LTPS TFT-Based Backlight Unit Using PWM Driving With Compensation for VTH Variation and Voltage Rise in Power Line

• Published in: IEEE transactions on electron devices 2024-01, Vol.71 (1), p.619-623
• Main Authors: Lin, Chih-Lung, Chen, Sung-Chun, Lin, Fang-Yi, Liao, Wei-Sheng, Deng, Ming-Yang, Peng, Chia-Tian
• Format: Article
• Language: English
• Subjects: Active matrix (AM); Current measurement; current–resistance rise/drop; Light emitting diodes; Liquid crystal displays; liquid-crystal display (LCD); low-temperature poly-silicon (LTPS); Mathematical models; mini-light-emitting diode (mini-LED); Pulse width modulation; pulsewidth modulation (PWM); Thin film transistors; thin-film transistors (TFTs); Threshold voltage; threshold voltage (VTH)
• ISSN: 0018-9383
• DOI: 10.1109/TED.2023.3330686

A novel mini-light-emitting diode (mini-LED) driving circuit based on the low-temperature polysilicon thin-film transistors (LTPS TFTs) for used in the active matrix (AM) backlight unit (BLU) of a liquid-crystal display (LCD) is proposed. By the pulsewidth modulation driving method, the mini-LED is operated at a high luminance efficacy point than that is achieved using the pulse-amplitude modulation (PAM) method to reduce the power consumption at a low gray level. To generate the uniform driving current of the mini-LED, the threshold voltage ( {V}_{\text {TH}} ) variation of the driving thin-film transistor (TFT) is compensated for by the diode-connection structure. Since the large driving current could exacerbate the current-resistance effect in the power source line, the negative supply voltage (VSS) {I} - {R} rise is also compensated for. The {V}_{\text {TH}} variation of the control TFT is compensated for by the matched TFT method to ensure the precise emission time of the mini-LED. The results of a simulation based on the fitting model of the LTPS TFTs indicate that the relative time shifts are below 0.581~\mu \text{s} . The samples of the proposed circuit are fabricated to verify the feasibility of compensation in the proposed mini-LED BLU. Measurements demonstrate that the maximum error rate among samples is 1.46%. When the VSS rises from −6 to −5 V, the measured current error rates are all less than 6.03%. Hence, the proposed circuit is verified to generate uniform driving currents for the mini-LED BLU of LCDs.