A Luminance Adjusting Algorithm for High Resolution and High Image Quality AMOLED Displays of Mobile Phone Applications

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1 H.-J. In et al.: A uminance Adjusting Algorithm for High Resolution and High Image Quality AMOED Displays of Mobile Phone Applications A uminance Adjusting Algorithm for High Resolution and High Image Quality AMOED Displays of Mobile Phone Applications Hai-Jung In, Student Member, IEEE, Kyong-Hwan Oh, Oh-Kyong Kwon, Member, IEEE, Chang Ho Hyun, Member, IEEE, and Sung-Chul Kim 9 Abstract A luminance adjusting algorithm using light sensing scanner is proposed for small-sized high resolution and high image quality active matrix organic light emitting diode (AMOED) displays such as smartphone applications. By using simple pixel structure with the proposed algorithm, high aperture ratio in high resolution display can be achieved. Experimental results show that the standard deviation of luminance improves from 7.7 to.8 SB when the proposed adjusting method is used to 3.5-inch AMOED display with 8- bit gray scale. Index Terms ight-emitting diode displays, thin film transistors, compensation, flat panel displays I. INTRODUCTION Active matrix organic light emitting diode (AMOED) displays with good image quality are generally used in mobile phones due to excellent color reproducibility, thin form factor, wide viewing angle, and low driving power of the panel []. The main problem for image quality in AMOED displays on polycrystalline silicon (poly-si) thin film transistor (TFT) backplane was non-uniform luminance of display image due to electrical characteristic variations of driving TFTs [], but good driving methods compensating these variations have been proposed [3-8]. However, in previous methods, many numbers of TFTs or capacitors are required in a pixel for compensating electrical characteristic variations of driving TFTs. This complex pixel structure decreases aperture ratio of pixel and increases emission current for organic light emitting diode (OED) in AMOED displays of bottom emission type. Increased emission current accelerates the degradation of OEDs and increases the power consumption of the panel. This situation gets worse as the resolution of the display panel increases. Furthermore, the increased number of pixels increases the deviation of the electrical characteristic of TFTs and optical characteristic of OEDs, so the image quality of the display is degraded. However, for smartphone applications, higher resolution and higher image quality displays are H. -J. In, K. H. Oh, and O. -K. Kwon are with the Department of Electronics and Communications Engineering, Hanyang University, 7 Haengdang-dong, Seongdong-gu, Seoul, 33-79, Korea (phone: ; fax: ; okwon@hanyang.ac.kr). C. H. Hyun and S. -C. Kim are with the Samsung Mobile Display Co., 58 Sungsung-dong, Cheonana city, Chungcheongnam-do, 33-3, Korea. required as the display contents increase. As a result, a new compensation method having a simple pixel structure and high image quality is required for small sized high resolution AMOED displays. In this paper, a luminance algorithm is proposed to increase the aperture ratio using simple pixel structure of TFTs and capacitor, and to compensate luminance deviation due to the electrical characteristic variations of not only TFTs but also initial optical characteristic deviation of OEDs. The proposed algorithm is verified by 3.5-inch AMOED panel of wide video graphics array (WVGA) resolution format. II. PROPOSED DRIVING METHOD Fig. shows the schematic diagram of the pixel structure for proposed luminance adjusting algorithm. The pixel consists of two p-type TFTs and one storage capacitor and OED device. Proposed luminance adjusting method can be divided into luminance sensing, parameter extraction, and display operations. When AMOED panel is fabricated, the luminance sensing operation is performed once for storing luminance of each pixel of the panel. Fig. shows the block diagram of the system during the luminance operation. uminance sensing operation can be divided into two steps. First, the control board sends the first reference digital data signal to data driver IC to apply the first reference voltage (V ref ) to the gate node of P of pixels on the first row line and EVDD to the gate node of P of the other pixels on the panel. A light sensing scanner senses the luminance of each pixel on the first row line and converts them to digital data. When the luminance value of each pixel of the first line is stored in the nonvolatile memory, control board controls the data driver IC to apply V ref to the gate node of P of pixels on the next row line and EVDD to the gate node of P of the other pixels on the panel. In the same way of the first row line, the luminance of each pixel on the next row line is sensed and digitalized by the light sensing scanner and stored in the nonvolatile memory. When the light sensing scanner scans all of the row lines on the panel, the luminance of each pixel on panel when V ref is applied to the gate node of P is stored in the nonvolatile memory. Second, the control block sends the second reference digital data signal to data driver IC to turn on the pixels line by line. Contributed Paper Manuscript received 7/5/ Current version published 9/3/ Electronic version published 9/3/ //$. IEEE

2 9 IEEE Transactions on Consumer Electronics, Vol. 56, No. 3, August of each pixel in the nonvolatile memory. Fig. (c) shows the block diagram of the system during the calculation operation. A calculation board extracts parameters containing electrical characteristic of TFTs and optical characteristic of OED of each pixel from the luminance data in the memory using the proposed algorithm. Extracted parameters restore luminance data of each pixel in the nonvolatile memory. The calculation operation is also performed only once like luminance sensing operation when the panel is fabricated. Fig. (d) shows the block diagram of the system during display operation. A data modulation block modulates video data signal using stored parameters of each pixel in the nonvolatile memory and sends the modulated video data signal to the data driver IC. When data driver IC drives AMOED panel using modulated video data signal, the panel displays clear image with the compensation of electrical characteristic variations of TFTs and optical characteristic variations of OEDs. memory (c) Calculation Board (d) Fig.. Schematic diagram of the pixel structure and block diagram of systems during luminance sensing, (c) calculation, and (d) display operations. The sensing sequence of the second step is same as the first sensing step but only the difference is the second reference voltage (V ref ) is applied to the gate node of P in each pixels. When the light sensing scanner scans all of row lines on the panel, the luminance of each pixel on panel when V ref is applied to the gate node of P is stored in the nonvolatile memory. When luminance sensing operation is terminated, calculation operation is performed using stored luminance data III. UMINANCE ADJUSTING AGORITHM During the luminance sensing operation, when the V ref is applied to the gate node of P, the drain current of P can be expressed as W I ( ) D, = μ PC EVDD Vref, () where P, C, W, and V thp are the mobility, the gate capacitance per unit area, the channel width, the channel length, and the threshold voltage of P, respectively. Because the luminance of OED is proportion to the drain current of P, the luminance of the pixel when the gate voltage of P is equal to V ref can be expressed as W ( ) = α μ PC EVDD V, () where is the current-to-luminance efficiency of OED. In the same way, the luminance of the pixel when V ref is applied to the gate node of P can be expressed as W ( ) = α μ PC EVDD V ref. (3) ref and ref of each pixel on the panel are sensed and memorized in the memory during the luminance sensing operation. et define s and t as s =, (4) αμ PC W t = EVDD V thp. (5) By using stored ref and ref, s and t can be calculated during the calculation operation using (6) and (7). V Vref s =, (6) t V ref V ref ref =, (7) ref

3 H.-J. In et al.: A uminance Adjusting Algorithm for High Resolution and High Image Quality AMOED Displays of Mobile Phone Applications 93 Extracted s and t parameter of each pixel replaces ref and ref of each pixel in the memory. During display operation, video data signal is modulated using (8). data data = t s, (8) mod MAX n where data mod, data, MAX, n are modulated video data signal, video data input signal, maximum target luminance of the OED, and the total number of gray level of the display, respectively. Modulated data signal is converted to analog data voltage using data driver IC and applied to the gate node of P in each pixel. In this time, the luminance of the pixel ( display ) can be expressed as W = α display μ P C ( EVDD V data ). (9) data = MAX n As shown in (9), the luminance of pixels does not depend on the threshold voltage and mobility of P and the current-toluminance efficiency of OED during the display operation. IV. SIMUATION RESUTS The proposed luminance adjusting algorithm is simulated on the variation conditions of the electrical characteristic of P and the optical characteristic of OED. The simulation models are extracted from the measured electrical and optical characteristics of TFT and OED devices. Fig. and show simulation results of the OED luminance in pixels using conventional driving and the proposed algorithm when the threshold voltage variation of P is from -.4 to.4 V, respectively. The OED luminance error of conventional driving ranges from to 6.3%, but that of the proposed algorithm ranges -.5 to.8%. Fig. 3 shows simulation results of the OED luminance in pixels using the proposed algorithm when the current-to-luminance efficiency variation of OED is from -5 to 5%. The luminance error using the proposed algorithm ranges -.6 to.7%. As a result, it is shown that the proposed luminance adjusting algorithm successively compensate variations of not only the electrical characteristic of driving TFT but also the optical characteristic of OED. V. MEASUREMENT RESUTS The proposed luminance adjusting algorithm is demonstrated on 3.5-inch WVGA (48 8) AMOED panel of poly-si TFT backplane. The PenTile Matrix TM structure [9] is used for pixel array. The specification of the panel is summarized in Table I. uminance % uminance % ΔVth = -.4 ΔVth = ΔVth = ΔVth = -.4 ΔVth = ΔVth = Fig.. Simulated luminance of OED using conventional driving and proposed algorithm when threshold voltage variation of P is from -.4 to.4 V. uminance % Δ OED efficiency = -5% Δ OED efficiency = % Δ OED efficiency = +5% Fig. 3. Simulated luminance of OED using proposed algorithm when current-to-luminance efficiency variation of OED is from -5 to 5 %.

94 TABE I SPECIFICATION OF THE PANE Specification Description Size 3.5-inch Resolution WVGA (48 8) Maximum luminance 3 cd/m Pixel VDD voltage 5 V Data voltage range.5 V ~ 4.

5 pf Although the unit pixel size is very small due to the high resolution format, 3% aperture ratio can be achieved using simple pixel structure in bottom emission type AMOED pixel structure when

DC-DC converter is used for generating EVDD and EVSS voltage of pixels on panel.

4 94 TABE I SPECIFICATION OF THE PANE Specification Description Size 3.5-inch Resolution WVGA (48 8) Maximum luminance 3 cd/m Pixel VDD voltage 5 V Data voltage range.5 V ~ 4.5 V Width by height of the unit pixel 48 m x 96 m Capacitance of C in pixel.5 pf Although the unit pixel size is very small due to the high resolution format, 3% aperture ratio can be achieved using simple pixel structure in bottom emission type AMOED pixel structure when the proposed algorithm is used. Fig. 4 shows the display test board for applying proposed luminance adjusting algorithm with the 3.5-inch AMOED panel. DC-DC converter is used for generating EVDD and EVSS voltage of pixels on panel. uminance of each pixel on panel is sensed using light sensing scanner, and s and t parameters of each pixel are extracted and stored in the flash memory. IEEE Transactions on Consumer Electronics, Vol. 56, No. 3, August The digital video signal input is modulated by the field programmable gate array (FPGA) using the stored s and t parameters in the flash memory. The modulated video data is applied to the data driver IC. The data driver IC converts 8-bit modulated video data to analog voltages and applies analog voltages to AMOED panel. Fig. 5 and show the photographs of the panel displaying middle gray level when the proposed algorithm is not used and is used, respectively. When the proposed algorithm is not used, the standard deviation of the luminance of the panel is 7.7 least significant bits (SB) due to the electrical characteristic variations of TFTs and optical characteristic variations of OEDs. The luminance deviation is improved to.8 SB when the proposed compensation algorithm is used. AMOED panel Data driver IC Digital video signal input Fig. 5. Photographs of the panel displaying middle gray level when the proposed algorithm is not used and is used DC-DC converter Flash memory FPGA Fig. 4. Display test board with 3.5-inch WVGA AMOED panel for applying proposed luminance adjusting algorithm. V. CONCUSION This paper proposes a luminance adjusting algorithm for high resolution and high image quality AMOED displays. The advantage of the proposed driving method is that both high image quality of the display and high aperture ratio of pixels can be achieved in small sized high resolution displays. Proposed method not only compensates the electrical characteristic variations of TFTs but also the initial luminance deviation of OEDs. Experimental results show that the proposed algorithm successively reduces the standard deviation of the luminance of 3.5-inch WVGA AMOED panel from 7.7 to.8 SB in middle gray level of 8-bit gray scale with 3% aperture ratio in bottom emission type pixel structure. As a result, it is expected that the proposed luminance adjusting algorithm is highly applicable for the high resolution and high image quality AMOED displays such as smartphone applications.

H.-J. In et al.: A uminance Adjusting Algorithm for High Resolution and High Image Quality AMOED Displays of Mobile Phone Applications 95 REFERENCES [] S. Forrest, P. Burrows, and M.

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5 H.-J. In et al.: A uminance Adjusting Algorithm for High Resolution and High Image Quality AMOED Displays of Mobile Phone Applications 95 REFERENCES [] S. Forrest, P. Burrows, and M. Thompson, The Dawn of Organic Electronics, IEEE Spectr., vol. 37, no. 8, pp. 9-34, Aug.. [] X. Guo and S. R. P. Silva, Investigation on the current nonuniformity in current-mode TFT active-matrix display pixel circuitry, IEEE Trans. Electron Devices, vol. 5, no., pp , Nov. 5. [3] S. -H. Jung, W. -J. Nam, and M. K. Han, A New Voltage-Modulated AMOED Pixel Design Compensating for Threshold Voltage Variation in Poly-Si TFTs, IEEE Electron Device ett., vol. 5, no., pp , Oct. 4. [4] H. -Y. u, P. -T. iu, T. -C. Chang, and S. Chi, Enhancement of Brightness Uniformity by a New Voltage-Modulated Pixel Design for AMOED Displays, IEEE Electron Device ett., vol. 7, no. 9, pp , Sep. 6. [5] C. -. in, T. -T. Tsai, and Y. -C. Chen, A Novel Voltage-Feedback Pixel Circuit for AMOED Displays, IEEE J. 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BIOGRAPHIES Hai-Jung In (M 4) received the B. S. and M. S. degrees in electronics and computer engineering from Hanyang University, Korea, in 4 and 6, respectively. He is currently working toward the Ph.D. degree in electronics and computer engineering at the same university. His research interests include analog circuit design, system-on-panel (SOP), and driving methods and circuits for flat panel displays. Kyong-Hwan Oh (M 6) received B. S. degrees in electronics and computer engineering from Hanyang University, Korea, in. He is currently working toward the Ph.D. degree in electronics and computer engineering at the same university. His research interests include system-on-panel (SOP), and driving methods and circuits for flat panel displays. Oh-Kyong Kwon (S 83-M 88) received the B.S. degree in electronic engineering from the Hanyang University, Seoul, Korea, in 978 and the M.S. and Ph.D. degrees in electrical engineering from the Stanford University in 986 and 988, respectively. From 98 to 983, he was with G Electronics Inc., Seoul, Korea, where he involved in the development program of telecommunication products including G-3 fax system and PCM system. From 987 to 99, he was with the Semiconductor Process and Design Center, Texas Instruments Inc., Dallas, Texas, where he was engaged in the development of multi-chip module (MCM) technologies and smart power integrated circuit technologies for automotive and flat panel display applications. In 99, he joined Hanyang University, Seoul, Korea, as an assistant professor in the Department of Electronic Engineering, and is now a professor in the Division of Electrical and Computer Engineering at Hanyang University. Dr. Kwon has been served the position of the dean of the College of Engineering at Hanyang University since 8. He was an IEEE IEDM subcommittee member on solid state devices from 997 to 998, a technical program chair of 999 IEEE International Conference on VSI and CAD, and a workshop co-chair of and Asia-Pacific Workshop on Fundamental and Application of Advanced Semiconductor Devices (AWAD). Dr. Kwon is serving the position of IEEE EDS Korea Chapter Chair. Dr. Kwon was the program manager of Korean TFT-CD Research and Development Program from 993 to 997. Dr. Kwon was the program manager of Korean Flat Panel Research and Development Program from 998 to. Dr. Kwon was a technical program chair of International SoC (System-on-a-Chip) Conference 4, and a technical program chair of International Meeting on Information Displays/International Display Manufacturing Conference 6. He is currently serving in the position of Vice-President of IEEK (Institute of Electronics Engineers of Korea), in the position of an executive chair of International Meeting on Information Display 7, and in the position of a technical program committee member of SID (Society for Information Displays), ISSCC (International Solid State Circuit Conference) and International Display Workshop. His research interests include interconnect and electrical noise modeling for high-speed system-level integration, waferscale chip-size packages, smart power integrated circuit technologies, mixed mode signal circuit design, and the driving methods and circuits for flat panel displays. He has authored and co-authored over 84 international journal and conference papers and 97 U.S. patents. Chang Ho Hyun (M 95) received the B.S. and M.S. degrees in Physics from Hanyang University, Korea, in 993 and 995, respectively. From 995 to, he worked for R&D center of Orion Electronic Company, where he developed Driving Methods and Circuits for PMOED. From to 7, he worked for R&D center of G electronics, where he developed Driving Methods and Circuits for Ultra-slim Projector, PDP, FED and AMOED. Presently, he is developing uminance Compensation Algorithms and Digital Driving methods for AMOED as a Principle Engineer of OED development department at Samsung Mobile Display. Sung-Chul Kim received the B.S. degree in physics from the Kyunghee university, korea, in 985 and the M.S. and Ph.D. degrees in physics at the same university in 987 and 99 respectively. From 99 to 998, he worked at Anyang R&D center and manufacturing R&D center of G electronics. In, he has been working for Samsung SDI Inc., Korea. At same year, he was involved in full color PMOED business and succeeded world's first commercialization of PMOED. Since, Dr. Kim has engaged in the new AMOED business development and managed AMOED technology and process development. In 6, he succeeded in the world's first mass production of AMOED (Top-emission structure and encapsulation technology). From 7 to 8, Dr. Kim has constructed full line-up for "~ 5" products of AMOED. Furthermore, he has led the development of large size AMOED. As a result, he has developed 4", 3" and 4" AMOED. In 9, he succeeded in the development of the world's first VGA AMOED for mass production. Currently, Dr. Kim is working as a senior vice president at Samsung Mobile Display.

3012 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 11, NOVEMBER 2010

3012 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 11, NOVEMBER 2010 An Advanced External Compensation System for Active Matrix Organic Light-Emitting Diode Displays With Poly-Si Thin-Film Transistor