. The brightness of each group was measured on the 50mA current. Select the group with the highest brightness and ensure that each LED has a brightness of at least 80mcd. For chips such as TLC5940, a resistor can be used to set the maximum current of each piece of IC, so that each piece of IC can drive 16 LED. The resistance value must be able to set the current high enough so that the darkest LED can also produce 80mcd brightness. Therefore, according to the LP E675 data, the chip must have a 43mA drive current to produce 80mcd brightness. By measuring the full current (43mA) brightness of the LED at the installation position, the LED brightness histogram is shown in figure 1. Where the X axis is represented by the LED mA current, while the Y axis is represented by the LED brightness of the mcd. As shown in Figure 1, in the absence of a point is before the measured brightness between each LED panel difference can be as high as 10%. Such a large brightness difference is unacceptable in the high-end display. The histogram gives the corresponding data for each LED to be adjusted or corrected to produce a uniform luminance. For example, when 80mcd is programmed for full brightness, the IC must adjust the brightness of the LED1 from 83mcd to. TLC5940 has a 6 bit point correction (64 step) step, full range resolution corresponding to each 1.56%.