A Highly Uniform Luminance and Low-Flicker Pixel Circuit and Its Driving Methods for Variable Frame Rate AMOLED Displays

This paper proposes a 7T2C pixel circuit for active-matrix organic light-emitting diode (AMOLED) displays using low-temperature poly-crystalline silicon (LTPS) thin-film transistors (TFTs) with two key features: high luminance uniformity and low flicker at a wide range of variable frame rates (VFRs). To achieve a high luminance uniformity, the proposed pixel circuit simultaneously compensates for the threshold voltage variation and subthreshold slope variation of the driving TFT with a sufficient compensation time regardless of the frame rate. Furthermore, flicker is reduced by employing a ramping reference voltage and current blocking method, which reduces the effect of the leakage current of TFTs and OLED capacitance, respectively. Therefore, these techniques reduce spatial emission current errors (ECEs) and temporal ECEs, enabling the circuit to be used in the VFR from 15 Hz to 360 Hz. The proposed pixel circuit was verified with unit sub-pixels designed for a 14-inch $3840\times 2160$ (4K) AMOLED display. The worst-case spatial ECEs of the proposed 7T2C pixel circuit at the frame rate of 360 Hz were measured as +4.1/-4.2 and +1.2/-1.1 LSB at the 255th and 15th gray levels, respectively. They show better luminance uniformity compared to those of the conventional 7T1C pixel circuit, which were measured as +6.1/-8.9 and +3.6/-3.6 LSB. Furthermore, the worst-case temporal ECEs at the frame rate of 15 Hz that shows flicker performance was measured as -0.3 and -3.2 LSB at the 15th and 255th gray levels, respectively, and they are better than those of the 7T1C pixel circuit, which show 11.2 and 2.41 LSB.