Low-dropout voltage regulator for stabilizing LEDs drives in dimmable group current sinks

The well-developed constant current sink (CCSK) supplied from a constant voltage source (CVS) has been widely qualified for driving group-parallel strings of power LEDs. This approach takes advantage of current balancing between multiple parallel strings of the LEDs for producing uniform luminous output. However, the drain-to-source voltage v_ds of the driver MOSFET may move into triode region when the sink current increases since the change of the forward voltage of the string LEDs and the voltage drop on source resistor cannot be compensated with the CVS. In this paper, a low-dropout controlled voltage regulator (LDCVR) is proposed to adaptively well bias the string LEDs and to prevent the sink MOSFETs from working in triode region when dimming for high luminous needs. An experimental to design approach is adopted in design considerations to facilitate the control means between CCSKs and LDCVR so as to stabilize the luminous output uniform with low conduction loss in the sink MOSFETs.