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Identifying the optimal operating point of an LED

04 Mar 2015  | Donald Schelle

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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: 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 colour shift. This is clearly illustrated in figure 4, which superimposes a digitized plot of the LED's colour-shift parametrics (figure 4) and and a MacAdam ellipse over the centre of the operating range. A one-step MacAdam ellipse encompasses all LED colours when operating between forward currents of 5 mA and 25 mA. Colours 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-per cent string-to-string matching is considered a key metric. A dual-boost architecture maximises electrical efficiency while minimising 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.

1. Nichia NNSW208CT datasheet.

2. Donald Schelle, Mark Brouwer, "Digitize graphical data easily and accurately," EDN, March 2013.

About the author
Donald Schelle is an analogue field applications engineer with Texas Instruments.

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