Abstract

iv Organic Light Emitting Diode (OLED) has emerged as a new generation of display techniques for mobile devices. Emitting light with organic fluorescent materials OLED display panels are thinner, brighter, lighter, cheaper and more power efficient, compared to other display technologies such as Liquid Crystal Displays (LCD). In present mobile devices, due to the battery capacity limitation and increasing daily usage, the power efficiency significantly affect the general performance and user experience. However, display panel even built with OLEDs is still the biggest contributor to a mobile device's total power consumption. In this thesis, a finegrained dynamic voltage scaling (FDVS) technique is proposed to reduce the OLED display power consumption. In bottom level, based on dynamic voltage scaling (DVS) power optimization, a DVS-friendly AMOLED driver design is proposed to enhance the color accuracy of the OLED pixels under scaled down supply voltage. Correspondingly, the OLED panel is partitioned into multiple display sections and each section's supply voltage is adaptively adjusted to implement fine-grained DVS with display content. When applied to display image, some optimization algorithm and methods are developed to select suitable scaled voltage and maintain display quality with Structural Similarity Index (SSIM), which is an image distortion evaluation criteria based on human vision system (HVS). Experimental results show that, the FDVS technique can achieve 28.44%~39.24% more power saving on images. Further analysis shows FDVS technology can also effectively reduce the color remapping cost when color compensation is required to improve the image quality of an OLED panel working at a scaled supplied voltage. FINE-GRAINED DYNAMIC VOLTAGE SCALING ON OLED DISPLAY Xiang BACKGOUND Current mobile devices become much more popular than ever in peoples' daily life. The general definition of mobile devices is also extended from cellphones into portable game consoles, tablets, laptops and other new electronic products. It is so amazing that the cellphones in past ten Moreover, the power consumption problem still exists in OLEDs. Unlike LCDs whose power is primarily consumed by BLUs, OLEDs' power consumption is decided by the colors tuned in each pixel. In a single OLED, that generates specific color, the added dopant to organic materials affects the luminance and power characters. Also, since an OLED pixel is composed of several different monochromatic OLED cells (or sub-pixel), and use particular luminance combination to produce chromatic colors, the power consumption will also vary a lot with colors. For instance, the power consumption of an OLED pixel displaying "black" is only around 40% of a LCD pixel, whose power consumption is constant in different colors. This is because almost no current is necessary to pass the organic materials to produce any light. However, an OLED pixel displaying Power difference also exists in different colors, blue OLEDs have much lower power efficiency than red and green ones MOTIVATION As an emerging display technology, OLEDs have been well promoted into practical mobile devices with increasing market share. Behind this fast development, more than 10 years of 5 dedicated researches have been invested in organic material, pixel arrangement, and manufacture process. These researches are mainly focusing on increasing OLEDs' display quality, accuracy and area to apply in devices application. Meanwhile, other research focusing on peripheral driving method, power efficiency, and system level optimization are still remained immature, compared to other display technologies such as LCD. As presented before, the power efficiency might be one of the most direct issues to improve the general mobile device performance and user experience. The basic idea behind this thesis is to implement dynamic down scaling the display panel's supply voltage and hence reduce the general power consumption. However, in conventional OLEDs, color will be distorted significantly when big down scaling is applied to supply voltage. Hence, modifying the OLEDs driver circuits to create more voltage scaling space without luminance distortion is the first key to achieve DVS and power saving. Also when applied into higher level of display panel, how to make most advantage of the DVS with different display content is another challenge. And when the voltage is applied into the display panel, how to evaluate and maintain the display quality is also an important issue to system level optimization. In this thesis, I am focusing to build up such a system solving these problems. New driver circuits are designed with a series of simulations; image analysis and quality evaluation are studied to develop suitable scaling algorithm and optimization method. Evaluations are made to prove the efficiency of proposed techniques. 6 PREVIOUS WORK Since power saving technology in OLED display panels is not fully developed in previous work, some ideas are derived from LCD voltage scaling techniques. Right now, DVS methods are already applied into LCDs, where the BLUs are dimmed to save power. This section will review these cases to present the mature concepts of display DVS techniques. Also previous display technologies in OLEDs are also introduced, including modeling, power analysis and some DVS related research. LCD Voltage Scaling A typical LCD panel is composed by multiple layers, such as BLUs layer, liquid crystal layer, filters and substrates. A basic LCD display panel diagram is shown as in With more complex crystal alignments and color filter films, the LCD panels can be made with specific pixel matrixes and produce chromatic colors In LCD panels, the amount of light perceived by human eyes depends on the color intensity, have already been widely adopted in present LCD devices. However these methods still can't 8 reduce the power consumption of display during normal display operation. In some more complex solutions, the BLUs dimming are not only associated with environment but also with the display content dynamically. Since an amplify factor contrast to the BLUs dimming can be applied to the transmissivity t, luminance L can be well compensated with dark display content. Hence the power consumption in BLUs can be also trade off by scaling down supply voltage dynamically with display content on runtime, and the display quality can be maintained. Nowadays, LEDs replaced CFLs to work as BLUs, which make dynamic local dimming possible. In LEDs based LCD panels, the LED bulbs are aligned as matrix under the liquid crystal layers. And each LED bulb can be controlled independently to achieve dynamic dimming for specific display area. Correspondingly, the display content on the LCD panel is virtually divided into multiple regions in order to adapt local content. If the DVS on BLUs is applied on some areas without visual focus, the dimming even with distortion may not affect human visual perception. And the dark region areas can be dimmed more aggressively than bright ones. Hence the local dimming can provide a chance to gain more power saving in LCDs [24]. OLED Voltage Scaling Although the OLEDs are much more power efficient than LCDs, they are still very power consuming comparing to other components in a mobile device. In From the table, we can tell that: OLED display is quite display content dependent; when the display contents fulfill the display screen, the display will directly affect the general power consumption; if we can aggressively dim the OLED screen in some areas, the power can be significantly reduced. With discoveries in practical measurement, it is optimal to apply dimming mechanism into OLEDs, which might be achieved by DVS. However, when DVS is applied into OLEDs' supply voltage, the displayed luminance will be directly affected. Hence color fix or color compensation becomes a very important part of OLED DVS and power saving. Moreover, since an OLED pixel's power consumption is positive correlated with the color displayed, the balance between DVS and compensated color will also become a problem. In some mobile devices, especially those heavily emphasize the operating systems, UI colors are mainly designed with low power consuming colors like black and green, while some high power consuming colors like white and blue are avoided 10 Similar with the dimming technologies from LCDs, global DVS was also introduced to OLED power management to adjust the display panel luminance CONTRIBUTION OF THIS THESIS Focusing on the circuit design, image processing, and image quality evaluation, this thesis gives a new solution to OLEDs' power saving. Compared to the existing works, this thesis's contributions are: (1) A new OLED driver design is proposed as DVS-friendly OLED driver. This OLED driver design can effectively maintain the color accuracy when supply voltage is scaled down, as long as the required luminance of the OLED cell is within the specific threshold. (2) The DVS performance is well evaluated, considering the relationship between voltage scaling and the color distortion. The DVS-friendly driver's performance is evaluated along with conventional driver design. 11 (3) Fine-grained DVS is proposed in OLED display panel. The display panel is virtually divided into multiple sections with independent voltage regulator to dynamically adapt the supply voltage with display content and achieve aggressive voltage scaling. (4) Corresponding voltage selection, display quality control algorithms are proposed. Based on human virtual perception image quality of the OLED displays index, i.e., Structural Similarity (SSIM), lots of image processing simulations are made to evaluate DVS affect to color distortion, display quality and color compensation cost. (5) A series of experiment are made to evaluate the FDVS performance. The experiment results are compared with those of previous works to evaluation its effectiveness and advantages. SUMMARY In this chapter, some display technology basics are introduced, including LCD and OLED. Also the advantages of OLED over LCD are presented in several aspects, especially for color tuning mechanism and power saving efficiency. Technologies issues are emphasized in the display power saving research like color tuning and voltage scaling, which are derived from LCD technology. The key idea to realize the OLED power saving is particular driver circuit and corresponding regulating method. Based on the proposed research, the contribution of this thesis is summarized. 12 DVS-FRIENDLY OLED DRIVER DESIGN In last section, the basic concepts of the OLED technique and dynamic voltage scaling have been introduced. As presented before, one of the most important key to explore the potential of DVS in power saving is to reduce the color distortion introduce by down scaled supply voltage. To achieve this goal, one of the most direct ways is to improve the driver circuit to be more resistant to color distortion when the supply voltage is scaled. In this section, the OLED cell structure and color tuning mechanism will be studied and the modeling of OLED will be built for later simulation. Also, several typical kinds of OLED pixel driver circuits will be discussed. Based on those studies, my own circuit design is proposed. Simulations are made to prove it is more suitable to OLED DVS. OLED CELL OLEDs' lighting mechanism is totally different from other display technologies, such as LCD. In a display image, the most basic display unit is a screen pixel. And a pixel might also be composed of different cells (or sub-pixels) according to color space or specific quality requirement. In a typical OLED pixel, several OLED cells emitting different colors are aligned together to form RGB color space. The OLED cell details will be presented in this section including physical structure, modeling, and performance characterization. In past few years, monochromatic OLEDs are mainly in mobile players, instrument panels and other small display screens. With advantages of OLEDs' low power consumption and high contrast, the display performance is highly improved than LCDs. In these OLED products, the cell structure is mainly based on a single layer of monochromatic OLED materials without any complex color tuning or sub-pixel matrix design. Similarly, people are also trying to build pure white lamp with OLEDs, which can have better power efficiency than CFLs and can be made flexible and transparent. However, there is no single dopant for OLED to produce pure white light. The technician from Phillips aligned yellow and blue OLED strips alternately with each other to build a white OLED lamp. Covered with light mix glass, the alternating two different color OLEDs can produce white similar color. The most popular design is still to align the three cells in a line to form a pixel. The discrete cells of red, green and blue are aligned side-by-side, sharing same voltage supply and substrate. However each cell in a pixel will be controlled by independent driver circuit and will be programed to tune different colors. In Samsung's recent years of OLED products, the sub-pixel color performance with specific area ratio. This design is widely adopted into the new Samsung products, like Galaxy Note2 and other tablets, which needs better display performance and bigger display area. The structure of such a RGB color tunable OLED pixel is shown in larger than the green cell. The red cell may be 10% smaller than the green OLED DRIVING Despite of the OLED cells, which consists sub-pixels and then the pixel matrix, a highly integrated OLED display panel consists of power suppliers, voltage regulators and selector switches for rows and columns. For a higher level view, the display panel is controlled by a MCU interface with current converters, timing controllers, graphic display data RAMs so on and so forth. When the display panel is working, graphic display data are bit mapped from static RAMs, and those data will be converted into voltage or current singles into the pixel driver circuit to program display content. In RGB color space, the display system is separated for three sets to control RGB color dimensions independently. That means each sub-pixel is driven independently. In OLEDs, the OLED driving methods and driver circuit design is totally specialized according to OLED's performance characters. OLED Driving Methods In OLED cell, despite of the color characters, which are determined by the metal dopants and sizes, the most important parameter of OLED cell is the luminance of the emitted light. In While the relationship between current and luminance is quasi-linear and does not depend much on temperature. The luminance suffers obvious distortion only at high temperature, but is still in linear relationship. Hence the best way to control the light output of the OLED is focusing on current modulating rather than voltage. With this control mechanism, the current through the OLED cell is the most important control object for an OLED driver. From a control perspective, the OLED cell is usually programed by two different driver methods, namely, pulse-width-modulation (PWM) and amplitude-modulation (AM). In PWM method, switching discrete signals are applied to the OLED cells. With sequentially closing and opening the driver switch, specific pulse patterns are applied to adjust the OLED cells' luminance. However, with quick switching signals applied on the OLED cells, the internal capacitances will be quickly charged and discharged and the OLED cells will suffer huge current spikes. That will case lethal affect to the OLEDs lifetime. Additionally, since the PWM method is working on high voltage/current amplitude range to prevent the descending along circuits, the power efficiency is not good enough. Rather than PWM method, the AM method is more preferred. In AM method, a constant current is applied to the OLED cell and maintained by its driver circuit. Only when the display 19 content is going to be changed, the OLED cells will suffer specific current change. The OLED