Due to their light output and long life, high-brightness
LEDs are well-suited for use in flashlights. Typically, the
LEDs are driven with a constant current. So when the battery
voltage drops, the flashlight just stops working. In some
situations, this could be dangerous. It would be nice to have a
flashlight that would automatically dim when the battery voltage
drops. Just like a traditional flashlight, it would indicate a
low-battery condition and would last longer.
Typically, the forward voltage of a white LED is 3 to 5 V.
Therefore, if the input voltage drops below 3 V, a boost converter
can drive the LED. Figure 1 shows such a boost converter
with automatic dimming based on battery voltage.
The L6920D is a high efficiency step-up controller requiring
very few external components to convert the battery voltage to the selected output voltage or current.
The device will start up at 1 V and
operate down to 0.6 V. Its quiescent
current is only 10 µA. The device also
includes an internal synchronous
rectifier implemented with a 120-m?
P-channel MOSFET, which replaces
the conventional boost diode, to
improve the 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. In
this way, the battery can last longer,
and the flashlight will offer dimming
characteristics. Assuming VFB = 1.23
V, determined by the L6920D:
If R5 >> R4, then R5||R4 ˜ R4, then:
Finally:
Figure 2 shows the resulting LED current calculated using the
resistance values in the example circuit.
We compared a constant LED flashlight and this auto-dimming
version, using two AAA batteries. Running times were
50 and 90 minutes, respectively. The auto-dimming function
nearly doubled the battery life and provided a warning when
the battery voltage dropped.