This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE JOURNAL OF SOLID-STATE CIRCUITS 1 A Hybrid AMOLED Driver IC for Real-Time TFT Nonuniformity Compensation Jun-Suk Bang, Hyun-Sik Kim, Member, IEEE, Ki-Duk Kim, Student Member, IEEE, Oh-Jo Kwon, Choong-Sun Shin, Joohyung Lee, and Gyu-Hyeong Cho, Fellow, IEEE Abstract—An active matrix organic light emitting diode Fig. 1 shows an AMOLED display system including col- (AMOLED) display driver IC, enabling real-time thin-film tran- umn driver ICs [6]–[8], [10], [11] and pixel circuits [3], [5]. sistor (TFT) nonuniformity compensation, is presented with a The AMOLED display-driving systems have been developed hybrid driving method to satisfy fast driving speed, high TFT current accuracy, and a high aperture ratio. The proposed in terms of three important aspects as follows. hybrid column-driver IC drives a mobile UHD (3840 × 2160) 1) High driving speed: As resolution and panel size increase, AMOLED panel, with one horizontal time of 7.7 µsatascan a one-horizontal (1 H) time for the driver to program frequency of 60 Hz, simultaneously senses the TFT current for data voltages (VDATA) into pixels in a row is continuously back-end TFT variation compensation. Due to external compen- reduced, which means that the driver must have a fast sation, a simple 3T1C pixel circuit is employed in each pixel. Accurate current sensing and high panel noise immunity is guar- driving speed. anteed by a proposed current-sensing circuit. By reusing the 2) Pixel-to-pixel luminance uniformity: This is mainly due hybrid column-driver circuitries, the driver embodies an 8 bit to temporal and spatial variation in thin-film transistor V I current-mode ADC to measure OLED – transfer characteristic (TFT) threshold voltage (VTH) and mobility (µ), which for OLED luminance-degradation compensation. Measurement results in difference among TFT currents (ITFT) determin- results show that the hybrid driving method reduces the maximum L current error between two emulated TFTs with a 60 mV thresh- ing pixel luminance ( ). Either internal pixel circuits or old voltage difference under 1 gray-level error of 0.94 gray level external column drivers must compensate for this. (37 nA) in 8 bit gray scales from 12.82 gray level (501 nA). The 3) Aperture ratio: Since the aperture ratio determines lumi- circuit-reused current-mode ADC achieves 0.56 LSB DNL and nance efficiency, it should be maximized by minimizing 0.75 LSB INL. the number (effective area) of pixel circuit components, Index Terms—Active matrix organic light emitting diode such as TFTs and storage capacitors. (AMOLED), circuit-reused current-mode ADC, data driver, A voltage-driving method with in-pixel VTH compensation hybrid driver, offset-compensated integrating comparator circuits, as seen in Fig. 2(a), is commonly used due to the fast (OCIC), OLED degradation, thin-film transistor (TFT), threshold driving speed [3]. However, the pixel circuit cannot compen- voltage shift. sate for mobility variation. Furthermore, a conventional five- TFTs/two-storage-capacitors (5T2C) VTH compensation pixel I. INTRODUCTION circuit reduces the aperture ratio. For a high aperture ratio, CTIVE matrix organic light emitting diode (AMOLED) the voltage driving with external TFT compensation method A displays have several strengths suitable for high- was presented [4]. This method has the advantages of both resolution flat-panel displays, such as a wide viewing angle, a fast driving speed and a high aperture ratio, because no a high contrast ratio, and fast response time [1]. Moreover, in-pixel compensation circuit is used. However, it requires addi- because the OLED is self-emissive and no backlight units are tional sensing time before or after displaying with additional needed, the AMOLED display consumes little power, making memory and computation resources. Accordingly, human-user its application thin and light. Also, it is noted for being a key would be very uncomfortable due to wait-time during sensing technology in displays of the future, such as flexible displays operation, and external components may lead to increase in and transparent displays [2]. production costs. Thus, it is thought that the additional com- pensation time has to be hidden by real-time sensing technique, Manuscript received September 01, 2015; revised October 23, 2015; and the solution to reduce external memory size is highly accepted November 17, 2015. This paper was approved by Guest Editor Masato required. Motomura. J.-S. Bang, K.-D. Kim, and G.-H. Cho are with the Department The current-driving method shown in Fig. 2(b) can guarantee of Electrical Engineering, Korea Advanced Institute of Science and high-current (luminance) uniformity over the panel regardless Technology (KAIST), Daejeon 34141, South Korea (e-mail: ghcho@ of the TFT characteristics [5]–[8]. A 4T1C pixel circuit is used ee.kaist.ac.kr). for the above operation. However, slow driving speed due to H.-S. Kim is with the Department of Display Engineering, Dankook (C ) (I ) University, Cheonan 31116, South Korea (e-mail: hs.kim@dankook. large line capacitance P and a low-level data current DATA ac.kr). makes it unsuitable for high-resolution display. Although sev- O.-J. Kwon, C.-S. Shin, and J. Lee are with Samsung Display Co. Ltd., eral proposed settling time reduction techniques have been Yongin 17113, South Korea. proposed, they require line-multiplexing for charge feedfor- Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. ward [7] or an elaborate feedback loop where the bandwidth Digital Object Identifier 10.1109/JSSC.2015.2504416 is strictly limited by data-line parasitic RC [8]. 0018-9200 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. 2 IEEE JOURNAL OF SOLID-STATE CIRCUITS Fig. 1. AMOLED driving systems. Fig. 2. AMOLED driving methods. (a) Voltage driving with in-pixel threshold voltage (VTH) compensation. (b) Current driving. (c) Digital driving. As an another alternative, a digital driving method in The hybrid-driving method is accompanied with the external Fig. 2(c) has emerged because it is also free from TFT variation compensation performed in real time; it means that the pro- with a simple 2T1C pixel circuit [9]. However, as resolution and posed driving method is able to maintain TFT current unifor- panel size grow, gate scan time becomes too inadequate to even mity under temporal VTH variation without any inconvenience turn ON the TFT gate. for users. In order to measure TFT characteristics in real time, This paper presents a new innovative approach: a hybrid driv- a hybrid column driver is proposed to accurately sense the TFT ing method in which the advantages of both voltage-driving and current while driving the data voltage. In addition, since the in- current-driving methods are combined. Thus, it achieves driv- pixel VTH compensation circuit is not required, a 3T1C pixel ing speed as fast as the voltage-driving method and high TFT for high aperture ratio can be employed for the hybrid-driving current accuracy as high as the current-driving method [10]. method. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. BANG et al.: HYBRID AMOLED DRIVER IC FOR REAL-TIME TFT NONUNIFORMITY COMPENSATION 3 Fig. 3. Functional diagram of proposed hybrid-driving method. Fig. 4. (a) Reduction of TFT current error (ΔI) under hybrid driving. (b) Gray-level grouping for mobility compensation. This paper is organized as follows. Section II describes operation. This iterative correction works as an external digital how the proposed hybrid driving method drives the data and feedback which minimizes error between ITFT and IDATA. compensates for TFT variation. Section III presents a dif- In the hybrid driving method, programming VDATA and sens- ferential sensing method for accurate current sensing under ing ITFT are performed for every pixel in a row within the 1 H display panel noise. Section IV shows circuit implementa- time. During the first half-period of the 1 H time (SCAN = L), tion of the hybrid column driver. Section V introduces the the voltage driver, which consists of a voltage DAC (VDAC) circuit-reused current-mode successive-approximation register and a buffer amplifier, drives VDATA to the TFT gate through (SAR) ADC to measure OLED V-I characteristics for OLED a scan switch. The VDATA corresponding to IDATA is initially luminance-degradation compensation. Section VI shows mea- defined in a look-up table (LUT) in a timing controller. At the surement results of a fabricated hybrid column-driver IC, and next half-period (SENSE = L), when a sense switch is turned Section VII concludes this paper. ON,theITFT corresponding to VDATA flows into the column driver, and the column driver compares it to IDATA. The com- parison results (CMP) differ among pixels, because every TFT II. HYBRID AMOLED DRIVING METHOD suffers from different variations and every column driver has an The hybrid AMOLED driving method shown in Fig. 3 is intrinsic offset (VOS). When the column drivers are in the same the voltage-driving method with real-time external TFT com- operation for the next rows, a TFT compensation block in the pensation. To determine the TFT V-I characteristics, the hybrid timing controller increases or decreases the correction voltage column-driver senses the TFT current (ITFT) and compares it (ΔV) for each pixel by 1 LSB voltage of the VDAC (ΔVLSB) to the target data current (IDATA) from a current DAC (CDAC) based on CMP to reduce error between ITFT and IDATA and then right after a voltage-driver programs the data voltage (VDATA) to stores ΔV to memory as a graph, as seen in Fig.