Real-Time External Compensation System With Error Correction Algorithm for High-Resolution Mobile Displays

This paper presents an external compensation system for QHD+ ( $3040\times1224$ ) mobile active-matrix organic light emitting diode (AMOLED) displays at a frame rate of 60 Hz. During vertical blank periods, current sensing AFE (CS-AFE) measures OLED currents to calculate threshold voltage ( $V_{TH}$ ) of driving thin-film transistors (TFTs). For precise $V_{TH}$ calculation against panel ground noise, a differential sensing scheme with 5-bit programmable capacitor array (PCA) is employed. In addition, digital correlated double sampling (CDS) removes an offset of the CS-AFE. However, recent advances in high efficiency OLED technology have led to increase in pixel density as well as the driving TFTs to operate close to subthreshold region. Therefore, the $V_{TH}$ calculation based on the quadratic model yields inaccurate results. To compensate for the modeling error, we propose an error correction algorithm, which establishes an error function using a relationship between the modeling error and calculated threshold voltage during the manufacturing process. The proposed external compensation system was verified using CMOS-modeled three transistors and one capacitor (3T1C) pixel circuit. The test chip, fabricated in a $0.18~\mu \text{m}$ BCD process, comprises 26 channels. Each channel consumes 78 $\mu \text{W}$ and occupies 1350 $\times 50\,\,\mu \text{m}^{2}$ . Measurement results show that current error at $64^{\mathrm {th}}$ gray level is reduced from 35.56 LSB to 6.03 LSB after error correction and four frames average.