High-brightness LEDs are ideal for use in flashlights due to their high light output efficiency and long life. A typical LED is driven with a constant current, so when the battery voltage drops, the flashlight will stop working. In some cases, this can be very dangerous. If the LED flashlight can be automatically dimmed when the battery voltage drops, it is easy to use. Just like a traditional flashlight, dimming the brightness of the light to indicate low battery voltage operation and lasting longer.
In general, the forward voltage of a white LED is 3V to 5V. Therefore, if the input voltage is lower than 3V, a boost converter can be used to drive the LED. The circuit in Figure 1 shows a battery-powered boost converter circuit with auto-dimming.
The L6920D is a high efficiency boost controller that converts battery voltage to a specified output voltage or current with very few external components. The device's quiescent current is only 10uA. The device also includes an internal synchronous rectifier with a 120-m? P-channel MOSFET that replaces traditional boost diodes for improved efficiency.
In this circuit, the LED current ILED is a function of the battery voltage VFB. When the battery voltage drops, the LED current drops, so that the battery can last longer, and the flashlight can provide dimming characteristics. Assuming VFB=1.23V (determined by L6920D), then:
Figure 2 is a graph of LED currents calculated from the resistance values ​​in the circuit of Figure 1. A comparison of a constant LED flashlight with an auto-dimming LED flashlight, powered by two AAA batteries, runs for 50 and 90 minutes, respectively. Auto-dimming LEDs almost double the battery life and provide a warning when the battery voltage drops.
Figure 1: Auto-dimming LED flashlight circuit designed with a high efficiency boost controller.
Figure 2: LED current curve calculated from the resistance value of the circuit of Figure 1.
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