Abstract
A weak voltage analog dimming method for high precision light emitting diode (LED) current control with N-Bits analog static offset calibration (N-Bits ASOC), dynamic offset cancellation (DOC) and K-times dimming duty to voltage (KD2V) conversion is presented. The N-Bits ASOC circuit decreases the analog dimming error to V OSMAX /2 N - 1 by pre-calibrating the error amplifier offset. Moreover, through a smart combination of KD2V circuits and DOC amplifier, the KD2V and DOC operational amplifier magnify the dimming reference and LED current sense voltage K-times, respectively, which reduces the residual dimming error K-times without any modification to the LED current sampling resistor. Based on the 0.18μm 5V/40V Bipolar-CMOS-DMOS (BCD) process, experimental results for a cell phone backlighting driver show that the analog dimming ILED error reduces to 30μA at maximum 200mV LED current feedback voltage. This deviation mostly keeps constant through a dimming duty range of 0.5% to 100% and VIN range of 2.7V to 5.5V. Furthermore, a 200-sample measurement data suggests that this method is effective even when dimming duty is smaller than 0.5% for mass production.
Highlights
The most widely used light emitting diode (LED) is the third generation solid state lighting source
EXPERIMENTAL RESULTS AND DISCUSSIONS The proposed high precision analog dimming method was used in a small power white LED driver, which is used for small size liquid-crystal-display (LCD) panel backlighting
The LED operates in DC current with the dimming duty cycle ranging from 1% to 100%
Summary
The most widely used LED is the third generation solid state lighting source. Compared to cold cathode fluorescent lamps (CCFL) and others lighting sources (such as incandescent light bulbs, high intensity discharge lamps), LEDs are more efficient, smaller, safer, have longer lifespan, and provide flexible dimming method. This white LEDs driver consists of two parts: one is the voltage conversion circuit block, which boosts VIN to VOUT to provide the power for LEDs. The LED current regulator block is illustrated in the red dotted line rectangle. A weak voltage analog dimming control method, which consists of a two-stage offset cancellation structure, is presented. Dynamic offset cancellation technology is applied to OP1 to achieve K-times attenuation of pre-calibrated residual error By this mean, the weak voltage VR1 controls the LED current accurately, even during the small duty cycle of the input dimming signal (DIM). This paper is organized as follows: Section II describes the proposed method principle; Section III presents the scheme and its circuit implementation; Section IV provides the experimental results; and Section V gives a conclusion for this method
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