Time-Multiplexed PWM LED Driver with Grayscale Enhancement Techniques for Signage Display

In this article, a pulsewidth modulation (PWM) LED driver is proposed to overcome the challenges of time-multiplexed driving and to enhance the quality of displays sufficient for signage applications. First, adaptive duty cycle restoration (ADCR) compensates for the brightness degradation in low grayscale, where a simple analog timer circuit measures the limited charging time and compensates its effect to ensure the desired emission time. Second, amplitude-segmented PWM (ASPWM) improves the grayscale resolution without increasing the grayscale clock frequency; thus, the electromagnetic interference performance is not sacrificed. The proposed LED driver is fabricated in a 0.18-μm Bipolar-CMOS-DMOS (BCD) process. The measurement results show that the driver Integrated Circuit (IC) achieves a duty cycle restoration factor (DCRF) greater than 0.95 out of ten samples, whereas the DCRF is limited to 0.67 on average when the ADCR scheme is disabled. Resolution enhancement with the proposed ASPWM is verified, achieving 16-b grayscale resolution. The prototype driver IC occupies 3.56 × 3.00 mm of chip area, operated from a 4.5 V supply voltage.