White LEDs are brighter and more powerful than ever. High-power white LEDs, because of their extreme luminous density and ultra-compact size, are replacing conventional bulbs in flashlights, headlamps, streetlights, and many automotive applications—anywhere a conventional bulb might be found. Some new white LEDs, such as Lumileds’ Luxeon™ series, improve on conventional bulbs in several characteristics, including greater luminescence, improved response time, and increased durability with decreased size and cost.
The challenge in using white LEDs in portable applications is powering them with the wide input voltage range that batteries present, such as 3.3V to 4.2V from a lithium-ion. LEDs require constant current to maintain constant luminosity. The battery-LED DC/DC converter must both step up and step down the source voltage to a 3.0V to 3.6V LED forward voltage range at a constant LED current such as 1A.
The LT3436EFE 800kHz boost converter in Figure 1 provides 1A driving current for the Luxeon III series white LED LXHL-PW09 from a lithium-ion battery. The Luxeon III white LED has a forward voltage range from 3.0V to 3.6V. By tying the LED from the output of the boost converter back to the input, as opposed to ground, the boost converter is capable of both stepping-up and stepping-down its input voltage to the LED. The effective output voltage of the converter is a boosted voltage of VIN plus VLED as shown in the schematic.
The LT1783 1.25MHz SOT-23 rail-to-rail op amp provides the current-sense capability and regulates the diode current to 1A when the LED ON switch is closed. When the switch is open, the LT3436 consumes only 6μA in shutdown.
Luxeon is a trademark of Lumileds Lighting