Implementation of a Differential Aging Measurement ...

Proceedings of the 7th WSEAS International Conference on SIGNAL PROCESSING, ROBOTICS and AUTOMATION (ISPRA '08) University of Cambridge, UK, February 20-22, 2008

Implementation of a Differential Aging Measurement and Compensation System for OLED Panel Kyungjoong Jeong POSTECH Info. Technology Dept. Pohang, Kyungbuk, 790-784 South Korea

Sungmin Woo POSTECH EECE Dept. Pohang, Kyungbuk, 790-784 South Korea

Hong Jeong POSTECH EECE Dept. Pohang, Kyungbuk, 790-784 South Korea

Abstract: This paper presents a real-time aging measurement and feasible compensation system for the prolonged lifetime of OLED panel. The proposed system is composed of four parts, a PC with a man-machine interface, a measurement block, an adaptive amplifier block, and a compensation block. We apply a tree algorithm for less complexity and convenience of measurement on the degree of aging. An adaptive multi-stage differential amplifier is also implemented to deal with a various range of input voltages at the same spot. Key–Words: OLED, Lifetime, Aging, Measurement, Compensation

1 Introduction Since C.W. Tang and S.A Van slyke of Kodak proposed an OLED scheme in 1987 [1], OLED technology became one of the popular fields of information displays in the recent. The OLED has unique characteristics, including self-emission, lightness, thinness, fast electronic response and wide viewing angle. Thus, the OLED panel can display any kind of real-time video without distorting colors or limiting the viewing angle. Due to its advantageous properties, the OLED panel could be the trend of next generation in information display. Electronic devices have already used OLED technology such as mobile phone, MP3 player, and monitor. Recently, the Sony Company opened an exhibition of world first OLED TV, XEL-1. [2] Although the excellent performance of the OLED panel has been proven from the experimental results, some technical issue must be solved for mass-production. They include back plane, encapsulation, power consumption, lifetime problem. One important issue of all the existing problems is the lifetime of an OLED panel. Differential aging is another problem which causes an image sticking problem. Consequently, a commercial system to measure and compensate the OLED lifetime must be developed for application in this new field of technology. Fig. 1 illustrates a new measurement and compensation system. It includes multiple parts of a current sensor to sense the currents of the OLED panel and an adaptive amplifier to amplify the voltage from the current sensor. A PIC microprocessor converts AC into ISSN: 1790-5117

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Figure 1: Measurement on aging and compensation system DC and adaptively captures the voltage from the adaptive amplifier. Man-machine interface program helps user observe the data communication and control the OLED panel. The compensation circuit stores original pixel’s information and offsets pixel’s brightness.

2 Method of measuring and compensating the differential aging OLED devices age and become less efficient as they are used. The loss of light output comes from a decrease in current-to-photon conversion efficiency and from an increase in OLED resistance that results in a reduction of current through the OLED for a given drive signal. By measuring the changes in current at ISBN: 978-960-6766-44-2

Proceedings of the 7th WSEAS International Conference on SIGNAL PROCESSING, ROBOTICS and AUTOMATION (ISPRA '08) University of Cambridge, UK, February 20-22, 2008

Figure 3: Current sensor location in the OLED panel

Figure 2: User friendly control interface a given drive signal, the loss in efficiency can be inferred, and a correction can be applied to the driving signal to compensate for OLED aging.

2.1 Man-machine interface

Figure 4: A tree structure for aging measurement

Fig. 2 shows a user-friendly environment. This program is oriented to exactly measure how a panel is aged in real-time. It provides several functions to control parameters. A user can see a list of test results graphically. The user also make a change of the color brightness of the OLED background and set basic test parameters such as the size and color of kernel through the window.

a current sensor on the power line. The current sensor senses the current and converts the amount of the current into a voltage value followed by sending it to the amplifier. At the final stage, insufficient amount of voltage is determined and compensated. More details will be discussed in section 2.4.

2.2 Measurement location and method

2.3 Algorithm of Measurement

Recently, some measurements on how aged a panel is have been reported. Eko T Lisuwandi developed CMOS camera method [3]. This method is powerful in that a user can find the area and degree of aging. However, a CMOS camera is in need and it is very sensitive to the background brightness. Photo-sensor method proposed by Michael E. Miller et al provides accurate measurement [4]. However, it needs additional circuits for measurement analysis and pay for expensive design cost as the panel size increases. We suggest an indirect method by measuring the amount of current change. In reference to the Fig. 3, we place

We propose a tree algorithm. To use it, we divided a panel into 24 blocks. A current flowing through each block was converted to a voltage value and memorized as a reference value for comparison. An aged pixel drops the original voltage since resistance of the panel increases. Therefore, the tree algorithm tries to find the smallest possible aged blocks whose voltage values has been deviated from the original ones as in Fig. 4. If a spot on one of the 24 blocks is aged, we split the block into four sub-blocks, and measure them again. To find an exact aged area, we keep splitting aged sub-blocks into another four sub-blocks until we

ISSN: 1790-5117

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ISBN: 978-960-6766-44-2

Proceedings of the 7th WSEAS International Conference on SIGNAL PROCESSING, ROBOTICS and AUTOMATION (ISPRA '08) University of Cambridge, UK, February 20-22, 2008

Figure 6: Adaptive amplifier circuit

2.5 Compensation The amplified voltage from the adaptive amplifier is compared with the stored reference. The RAM on the Compensation Circuit memorizes the voltage difference between the amplified and the referenced as a compensated voltage in real time. The compensated voltage is then added to video signal at the same time, intensifying decreased pixel brightness. Since the proposed system employs the current sensor, the realtime measurement and compensation are realizable as well as less effect on voltage drop on panel. Figure 5: Tree algorithm find the smallest aged areas on the panel. Pseudo code for the proposed tree algorithm is described in Fig. 5. Incorporating the tree algorithm reduces complexity and provides fast measurement due to the block-based method. Complexity of the proposed tree structure is ) in O(  ) , while it results in O(   the case that all pixel values are measured. Note that , , and  are the horizontal size, vertical size of a panel, and the horizontal size of divided sub-blocks, respectively.

    



2.4 Adaptive Amplifier



Voltage range obtained from the proposed measureto since the searched blocks ment varies from include a different number of aged pixels. Therefore, the various amplification levels for compensation have to be applied to each corresponding aged pixel. The adaptive amplifier described in Fig. 6 is designed to deal with a various range of voltage amplifications. First, a voltage value converted from current sensor is amplified through a multi-stage differential amplifier. Next, the PIC microprocessor decides one of the five different paths (   ) according to the voltage range.



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3 Experimental results and Analysis We made two experiments with the proposed measurement and compensation system. We used Samsung OLED panel with 1280*768 resolutions. The panel was already aged in the middle of production. Fig. 7 shows the measurement result on aging according to red, green, and blue colors. 24 picks in each paragraph suggest how age the spot is. To the aged areas on the panel, we employed compensation process with the proposed tree algorithm. We implemented a compensation circuit on our FPGA board which communicates a PC through universal serial bus. Compensation data are originally written on the RAM mounted on the board. In real-time, panel’s aging degree data are transmitted and compared with the originally stored one. FPGA does compensation and send the offset pixel values to the panel. We simulated the compensation results in Fig. 8. Note that the compensating and input data are summed and appears to be output brightness.

4 Conclusion We introduced a new method of indirectly measuring how aged pixels are and compensating those pixISBN: 978-960-6766-44-2

Proceedings of the 7th WSEAS International Conference on SIGNAL PROCESSING, ROBOTICS and AUTOMATION (ISPRA '08) University of Cambridge, UK, February 20-22, 2008

els, which prolongs the panel’s lifetime. The proposed tree algorithm reduces complexity and provides fast measurement. The adaptive amplifier consists of multi-stage differential amplifiers and has a wide range of input voltage levels, allowing direct comparison with predefined reference values. The experiment proved that the proposed system effectively offsets aged pixels with block-based operations. Our future work may include measurement timing problem which will be an important issue for a commercial system. Acknowledgements: The research was supported by SAMSUNG, Korea References:

Figure 7: Red, Green, and Blue component measurement results

[1] C. W. Tang, S. A. Van slyke, Organic Electroluminescent Diodes, Appl. Phys. Lett. 51, 1987, pp. 913–915. [2] http://www.sony.jp/products/Consumer/oel/ special/index.html verified January, 2008. [3] Eko T Lisuwandi, Feedback Circuit for Organic LED Active-Matrix Display Drivers, Massachusetts Institute of Technology M.S. thesis, 2002. [4] Michael E. Miller, Ronald S. Cok, Andrew D. Arnold, Michael J. Murdoch, Color OLED Display System having Improved Performance, United States Patent, 2004.

Figure 8: Simulation result

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ISBN: 978-960-6766-44-2