Modern high-power LEDs are more efficient and have longer operational lifetimes than incandescent or compact fluorescent light bulbs. In addition, their lack of flicker helps to reduce eyestrain, making LEDs far superior to fluorescent lighting.
This analog tip shows how the ADP2370 adjustable-output-voltage buck converter can regulate the LED current from a variety of power sources to make a simple, robust, high-efficiency dimmable LED driver.
Figure 1. Regulated LED driver using the ADP2370
Regulated current LED driver
Buck converters such as the ADP2370 normally regulate a lower voltage from a higher voltage source, but the LED driver circuit shown in Figure 1 uses the ADP2370 to regulate the LED current instead of an output voltage.
R7 and R8 function as both ballast and current sense resistors; R6 and R9 average the current sense voltages at the input of op amp U2 and equalise the LED currents. U2 amplifies the voltage across the current sense resistor, with the gain set by R2 and R3, and drives the FB input of the ADP2370, thereby “fooling” it into regulating the LED current. A gain of 5.32 sets the current in each LED to about 320 mA.
R4, R5, R6, and R9 provide a dimming function. As R4 is decreased, the current-sense voltage is slightly offset above 0V, reducing the LED current. A photoconductive sensor could replace R4 to dim the LEDs as a function of the ambient light level.
Over-temperature protection can be added by replacing or paralleling R4 with an NTC thermistor to reduce the drive current if the LEDs exceed a certain temperature. The thermistor must be in good thermal contact with the LEDs for accurate temperature sensing.
Glenn Morita [ firstname.lastname@example.org] graduated from Washington State University with a BSEE in 1976. His first job out of school was at Texas Instruments, where he worked on the infrared spectrometer instrument for the Voyager space probe. Since then, Glenn has