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External compensation is an effective scheme to adapt the threshold voltage variations of the driving Thin-Film Transistors (TFTs) and the deterioration of the Organic Light-Emitting Diodes (OLEDs) in pixel circuits of Active-Matrix Organic Light-Emitting Diode (AMOLED) display systems. In this paper, an accurate current comparator is proposed to sense the difference between the current trickling through the OLED and the reference current, so that the deterioration and variations of TFTs and OLEDs in the AMOLED pixel circuits can be detected and compensated effectively. The proposed current comparator is demonstrated in a source driver chip for AMOLED panel with external compensation and fabricated in a 0.25-μm high-voltage CMOS technology. The sensing accuracy of the proposed current comparator is less than 5.3 nA by chip measurement with 500 nA reference current. Furthermore, the sensing error of the proposed current comparator is lower than 1.05%.

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The externally compensated display system has been proved to have greater potential to completely solve aging and uniformity issues of the AMOLED display system than the internally compensated display system. However, the traditional compensation method is time consuming and responses slowly to the mobility change of TFT. The traditional compensation method performs calibration during the inter-frame blanking period, the length of which is limited. Only a few rows of pixel circuits can be calibrated per frame. Furthermore, each pixel circuit needs multiple times of calibration to reach compensated state. In this article, a fast progressive compensation method is proposed. The calibration of a row of pixel circuits is performed K rows ahead of the display line during the display data updating period. By doing so, the proposed fast progressive compensation method can approach compensated state faster than the traditional method because display data updating period is significantly longer than the inter-frame blanking period. For the case of 10% inter-frame blanking period and 20 µs waiting delay for feedback data line to be stable, the proposed method reduces the compensation time by ∼14x than the traditional method.INDEX TERMS Thin-film transistor (TFT), external compensation, fast calibration.