Low-Dropout, Constant-Current Triple White LED Driver
In conventional white LED design, the Max1916 low-dropout bias supply for white LEDs is a high performance alternative to the simple ballast resistors. A single resistor is used by this device to set the bias current for three LEDs, which are matched to 0.3%. On enable condition, the MAX1916 consumes only 40uA of supply current and 0.05uA when disabled.
The MAX1916 has a lot of advantages over ballast resistor such as much lower bias variation with supply voltage variation, significantly better LED-to-LED bias matching, significantly lower dropout voltage and improve efficiency in some application. 200mV dropout at a 9mA load on each output is required by the MAX1916 to match the LED brightness. The MAX1916 is available in a space-saving 6-pin Thin SOT23 package. The LED bias current is controlled by SET. Than the current flowing into SET, current flowing into LED1, LED2, and LED3 is 230 times greater. The output current can be set as follows:
Iled = 230 (Vctrl-Vset)/Rset
where Vctrl is an external voltage between 1.8V and 5.5V, Vset=1.215V, and Rset is the resistor connected between Vctrl and SET. [Circuit’s schematic diagram source: maxim-ic.com]