Automatic dimming LED flashlight circuit design

By the light output of high efficiency and long life, high brightness LED is very suitable for use in a flashlight. A typical LED with constant current driver, so when the battery voltage drops, flashlight will stop working. In some cases, it can be very dangerous. If the battery voltage drops under the condition of the LED flashlight can automatically dim, that is good. Just like a traditional flashlight, in dim light brightness to indicate low battery voltage working state, and can last longer.

in general, white LED forward voltage 3 v to 5 v. Therefore, if the input voltage is lower than 3 v, can use a boost converter drive leds. Figure 1 circuit is given by the battery power supply, which has the function of automatic dimming boost converter circuit.

L6920D is an efficient booster controller, only very few external components to transfer the battery voltage to a specified output voltage or current. The static current of the device is only 10 ua. The device also contains 120 - m ? P - The interior of the channel MOSFET synchronous rectifier, can replace the traditional boost diode, improve efficiency.

in this circuit, the LED current ILED is a function of the battery voltage VFB. When the battery voltage drops, the LED current decline, so that the battery can last for a longer time, can provide the flashlight features. Assuming that VFB = 1. 23V( Be determined by L6920D) Is:

figure 2 is calculated according to the figure 1 resistance in the circuit of LED current curve. For constant LED flashlights and automatically compares the dimming LED flashlight, both the two AAA batteries, running time of 50 and 90 minutes respectively. Automatically dimming LED battery life almost doubled, and provides a warning when the battery voltage is falling.

figure 1: using high efficiency booster automatic dimming LED flashlight circuit of controller design.

figure 2: according to the calculated resistance value in figure 1 LED current curve.