The role, and future, of the display driver integrated circuit in OLED displays
02 August 2018
This piece from MagnaChip covers the role that DDIC (display driver IC) technology plays in the function of modern display technology, and ultimately considers the impact it will have on the next generation of OLED screens in mobile devices.
Flat panel display technology is divided into two types: non-emissive display, which requires an external light source; and emissive display, which produces its own light organically. Thin-film transistor (TFT) LCD is a non-emissive and older display technology which requires an external light source to work, while OLED (organic light emitting diode) is considered an emissive display technology that does not require a backlight, because each pixel provides its own illumination.
OLED (organic light emitting diode) has three self-emitting phosphorescent organic compounds: red, green and blue (RGB). An OLED display utilises the emissive phenomenon of three-colour phosphorescent organic compounds combined with electrons emitted from positive anode and negative cathode particles.
An OLED display driver integrated circuit (DDIC) is a component that controls the OLED display panel: it enables thinner and bezel-less displays that are also more flexible and foldable and provide a wide range of colours 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 the required 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 the former, by supplying a current into both vertical and horizontal panel ends, the pixels will emit light where the currents cross; therefore, by controlling the amount of crossing current, the intensity of light is also controlled.
As for AM OLED, moreover, each pixel in the panel has a TFT (thin film transistor) 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 commands all of the AM OLED’s pixels, each is controlled through TFT. Display pixels consist of subpixels that represent RGB. With such sub-pixels being controlled by the TFT, and given that the DDIC sends signals to the TFT, it is the DDIC that ultimately controls the pixels directly. Consider, therefore, the TFT functioning as a switch that drives RGB sub-pixels – while the DDIC functions as a type of ‘traffic light’ to instruct such a switch on how to operate.
Technological trend of OLED DDICs
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 in two ways: rigid or flexible. Rigid type uses a rigid glass substrate, while flexible type employs a flexible substrate based on a plastic material called polyimide, which 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).
To realise such flexible displays, DDIC COF (chip on film) technology is a requirement. COG (chip on glass) is a method of directly mounting the DDIC onto a rigid glass substrate, whereas COF, and also COP (chip on plastic), sees the DDIC being directly bonded onto the flexible substrate – to ensure the realisation of flexible displays.
To specify, 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 the former make it possible to design the side area of a screen (often called the bezel), to be narrower than COG. This results in a relatively larger screen-to-body ratio. In other words, it can create a ‘bezel-less’ (or full screen) display. Moreover, in order to realise flexible displays where the screen itself bends, the DDIC package must also be flexible; and this is why it is imperative to apply COF technology. By contrast, LCD drivers can not physically fold or bend.
With the increasing resolution of smartphone displays, the number of DDIC channels connected to an individual pixel of the display panel also increases. In order to support high resolution, a ‘double-sided 2 Metal COF’ package technology is required. In general, while the resolution of FHD (full high definition) and below can be achieved by 1-layer metal COF, but resolution of QHD (quad high definition) and above – with a 30 percent increase in the number of channels, requires 2 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, the downside is that this leads to more power consumption, therefore reducing the battery life of mobile devices. 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. RAM capacity must therefore increase accordingly, which also 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 – reducing the entire chip size and enabling the design of smaller and thinner products. Also, finer-scale processes translate to using relatively less energy, which reduces 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, as of recently, DDIC makers are striving to develop finer processes.
DDIC technology will power the flexible display capabilities of OLED and, as a result, it will play a key role in the growing and highly competitive mobile market – where differentiated design is of course critical for success. Since flexible AM OLED display successfully entered mass production in 2013, products in various form factors have been launched and the design freedom that OLED enables will bring even more innovation in the years to come.
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