LED has now developed into two categories: LED lighting and LED display screens. Advertising display screens are used in a wide range of applications, such as transportation, banks, stages, hotels, stadiums, shopping malls and other places. The color quality of the display screen is also the focus of everyone's attention. This article focuses on how the LED driver affects the color quality of the LED screen.

A full-color LED display is a display that can display images and animations in millions of colors. The video input signals provided by different types of video sources are processed and sent to the LED display. The entire display screen is usually composed of several square or rectangular dot matrices, and each dot matrix is composed of several pixels. Each pixel is composed of RGB (red, green, blue) LED light tubes. By mixing the light colors of these LEDs, the required pixel information can be generated. The LED driver is equivalent to the interface between the processed video data and the colored lights emitted by the RGB LED (the image is composed of these colored lights).

Considering the target application of large LED video displays, we can easily realize the important role played by picture quality. This concept includes the following aspects:

The signal quality of the original image and data processing capabilities are prerequisites for achieving good image quality.

The size of the display, resolution, pixel spacing and LED selection are key parameters that determine the image quality perceived by the human eye.

You need to choose a suitable LED driver that can convert the processed image data into the required color and animation effects.

1. LED display requirements for LED drivers

In terms of color and animation, in order to achieve excellent image quality, a variety of different functions need to be integrated: high frame rate, high refresh rate, color reproduction accuracy, etc. However, the quality of the display refers not only to the image itself, but also to the overall quality of the solution, such as interference immunity and reliability. Therefore, all color LED displays need to choose technically sophisticated LED drivers: good color rendering depends on the effective brightness of each RGB (and each color): the higher the effective brightness, the richer the colors displayed. Reasonable and fine control of brightness requires the use of PWM dimming technology to control a large amount of brightness.

High frame rates and high refresh rates require processing large amounts of data at high rates. LED drivers can meet these requirements using high-frequency serial interfaces and flexible data format management capabilities.

The failure of one or more LED tubes will affect the accuracy of pixel color, endanger the overall visual effect of the image, and affect the image quality of the system. Maintaining ideal visual performance on displays requires a reliable method of detecting LED failure conditions.

2. LED driver solution

STMicroelectronics has a variety of display and digital signage LED driver products, among which STP1612PW05 (see Figure 2) is an LED driver specially developed by STMicroelectronics for full-color LED video displays. This product is a 16-channel LED driver with a built-in high-frequency serial data interface. The maximum output current of each channel is 60mA. The output current can be adjusted through an external resistor. An 8-bit gain register can be used to accurately adjust 256 levels of grayscale.

Improved current accuracy at lower currents (typically ±1.5% between channels) meets the luminous efficiency improvement requirements of LED technology. In fact, the advent of energy-efficient LEDs allows display manufacturers to use lower current LED drivers to achieve the same brightness. This product is equipped with independent adjustable PWM brightness control function for each output channel. The well-known advantages of PWM dimming function in LED color protection and 4096 (12-bit) or 65536 (16-bit) levels of adjustable brightness can ultimately lead to significant improvements in color aberration and picture quality.

In addition, to further enhance the dimming control function, the product also adopts ScrambledPWM mode. The total PWM cycle basically covers many refresh cycles. The effect is to increase the dimming frequency while keeping the dimming resolution unchanged, thereby reducing screen flickering.

STP1612PW05 built-in serial interface meets the high-speed data rate and data format flexibility requirements of the display: the data transmission clock frequency can be as high as 30MHz, using 16x16-bit format (if the brightness level is selected 4096, the data format is 6x12-bit) or higher compression 256-bit format. As mentioned earlier, a highly interference-resistant and reliable LED driver can significantly improve the overall quality of the system.

When a failed LED tube (open circuit or short circuit) is found, the STP1612PW05 will send the detection results to the microcontroller through the serial interface, thereby avoiding the color change or shutdown of the relevant pixels and protecting the consistency of the image.

At the beginning of a frame of video image, all LED tubes are turned on at the same time, which will produce an inrush current with a large rising slope, which means a large input capacitor is required, parasitic channels will oscillate, and the system will generate noise. Group four continuous outputs into one group (the first group is OUT0, OUT1, OUT2 and OUT3; the second group is from OUT4 to OUT7; arrange them in this order), and there is a continuous delay of 40ns between each group to make the output channel conductive. The times are staggered so that the inrush can be significantly reduced. If the PWCLK clock signal (PWM counter clock signal, used for PWM dimming control) is not present for any reason (line disconnection), this will cause all LEDs on the display to turn on indefinitely.

In addition to affecting the correct display of images, such accidents can also burn or damage the LED lamp. STP1612PW05 has a built-in protection function that can activate the PWM clock timeout disconnection: when the PWCLK signal disappears for more than 1 second, the signal generator will be forcibly turned off.

The LED forward voltage and settable current to achieve the target brightness vary with the LED color: among the three color LEDs of red, green and blue, the red LED has the lowest forward voltage and the blue LED has the lowest driving current. . Therefore, it is recommended to use a different driver for each light color.

Each LED display lattice is composed of 16 pixels, and each pixel is composed of 4 colored LED tubes (1 red, 1 blue, 2 green). 16 LEDs of the same color are driven by an STP1612PW05. The choice of LED supply voltage (VLED) is a balancing consideration between several, often competing, requirements. The VLED voltage should be high enough to allow the LED to maintain the correct on-state, but the voltage drop of the LED driver's current generator should be minimized, otherwise it will cause useless power consumption and cause the chip and system to overheat.

The color quality of the full-color LED display is the most intuitive way to identify the quality of the product. LED display technology has been further developed some time ago, which is very encouraging. Although my country's LED display screens occupy 80% of the global market, it is not our country that leads the development of display screens. For this reason, Shenzhen LED manufacturers are We are actively engaged in the development of LED technology.

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