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

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.