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06-28-2016

Optical feedback extends white LEDs' operating life

Intersil » ISL29000, EL7630

Bjoy Santos

EDN

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 R1 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.

Optical feedback extends white LEDs' operating life
Figure 1. An LED’s light output changes considerably as a function of
its forward current, even within the sweet spot (oval area)
of its nominal operating current.

Driving series-connected white LEDs with a current source relies on the assumption that, at constant current, an LED's 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 illuminator's performance over an extended-temperature range (upper curve, Figure 3).

Optical feedback extends white LEDs' operating life
Figure 2. One method of driving an LED illuminator samples current
through a string and adjusts the voltage across the entire
string to maintain a constant current.

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

Optical feedback extends white LEDs' operating life
Figure 3. Even at a constant forward current, an LED’s light output correlates
strongly with temperature and can vary by as much as 100% over
the entire operating-temperature range (upper curve).

The circuit in Figure 4 includes an optical-feedback loop based on Intersil's ISL29000 light-to-current optical sensor, IC2, which senses changes in the LEDs' light output and decreases the feedback voltage applied to IC1, the current controller, an Intersil EL7630. The pulse-width-modulated controller then increases the LED-drive current's 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.

Optical feedback extends white LEDs' operating life
Figure 4. Photosensor IC2, an Intersil ISL29000, resides near an LED to detect
brightness fluctuations and provides compensating feedback to IC1,
the current controller, which is an Intersil EL7630 pulse-width regulator.

Materials on the topic

  1. Datasheet Intersil ISL29000
  2. Datasheet Intersil EL7630

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