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Optical feedback extends white LEDs operating life

Bjoy Santos, Intersil, Milpitas, California

Regardless of its color, an LEDs light output varies as a function of forward current and ambient temperature. As Figure 1 shows, this light output can vary by as much as 150% over its operating-current range. In response, a designers first attempt to solve the problem focuses on driving the LEDs with a constant current. The most common white-LEDdriver 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 R1 remains constant. As a result, the controller varies the voltage across the entire string to maintain the LEDs current constant without regard tothe LEDs actual light output.

Driving series-connected white LEDs with a current source relies on the assumption that, at constant current, an LEDs light output remains constant. Unfortunately, all LEDs exhibit a nonlinear decrease in brightness as a function of operating time. Although less obvious in colored LEDs that find use as indicators, the decrease in brightness of a white-LED-illuminator-array source becomes noticeable over an extended period. Brightness also varies as a function of temperature, which can affect an illuminators performance over an extended-temperature range (upper curve, Figure 3 ).

To compensate for LED-output variations due to aging and temperature fluctuations, the control loop needs more information than just to voltage or current data. Adding an ambient-light sensor and optical feedback to the control loop can ensure that a white LEDs light output remains uniform and consistent over time and temperature variations.

An optical sensor can measure the LEDs light-output intensity and provide a feedback signal for the control loop, which can adjust the current to produce a relatively constant light output. As the LEDs light outputs decrease, increased current compensates for aging and temperature-induced variations (lower curve, Figure 3).

The circuit in Figure 4 includes an optical-feedback loop based on Intersils (www.intersil.com) ISL29000 light-tocurrent optical sensor, IC2, which senses changes in the LEDs light output and decreases the feedback voltage applied to IC1, the current controller, which is an Intersil EL7630. The pulse-widthmodulated controller then increases the LED-drive currents duty cycle, boosting the LED current until the feedback voltage reaches its nominal value.

As ambient temperature decreases, the LEDs light output tends to increase, and IC2 delivers a higher feedback voltage to the controller, which responds by lowering the duty cycle to decrease the LEDs current and thereby compensates for the decrease in temperature.