Feedback circuit for organic LED active-matrix display drivers

Abstract

A feedback circuit for an Organic Light Emitting Diode (OLED) based display is proposed and demonstrated. An OLED-based flat panel display is brighter, much lower power, has no viewing angle limitation and potentially cheaper compared to available Liquid Crystal (LC) based displays. Despite these advantages, an OLED-based display is not widely commercialized mainly due to its short practical lifetime. The I-V characteristics of the individual OLED pixels vary over time, temperature and processing-dependent parameters. Moreover, the variation is not uniform across an array of OLED pixels, causing OLED based displays to lose brightness accuracy after a few thousand hours of operation. The proposed feedback circuit is used to compensate for the non-uniformities in the individual OLED characteristics. The resulting display leverages the beneficial aspects of OLED display technology, while maintaining pixel uniformity and grayscale reproducibility. A demonstration system is built proving the feasibility of a flat panel display using direct optical feedback. The feedback loop monitors the output light level using a sensor and adjusts the current fed to the pixels to set the output light power to a digitally set reference level. The system shares a single feedback loop among a number of pixels, saving power and real estate. The demonstration system consists of a 5x5 array of LEDs, a CMOS camera, analog pixel circuitry, driver and feedback loop, as well as a digital controller. The demonstration system also shows the feasibility of time-sharing a feedback loop among a number of output devices.