Text. Jerry Han/Jeffrey Kwon
Today, many high-end mobile phones support high-pixel photography, mobile digital TV, MPEG-4 video playback, and video conferencing. In order to improve the display quality of mobile phones, designers generally use red, green and blue (RGB) light-emitting diode ( LED ) backlight technology. Although this new technology can significantly enhance the color gamut of the display, it is much more difficult than the white LED for the actual design. This article will introduce the functional advantages and design challenges of RGB backlighting and the best solution for driving RGB LED backlights on mobile phone displays.
Portable device applications generally contain multiple subsystems. As the size of the display continues to increase, the quality of the display becomes a weapon for differentiation in the market. Several important components of a mobile phone, including backlights, color management chips, flash LEDs, and display interfaces, can affect the overall quality of the display.
Fidelity/energy saving/size is indispensable
In the challenges faced by backlighting designs for portable devices, the color gamut can affect the "fidelity" of the picture.
Consumers want the picture displayed on the portable device to be as close as possible to the real or live view. At the same time, the power consumption of the display can not be too large to affect the use time of the mobile phone, so the energy-saving technology of the display is also important. In addition, display size is an important consideration from a portable device perspective. Therefore, in order to meet the needs of the market and design successful products, today's engineers must overcome the severe challenges.
Although cold cathode fluorescent lamps (CCFLs)/white light emitting diodes (WLEDs) are currently the most common display backlighting design technology, RGB LED backlighting design is rapidly emerging. The advantages and disadvantages of these two backlighting technologies and the design challenges they face will be evaluated separately below.
RGB fully meets the needs of use
At present, most CCFL-lit displays can cover 40 to 76% of the color TV broadcast standard (NTSC) color gamut, while RGB LED backlights display NTSC gamut coverage can exceed 100%. The difference can be distinguished by the naked eye.
But for the more high-end mobile displays, there are several major design challenges: how can you reproduce the most realistic images? How to achieve high image quality under high efficiency and small size? And how to solve the backlight design problem of the flip phone and the cap phone.
In general, a white LED contains a blue or near-ultraviolet light-emitting chip coated with a phosphor material that absorbs light emitted from the LED and emits it at a longer wavelength. However, more than half of the photons produced by the phosphor are scattered back to the LED chip , causing a significant portion of the light to be absorbed, resulting in a reduction in the overall light output of the white LED (Figure 1).
Figure 1 Typical emission spectrum of a phosphorus-containing white LED
White LEDs have a strong blue-ray peak and a relatively low-luminosity phosphorescent peak than blue light, and different combinations of blue and yellow light intensity can produce different light-frequency colors, which can make the colors appear whiter or compare. blue.
As shown in Figure 2, the spectrum of the white LED does not match the color filter of the color LCD, resulting in partial energy dissipation. In addition, since gamma correction must be performed for the spectrum of the WLED, the depth of the color is lost, especially in the red portion.
Figure 2 Spectrum distribution of WLED and color LED filters
The RGB LED response (Figure 3) has three peaks, one from red, blue, and green. The amplitude of each peak can be independently controlled. From the comparison of the spectrum, it can be seen that the white LED has a weak response in the red spectrum range, so the visual sensitivity in this range is also low.
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