A High Accuracy Weak Voltage LED Analog Dimming Method

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 $0.18\mu \text{m}$ 5V/40V Bipolar-CMOS-DMOS (BCD) process, experimental results for a cell phone backlighting driver show that the analog dimming I_LED error reduces to $30\mu \text{A}$ at maximum 200mV LED current feedback voltage. This deviation mostly keeps constant through a dimming duty range of 0.5% to 100% and V_IN 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.