A novel dimmable LED driver with soft-start and UVLO circuits

A fully integrated LED driver based on a current mode PWM boost DC-DC converter with constant output current is proposed.In order to suppress the inrush of current and the overshoot voltage at the start up state,a soft-start circuit is adopted.Additionally,to adjust the LED brightness without color variation over the full dimming range and achieve high efficiency,a PWM dimming circuit is presented.Furthermore,to keep the loop stability of the LED driver,an internal slope compensation network is designed to avoid the sub-harmonic oscillation when the duty cycle exceeds 50%.Finally,a UVLO circuit is adopted to improve the reliability of the LED driver against the input voltage changing.The LED driver has been fabricated with a standard 0.5μm CMOS process,and only occupies 1.21×0.76 mm;.Experimental results show that the brightness of the LED can be adjusted by an offchip PWM signal with a wide adjusting range.The inductor current and output current increase smoothly over the whole load range.The chip is in the UVLO condition when the input voltage is below 2.18 V and has achieved about 137 μs typical start-up time.