Increasingly organic light emitting diode (OLED) technology is appearing on the bill of materials (BOM) for many buyers. As technology evolves, the bill of material (BOM) for some new products will change along change as well, bringing new product lines, new vendors, and a variety of supply chain changes. We want to help buyers get up to speed without getting an engineering degree.
This week, in week two of our four-week summer school for buyers, we’ll look at the display drivers used with OLED displays. We’ll be offering a primer on the current state of the art for OLED display driver integrated circuits (DDIC) technology.
How it works
An DDIC controls the OLED display panel. These devices enable thinner and bezel-less displays that are thinner, flexible, and foldable and provide a wide range of colors that are true to the content being displayed. OLED also requires less power consumption than LCD, which causes less drain on the battery and extends the useful operating time of a device.
A DDIC sends a driving signal and data to the display panel in a form of electrical signals, to represent image signals such as letters and images. The DDIC resides in the OLED panel and differs between PM OLED and AM OLED panels.
In the case of PM OLED, by supplying a current into both vertical and horizontal panel ends, the pixels will emit light where the currents cross, so by controlling the amount of crossing current, the intensity of light is controlled. As for AM OLED, each pixel in the panel has a thin film transistor (TFT) and data storage capacitor, which is capable of controlling the brightness of each pixel in “degrees of gray,” which leads to lower power consumption and a longer panel lifetime. When the DDIC for AM OLED commands each pixel, the pixel is controlled through TFT. Display pixels consist of subpixels that represent red, green, and blue (RGB), the three primary colors of light. Those sub-pixels are directly controlled by TFT. By sending signals to the TFT, the DDIC controls the pixels directly. Therefore, the TFT functions as a switch that drives RGB sub-pixels and the DDIC functions as a type of “traffic light” to instruct the switch how to operate.
Technological trend of OLED DDIC
Since Samsung Display first mass produced the world’s first curved display for smartphones in 2013, flexible display technology has advanced rapidly. Overall, display type is classified two ways: rigid or flexible. The rigid type uses a rigid glass substrate while flexible type employs a flexible substrate based on a plastic material called polyimide. Flexible technology offers the advantage of achieving a variety of form factors including bendable, foldable, and rollable displays. Currently, the high-end smartphone market has bendable displays which curve around the edge of the smartphone and foldable or rollable smartphones are widely believed to be on the drawing board.
Realizing flexible displays requires DDIC chip on film (COF) technology. Chip on glass (COG), meanwhile, offers a method of directly mounting DDIC onto a rigid glass substrate, whereas COF or chip on plastic (COP) is where the DDIC is directly bonded onto the flexible substrate to ensure realization of flexible displays. COF is a packaging method of attaching DDIC to a panel substrate by bonding thin film, whereas COP is a method of mounting DDIC directly onto the substrate.
The flexible qualities of COF make it possible to design the side area of a screen, often called the bezel, to be narrower compared with COG. This results in a relatively larger screen-to- body ratio. In other words, it can create a “bezel-less” or full screen display. Also, in order to realize flexible display where the screen itself bends, the DDIC package must also be flexible. This is why it is imperative to apply COF technology. By contrast, LCD drivers can not nr physically folded or bent.
With the increasing resolution of smartphone displays, the number of channels of DDIC connected to an individual pixel of the display panel grows. In order to support high resolution, a ‘double-sided 2 Metal COF” package technology is required. In general, resolution of full high definition (FHD) and below can be achieved by one-layer metal COF, but resolution of quad high definition (QHD) and above, with a 30% increase in number of channels, requires two-layer metal COF. Therefore, for these high-resolution formats, it is essential that “fine-circuit” technology be embedded on both sides of the COF package for DDIC.
While smartphone display resolution has continued to improve, it, ironically, leads to more power consumption, thus reducing battery life of mobile devices such as smartphones. Also, when it comes to the RAM (Random Access Memory) for storing display pattern data within a DDIC, a higher resolution increases the amount of display pattern data stored in the RAM. Therefore, RAM capacity must increase, which increases the chip size of the DDIC.
In semiconductor processes, numbers like 55nm, 40nm and 28nm refer to the minimum device length between the source and the drain in a transistor, which functions as a switch in a digital circuit. The smaller the number, the better the switch performs. In other words, the switch operates at a faster speed and consumes less power, making it easier to design products of high performance and lower power consumption.
Finer-scale processes mean higher integration density where each and every circuit and RAM in a DDIC become smaller, making the entire chip size smaller and enabling design of smaller and thinner products. Also, finer-scale processes translate to using relatively less energy, reducing power consumption. In addition, DDIC manufacturers will take advantage of 28nm and beyond to make many more DDICs out of a single silicon wafer, raising cost effectiveness. This is why recently DDIC makers are striving to develop finer processes
Paul Kim, vice president of marketing, Standard Products Group, MagnaChip Semiconductor Corp., co-authored this article. Kim became vice president of marketing, Standard Products Group in December 2015. He joined MagnaChip in August 2011 and served as vice president of Display Design, Display Solutions Division. Prior to joining MagnaChip, Kim served as principal engineer of SOC & Display Driver IC Design Group of Samsung Electronics, where he worked from 1994 to 2010. Kim holds B.S degree in Electrical Engineering from Inha University, Korea.