Introduction to the management solution of display backlight

Backlighting is the largest power consumer in a display system. LEDs have replaced CCFLs in almost all displays except for very large screens, significantly reducing power consumption, but backlighting can still consume excessive power if not managed effectively. This article focuses on the rise of LED backlighting and analyzes OLED display technology that can eliminate the need for backlighting.

OLED Power Management

The energy consumption of OLED displays is not fixed, but depends on the image being displayed at the time. In typical video and image display applications, the power consumed to fully illuminate all pixels is only 25% of the theoretical "maximum" value. For applications with strict power requirements, power consumption can also be reduced through image design. For example, displaying images in negative mode (white text on a black background) is more energy-efficient than in positive mode (black text on a white background) because only about one-tenth of the pixels need to be activated.

Pixel brightness also affects power consumption. Energy saving methods include reducing the brightness of all pixels when ambient light allows, as well as brightness management that is sensitive to ambient light, such as dimming options that cannot be selected in a menu. Reducing brightness can also extend the life of the display. When power requirements are very demanding, you may also want to consider changing the common menu colors, as red and green pixels are more energy efficient than blue.

Due to the capacitive nature of OLED pixels, the refresh rate also has an impact on power consumption. Very high frame rates increase the number of charge cycles, which in turn increases power consumption, and also makes the display dimmer, as the pixels do not have time to fully charge between refresh cycles. The refresh rate setting should be as low as possible without causing "jitter" in the visible image. Generally speaking, a refresh rate of 75 Hz is appropriate, but sometimes you can get away with as low as 60 Hz.

Because OLED is an emissive technology, the pixels will degrade very slowly with continued use. Standby and timeout modes can be used to slow down the aging process, or a screen saver can be considered. The number of pixels used in a screen saver and their brightness can be managed as described above.

Figure 1: Nokia N85 phone, OLED displayOLED -

no backlight technologyLCD

drivers consume very little energy themselves, and the display system consumes almost all of its energy for the backlight, which typically illuminates all display areas as long as the power is on.

In contrast, OLED is an emitting technology. Each pixel emits its own light, and when not activated, it does not emit light, consuming almost no energy. Unlike backlit LCD displays, OLEDs can produce true blacks and have a much higher contrast ratio, typically 10,000:1, compared to 400:1 for traditional TFT displays.OLEDs are also brighter, one reason being that OLEDs do not require pairs of polarizers, which filter out half of the light emitted by the backlight, as is done with LCD displays.

Figure 2

From an aesthetic point of view, OLED technology is also definitely superior to LCD due to its enhanced brightness and contrast. The response time of OLED displays is typically 50μs, while that of LCD is 25ms, which means that OLED full-motion video speed is improved and grayscale reproduction quality is greatly improved.

For the above reasons, although OLED is more expensive and has a shorter lifespan, it is increasingly being adopted by consumer products with cool looks/performance, such as Sony's X-series Walkman, Nokia's N85, and Microsoft's Zune HD. OLED technology is also available in the industrial market, with development and evaluation kits for a variety of models (0.79 to 7.0 inches) and resolutions (64×48 to 480×272). Advantages and Disadvantages of LED Backlighting

LED backlighting is now the standard for industrial display panels below 10.4 inches, and is even an option for panels below 15 inches. Because LEDs are powered by DC, they are simpler to drive and do not require inverters, which improves efficiency and increases power consumption approximately linearly with brightness, simplifying power management. Therefore, most small and medium-sized displays are currently equipped with LEDs.

Although LED backlighting has obvious advantages, it also has some disadvantages. White LEDs are not really white, but are actually blue LEDs plus yellow phosphors to produce white light, with gaps in the green and red parts of the spectrum. To achieve the best color balance, high-quality LCD displays are equipped with RGB backlights.

Regardless of the size, most displays require more than one LED to achieve acceptable brightness, and uniformity between different LEDs is even more difficult to achieve, especially as the LEDs age. Power efficiency is also a consideration. Manufacturers use MEMS and other light-guiding technologies to spread the light over a larger area to achieve maximum brightness and uniformity.

Maximizing LED Backlight Utilization LCD display performance varies, with key parameters including brightness, contrast, and viewing angle. Brightness is a more standardized standard, determined by the number of candelas per square meter in a dark room with the backlight driven to maximum and all pixels white. Contrast ratio numbers are more difficult to compare because there are different interpretations of contrast, but basically they compare the brightness of the brightest color (white) to the darkest color (black) that the system can produce. Viewing angle is more subjective.

Display brightness is affected by the transmission rate of the TFT display. Each pixel is controlled by a thin-film transistor, which shades a small part of the pixel. The size of the transistor can be reduced by using technologies such as low-temperature polysilicon (LTPS).

Based on performance data, a number of display options can be listed, but if good visibility under demanding conditions and power consumption are also required, the best advice is to simulate the actual application on several different displays and measure the backlight power in each case, taking into account factors such as the ambient light in the environment where the display will be used and the possible viewing angle. Sometimes changes to the color that carries the key information can also have an impact on the display performance.

Summary

In today's world of portable products, there is a conflict between the requirements of reducing power consumption, weight and cost on the one hand, and adding multimedia functions on the other hand, which requires even the most monotonous measurement and test systems to be aesthetically pleasing. The mobile phone market is like a large testing ground where new technologies aimed at reconciling the two contradictions can be brought to the market for experimentation. LED backlighting was first used in mobile phone displays and has now become a universal standard for small and medium-sized displays in the industrial market. OLED technology seems to follow the same development path.