Development and application of OLED drivers

Ten years after the discovery of the mechanism of electron light emission, organic light emitting diode (OLED) technology is finally commercialized in mobile phones, MP3 players and digital cameras. According to Display Search, since the first single-chip OLED driver in 2001, more than 17 million ICs were used in mobile phone displays in 2003. This year, OLED also began to be used in the main display of mobile phones. The application of OLED in mobile phone displays is achieving rapid growth, and it is expected that the number of OLED modules used this year will exceed 330 million.

In line with OLED technology and development, OLED drivers are playing an increasingly important role. Not only has it risen from low duty cycle to support high duty cycle, but also some features such as control of each RGB current, wider IC operating temperature (-45 to 80℃), internal DC-DC boost, and graphics acceleration instructions have been applied. Solomon Systech's OLED drivers have all these features, which improve the service life and reliability of OLED and enhance the display effect of OLED.

Common display resolutions for mobile phones

There are three display resolutions on mobile phone sub-screens: 80x48, 96x64, and 96x96. In 2003, the main type of OLED used in mobile phone sub-screens was regional color, which is a single display type with two or three colors. It has been proven to be a good choice for mobile phone sub-screens. In the future, regional color OLED will be used in low-cost mobile phones, while full-color OLED sub-screens will be used in high-end products such as mobile phones with cameras, 3G mobile phones, and smart PDA phones.

Home Screen

There are many display resolutions for mobile phone main screens, ranging from 96x64 to 640x320. On some candy-bar phones, 96x65 and 101x80 display resolutions are common; on folding phones, 128x128 is common; 132x176 and 176x220 are used in many folding phones and camera phones; 320x240 is used in 3G phones, etc.

This year, the first mobile phone with OLED main screen appeared in China. This technology, including driver IC, is now fully commercially available. More and more mobile phone manufacturers are beginning to consider using OLED main display screens, and some of them have already added this technology to new products. In the near future, OLED will be an important member of mobile phone main displays.

Compared with LCD modules, self-luminous OLED displays do not require backlights and LED driver circuits. The thickness of a typical OLED module is only 1 to 1.5 mm, while the thickness of an LCD module is generally 3 mm. Therefore, OLED modules are suitable for ultra-thin flip covers on folding machines.

A highly integrated OLED driver/controller IC includes row and column drivers, DC-DC conversion, timing control, display memory and MCU interface circuits, providing a simple solution for OLED module manufacturers to use in mobile devices. In addition, software engineers can also save time in mobile phone development by using the built-in graphics controller function (as shown in Figure 1).

As the duty cycle of display resolution increases, the difficulty and technical requirements of using passive matrix OLEDs also increase. Therefore, some OLED module manufacturers intend to use active matrix OLEDs for displays with a duty cycle greater than 132. This is somewhat similar to the situation of STN and TFT encountered in LCD technology. One speculation is that in the future, large-size displays will be dominated by active matrix OLEDs (AMOLEDs) considering the quality of the display and the size of the screen, while low duty cycle displays will be dominated by passive matrix OLEDs (PMOLEDs) due to cost and flexibility. However, most AMOLED products are still in the laboratory stage and have not yet been fully commercialized. PMOLED manufacturers are also working hard to produce products with larger sizes and higher duty cycles, trying to share the huge mobile phone market with STN LCD and TFT LCD.

Although PMOLED faces some technical problems in high duty cycle applications, this can be solved by using a suitable driver IC to achieve high duty cycle display. For example, by driving two separate screens with a driver IC that supports cascading, an 88x176 display can be doubled to 132RGBx176 (as shown in Figure 2). To implement this solution, the driver IC needs to have the following functions: (a) Unlike LCD drivers, current driving technology must be used; (b) Due to the high data transfer rate and high power consumption of full-color applications, data memory and control functions, grayscale tables, and power saving modes must be integrated into the IC, which also helps the life and reliability of OLED; (c) In order to reduce external components and save costs, an internal power control system must be built in. With the above technologies and features, PMOLED will be more likely to enter the arena of full-color main screens for mobile phones.