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.
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 using available IF versus VF and luminosity versus IF curves. These calculations also require a typical lumen output at a given IF (8.4 lumens at IF = 20 mA). All the required data is readily available in the manufacturers’ datasheets.
The procedure begins with importing/digitizing the datasheet graphs (Figure 1) into a spreadsheet using predefined increments of LED current. Free software tools are available which speed the process and digitize Y data on pre-determined X increments, enabling the calculations required to derive efficacy.
Figure 1. Cornerstone plots used to derive the optimal LED operating point
Once digitized and tabulated, LED flux output (V), LED power consumption (PLED) and efficacy () are calculated versus LED forward current (IF) (Figure 2). Peak efficacy is reached at a relatively low forward current and drops off steadily as forward current approaches the maximum rated amount. Battery powered applications greatly benefit by reducing these power requirements. Operating more LEDs at a lower forward current results in a net reduction of power for a given fixed-light output. Table 1 summarizes the original application requirements while comparing three alternative LED configurations.
Cost and mechanical volume requirements may limit the final configuration; however, doubling the number of LEDs yields a power savings of 160 mW. This equates to a 6.3% net power reduction. Additionally, the backlight can be operated at a much higher brightness (with increased power consumption) when ambient light conditions (outdoors/ daylight) dictate a brighter image.
Figure 2. LED flux output, power consumption and efficacy can be calculated and plotted
Table 1. A comparative backlight design study
Figure 3 highlights the increasing power savings trend over a number of LED data points. Note that the LED knob turns both ways. In other words, it’s possible to decrease the total number of LEDs required for a particular design by overdriving each of the LEDs. This approach is often used less to reduce BOM and assembly costs of display modules and other highly cost-competitive applications.
Figure 3. LED power analysis shows lower power with greater number of LEDs
Operating the LEDs at a reduced brightness requires 100% duty cycle and a reduced current. Driving the LEDs using a traditional pulse-width modulation (PWM) architecture at maximum LED current yields no performance improvements. The white-point shift which occurs in LEDs at lower currents is a potential complication for backlight applications but, thanks to improvements in LED design and manufacturing techniques this phenomenon has been greatly reduced. As a result, modern LEDs exhibit minimal to negligible color shift. This is clearly illustrated in Figure 4, which superimposes a digitized plot of the LED’s color-shift parametrics (Figure 4) and and a MacAdam ellipse over the center of the operating range. A one-step MacAdam ellipse encompasses all LED colors when operating between forward currents of 5 mA and 25 mA. Colors inside a one-step MacAdam ellipse are perceived as the same to the average observer.
Figure 4. White point shift versus LED current
Figure 5. The LP8555 can power large matrices of LEDs
Powering a large array of LEDs is relatively easy using an LED driver such as the LP8555 (Figure 5). This device drives up to 96 LEDs, which is suitable for the largest of mobile displays and capable of driving all the configurations mentioned above. Maintaining a uniform image quality requires close matching of inter-string variation. For many applications, two-percent string-to-string matching is considered a key metric. A dual-boost architecture maximizes electrical efficiency while minimizing the physical height of the associated inductors. Additionally, this device features 12 current-sink inputs, enabling shorter series-LED strings.
This configuration allows the boost converters to power the LEDs at a more efficient electrical operating point. Key features such as adaptive dimming and content-adjustable backlight control (CABC) yield further electrical efficiency gains over all operating modes.
For applications where achieving maximum power savings is the key objective, the designer must use the techniques we’ve explored here to tailor the operating point of the LED to the application’s most typical operating mode. While LCD backlight applications have been the main focus, the concepts presented here can be easily applied to any LED lighting application which requires efficiency as a key performance metric.
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