Optical feedback extends white LEDs' operating life

摘要

Regardless of its color, an LED's light output varies as a function of forward current and ambient temperature. As Figure 1 shows, an LED's light output can vary by as much as 150 over its operating-current range. In response, a designer's first attempt to solve the problem focuses on driving the LEDs with a constant current. The most common white-LED-driver circuits use an inductor-based dc/dc boost-converter topology similar to the circuit in Figure 2. A current-feedback controller ensures that the voltage across current-sensing resistor R{sub}1 remains constant. As a result, the controller varies the voltage across the entire string to maintain the LEDs' current constant without regard to the LEDs' actual light output.