A parallel light-emitting diode string driver with high dimming ratio

Abstract

Light-emitting diodes (LEDs) have become a good alternative to traditional cold cathode fluorescent lamps (CCFLs) for backlighting sources in liquid crystal displays (LCDs). LED backlighting provides a number of benefits over CCFLs, such as greater efficiency, better brightness, longer lifetime, higher dimming ratios, and lower drive circuitry voltage [1]. One application where a higher dimming ratio is desired is in aerospace cockpit LCD modules, which need to provide both high brightness for sunlight readability and also a high dimming ratio for very low brightness during nighttime operation. Two methods commonly used to dim LEDs are constant current reduction(CCR) and pulse width modulation(PWM) dimming. The downside to using CCR is the limited dimming ratio and nonlinearities in light output that occur from variations in luminous efficacy over the dimming range [2]. To obtain a high dimming ratio using PWM dimming, the LED current rise time must be sufficiently faster than the minimum PWM. This ensures that there will be no nonlinearities in the light output, which could result in visible flicker when operating at low-brightness settings. To provide higher system efficiency, a method for soft starting the LED regulator is presented. This method allows for reduced head room operation by ramping the LED current during turn-on, which results in a reduced voltage dip on the LED power supply. Finally, a method for head room adjustment that allows for adjustment of the LED supply voltage for variations in forward voltage over temperature and time is presented.