Time-multiplexing PWM LED driver with grayscale enhancement techniques for signage display

Abstract

This paper proposes a PWM LED driver with adaptive duty cycle restoration (ADCR) and amplitude segmented PWM (ASPWM), which are specialized for a signage application. Time-multiplexing driving is commonly used for signage systems because it significantly reduces the number of driver-ICs. However, such systems require several methods to enhance the brightness and the resolution for the application of time-multiplexing driving. ADCR improves the brightness of LEDs in the low grayscale region with a simple analog timer circuit composed of two switches and capacitors. Because ADCR measures the charging time of a forward voltage in the LEDs periodically and adds the time to the original PWM signal, the actual emission time of the LEDs is individually compensated. With ASPWM, the system increases the grayscale resolution without the EMI problem caused by high-frequency clocks to achieve higher resolution for the same frame time. ASPWM controls the duty cycle as a small segment vertically and horizontally. Because these techniques are compatible with conventional methods, the quality of the display can be easily improved further. The proposed driver supports 48-channels with 48-scans and achieves a duty cycle restoration factor (DCRF) greater than 0.9. The proposed driver is fabricated in a 0.18-μm BCD process, and experimental results verify its effectiveness.