51.1: A 2.5‐inch OLED Display with a Three‐TFT Pixel Circuit for Clamped Inverter Driving

Abstract

A new pixel circuit for "clamped inverter driving" was developed. Since a PMOS inverter is applied instead of the conventional CMOS for the pixel circuit, the circuit is simplified and is composed of only three TFTs. A simulation of gray-scale characteristics verified that this pixel circuit can compensate variation in V th and carrier mobility. A 2.5-inch OLED display based on the new pixel circuit was fabricated by LTPS processing and achieves 6-bit gray-scale and good uniformity. IntroductionSeveral designs of pixel circuits for organic light-emitting-diode (OLED) displays have been reported [1][2][3]. Regarding the driving scheme for OLED displays, it is known that the voltageprogramming method has advantages of high-speed writing and compatibility with existing driver ICs. However, this scheme has one problem that must be solved; namely, V th and carrier mobility of the TFTs in the pixel circuit must be compensated effectively. At SID'02 and '03, we proposed a "clamped inverter driving" method, which uses sweep-signal comparison to control the duration of light emission [4,5]. This method is a kind of voltage programming. It compensates V th and carrier mobility of a TFT effectively and achieves not only luminous characteristics of 6-bit gray-scale and good uniformity but also gamma correction and peak luminance. The pixel circuit for this driving method, however, requires more than four TFTs. Accordingly, in the present work, we designed a simplified pixel circuit with only three TFTs. Such a pixel circuit composed of fewer TFTs has advantages of high aperture ratio and high yield ratio. We applied it to a 2.5-inch OLED display and evaluated the display characteristics of the OLED panel. Figure 1 shows the configuration of the pixel circuit for the previously developed "clamped inverter driving" scheme, which controls the brightness of the OLED by means of changing the length of the "ON" and "OFF" states of the inverter. TFT T1 and TFT T2 compose the CMOS inverter, and TFT T3 and TFT T4 -Fig. 1 Configuration of previous pixel circuit Fig. 2 Timing chart for OLED driving V data =High Writing mode Illumination mode V data =Low V data =High V data =Low OLED ON OFF ON OFF Vdata Bright Dark Data-line voltage Sweep signal Data signal OLED T4 T3 T1 T2 Illumination line Reset line Data line C1 CMOS inverter Clamped inverter driving method

Threshold‐voltage‐compensation methods for AMOLED pixel and analog buffer circuits

“…Although the offset voltage of the source follower with an active load is larger than that for a conventional source follower, it can be eliminated by gamma correction. 19,22 According to the results, the charging time variation-tolerant characteristic of the source follower with an active load is superior to that for a conventional source follower.…”

Anode‐voltage‐control circuit for compensation of luminance deterioration

“…1 Regarding the problem of image non-uniformity, we proposed a driving scheme refered to as clamped inverter driving at the 2003, 2004, and 2005 SID Symposia, and in the Journal of the SID. [2][3][4][5][6] Clamped inverter (CI) driving is a type of constant-anodevoltage (CV) driving, so it also achieves low power consumption and peak luminance. For the other problem of aging, we suggested a luminance compensation that increases the anode voltage when the OLED-display luminance deteriorated at SID '05.…”

Investigation of source-follower type analog buffer using low temperature poly-Si TFTs

“…Low temperature poly-Si (LTPS) TFTs have attracted much attention in the application on the pixel circuits and integrated peripheral circuits of active matrix liquid crystal displays (AMLCDs) and active matrix light emitting diodes (AMOLEDs) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. In a poly-Si TFT-LCD, poly-Si TFT is not only used to implement pixel circuit but also the driving circuit on a single glass substrate to reduce system cost and possess compact module which a-Si TFT is hard to achieve.…”

“…Among output buffer circuits for displays, source follower is considered an excellent candidate for the output buffer circuit for the ''System on Panel'' application because of its simple schematic and low power dissipation [10][11][12][13][14][15][16][17][18].…”