The LM3699 is a three-string, high-efficiency, PWM-controlled power source for display backlight or keypad LEDs in smartphone handsets. The high-voltage inductive boost converter with integrated 1-A, 40-V MOSFET provides the power for three series LED strings. The boost output automatically adjusts to LED forward voltage to minimize headroom voltage and effectively improve LED efficiency. The ILOW terminal provides a method to quickly reduce LED brightness during camera flash operation. The LM3699 has integrated overvoltage, overcurrent, and thermal protection. The device operates over a 2.7-V to 5.5-V input voltage range and a −40°C to 85°C temperature range.