Pixel Circuit With Leakage Prevention Scheme for Low-Frame-Rate AMOLED Displays

Abstract

This work proposes a new pixel circuit for active-matrix organic light-emitting (AMOLED) smartwatch displays with a low frame rate. Within the long emission period, the leakage current of a lowtemperature polycrystalline silicon thin-film transistor (LTPS TFT) is reduced to suppress the distortion of the driving voltage at the gate node of the driving TFT. Based on the measured electrical characteristics of a fabricated p-type LTPS TFT, the HSPICE model is established to verify the feasibility of the proposed circuit. The analytical results indicate that the relative OLED current error rates are all below 4.73%, as the threshold voltage of TFT varies by ±0.5 V. Notably, the OLED current varies by only 2.94% during the emission period of 66.7 ms at a medium gray level, demonstrating the effectiveness of the leakage prevention scheme. INDEX TERMS Active-matrix organic light-emitting diode (AMOLED), low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs), leakage current prevention.

New Low-Frame-Rate Compensating Pixel Circuit Based on Low-Temperature Poly-Si and Oxide TFTs for High-Pixel-Density Portable AMOLED Displays

“…Based on the proposed pixel structure with two in-pixel signal lines, the subpixel layout area is estimated to be 42 µm × 21 µm, and is suitable for portable displays. Moreover, it shows that the error rate of 2.71% in the study is better than that of the reference [13] and is comparable to the references [15,16]. It was also found that the error rate is increased with the increasing ∆V TH_DTFT , because more compensation time would be needed for our pixel to achieve the varied V TH_DTFT .…”

“…It was also found that the error rate is increased with the increasing ∆V TH_DTFT , because more compensation time would be needed for our pixel to achieve the varied V TH_DTFT . Although these reported circuits [13][14][15][16] could compensate for the ∆V TH_DTFT , only the presented pixel circuit has the ability to simultaneously achieve low-voltage driving and low frame rate, with only two signal lines including SN and EM in a pixel. Thus, the presented pixel circuit using an LTPO TFT is suitable to compensate for the ∆V TH_DTFT for high-pixel-density portable AMOLED displays.…”

“…Several compensation pixel circuits using LTPS or oxide TFTs as driving devices have been reported to solve issues of threshold voltage variation of the driving TFT (∆V TH_DTFT ) [13][14][15][16][17]. Apple Inc. [14] developed the first LTPO compensation pixel circuit for an AMOLED display, in which pixels could be operated at 1 Hz with no flicker phenomenon.…”

“…However, the pixel density was not high enough for use in high-resolution displays, because it had too many components, including six TFTs, one capacitor, and seven lines. Lin et al [15] reported a 9T2C pixel circuit using leakage prevention scheme to make AMOLED displays be driven at a low frame rate; however, this circuit was difficult to fabricate because it included nine TFTs, two capacitors, and six lines. Portable AMOLED displays must have low power consumption and high pixel density.…”

A Low Power and IR Drop Compensable AMOLED Pixel Circuit Based on Low-Temperature Poly-Si and Oxide (LTPO) TFTs Hybrid Technology

“…is always one of the most important requirements. Low frame rate operation has been considered as one very effective scheme to reduce the power consumption of display panels [9]. However, it is very difficult to have the LTPS TFT-based AMOLED displays operating at a low frame rate, as the LTPS TFT usually has a high off-current, which restricts the low limit of the frame rate [7], [10]- [12].…”