A simple method for determining the optimal operating point of an LED

March 02, 2015 //By Donald Schelle
A simple method for determining the optimal operating point of an LED
Donald Schelle, Analog Field Applications Engineer, Texas Instruments outlines a simple method for determining the optimal operating point of an LED.

Achieving optimal performance of an LED luminaire or LED backlight design requires numerous trade-offs. Understanding an LED’s power transfer characteristics empowers intelligent choices regarding cost, power consumption, and weight. While most LED datasheets publish pertinent data that can be used to make these decisions, data may not be formatted in a way that is readily applicable to the chosen application.

Optimal performance requires finding pertinent information from manufacturer’s LED datasheets and utilizing methods to capture, reformat and analyze the data. A relevant case study involves a typical tablet LCD backlight application that drives a 10-inch display with a 16:9 aspect ratio. Driving the backlight, the LED chosen for our example is the Nichia NNSW208CT [1].

Typical displays in modern mobile devices emit approximately 650 nits of light when driven at maximum brightness. Most of the LED light produced is lost as it passes through the physical elements integrated into the display (light diffuser, polarizers, RGB color filter, touch-panel ITO, and so on). Modern display stack-ups loose approximately 95% of the light produced by the LED. This device in this case study emits 10.398 lumens when driven at the recommended continuous drive current of 25 mA. Calculate the minimum number of LEDs using Equation 1.

Using a conversion constant of K = 1550.0031 and the design requirements listed above, the calculated minimum number of LEDs is 35. While seven strings of five LEDs satisfies the design requirements, most LED driver ICs in his market are tailored to drive only six strings of LEDs. Adjusting the LED count to 36 enables an off-the-shelf LED driver. Assuming 100% driver efficiency, driving 36 LEDs at maximum brightness consumes 2.56 W of power. LED efficacy, color shift, and thermal properties are key data metrics. Efficacy versus forward current is rarely provided in an LED datasheet. Tabulated efficacy data is also difficult to find in specifications. But it's relatively easy to calculate this key metric

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