HIGH_INTENSITY_LED_CIRCUIT

Published:2009/7/13 4:08:00 Author:May | From:SeekIC

The forward voltage for high-intensity LEDs (1.5 to 2.5 V) is too large for operation with one-cell batteries. The circuit shown overcomes this limitation with a boost-regulator technique-it drives controlled current pulses through the LED, regardless of the LED's forward voltage, and operates on input voltages from 6.2 V to below 1 V. The circuit is useful for bicycle lights, beacons, alarms, flashlights, and low-power indicators. IC1 is normally part of a regulated boost converter, but, in this case, it simply transfers energy without regulating the output. Omission of the usual rectifier and output filter capacitor makes the circuit compact, as does the high switching frequency (about 175 kHz).Programming resistor R1 sets the LED intensity by setting a peak current for the inductor and LED.A 10-kΩ value for R1 sets the approximate peak at 75 mA, and the average LED current at about 26 mA. A shutdown command tums off the OUT terminal completely, even if cell voltage exceeds the LED's forward voltage, by turning off the diode internal to IC1. (During shutdown, most step-up converters exhibit a troublesome dc path from the battery through the coil and diode to the load.) This circuit draws about 8 μA during shutdown and about 60 mA during normal operation. It operates for 35 hours continuously on one AA (or R4 size) alkaline cell.



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