LTPS TFT-Based Backlight Unit Using PWM Driving with Compensation for V_THVariation 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 (VTH) 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 VTH 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 μs. 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.