LED lighting applications can be subcategorized in to three groups; <25W, 25W-100W and 100W. It is fair to say the <25W LED lighting applications can be usually found in portable handheld products such as PMPs, or even automotive infotainment displays. Similarly, 25W-100W LED lighting applications can be found in automotive headlamps and architectural lighting applications. And finally, 100W LED lighting applications will commonly occur in large panel LCD-TVs, video walls and industrial lighting.
Thus, it is useful to think of these LED lighting power levels as they relate to some of these applications as the best way to discuss the challenges of the technical implementation and commercial viability aspects.
< 25W LED Lighting – Handheld Devices
Many of today’s mobile phones have a built-in digital camera capable of high-resolution still and video images. Gains in camera performance have also created the need for a high power white light source for camera use indoors or in dim ambient light. White LEDs have emerged as the primary light source in cellular phones equipped with cameras. Since they possess a desirable combination of features for the modern cell-phone designer: small size, high light output, and the ability to provide both “flash” and continuous “video” subject lighting. High output power LEDs have been developed specifically for use as integrated camera lights.
Furthermore, just about any consumer battery-powered handheld device uses a color active-matrix LCD to display the different types of information and data needed by the user. However, manufacturers are faced with the challenge of ensuring that a user can read the information from these displays in any type of environment. To achieve this, they must provide the color LCD with the correct amount of backlighting. This backlighting is normally provided by white LEDs. This created the demand for compact, efficient and low noise LED Driver ICs to power them.
25W-100W LED Lighting – Automotive Headlights
Consider if you will, a typical LED headlamp assembly used by any automobile manufacturer. The overall lighting configuration is similar for all of these vehicles. Each headlamp assembly contains five LED powered beams optimized for all lighting requirements which include: low beam, high beam, cornering assist beam, daytime running light and turn signal indicator.
The standard beams will generally require between 35 and 50 watts of power. This might not seem like a lot of power; however, a LED delivers 10 times the lumens when compared with a HID halogen, so the LED’s light output is equivalent to 500 watts of halogen. The high beams usually requires the same, or slightly more power than the standard beam, whereas the cornering assist beam, daytime running lights and turn signal indicators will require less. Nevertheless, this total headlamp housing can consume over 200 watts of electrical power – potentially creating a huge thermal power dissipation problem. This is not good, since a LEDs light output and operational lifetime degrade rapidly with high temperatures.
There are a number of ways to deal with this thermal problem. One is to add plenty of heat sinking to take the heat away from the lamps. However, this creates another set of issues including more cost and weight for the heat sinking material. The most effective way to deal with this issue is to minimize the thermal dissipation of the LED driving circuitry by having an extremely efficient driver – one that is over 93% efficient. This is not as difficult as it sounds since a 50W high beam usually consists of 14 x 1A LEDs in a series string. Since the forward voltage drop over temperature is approximately 4V per LED, a boost converter LED driver topology can take the nominal battery voltage of 12V to just over 56V with 93% efficiency. This leaves only 3.5W of power to be dissipated, a value easily accommodated with a low level of copper heat sinking within the printed circuit board upon which the LED headlamp is mounted.
100W Plus LED Lighting – Large Panel LCD-HDTVs
LED Driver ICs bring many advantages when being used to drive LEDs used for the backlighting of a high definition TV panel. They allow LED driver circuit solutions to be tiny, compact and low profile. These also operating at high switching frequencies to decrease the value, size and cost of the output capacitors in the case of a charge pump based topology. In an inductor based DC/DC switching topology, these high switching frequencies decrease the value, size and cost of the inductor and output capacitors. Furthermore, in some instances, the LED drive ICs can include both the Schottky diode and boost diode on-chip, thereby reducing the number of external components. This, in turn, reduces design complexity, solution size and cost.
LCD HDTVs have a variety of shortcomings ranging from motion blur to color reproduction. Namely, with the current generation of LCD HDTVs, true blacks cannot be attained and offer a lower dynamic range of all colors. Conventional HDTVs are backlight with CCFL tubes and can only offer contrast ratios between 450 and 650cd/m2. The primary problem of these HDTVs is the inability to completely turn off or locally dim the CCFL backlighting.
Conversely, with HB LED backlighting, an array of LEDs (up to 1,600 for a 46” display) that can be dimmed or turned off locally in backlighting “clusters”, offering contrast ratios almost an order of magnitude higher (>4,000cd/m2), than CCFL designs. Additionally, by adjusting the brightness of the backlighting LED clusters, more mid-tones of colors can be replicated adding a more vivid picture. Another benefit is being able to completely turn off the LEDs locally, thereby reducing motion blur. By turning the LEDs completely off between frames, the blur associated with fast moving objects in virtually eliminated. The LEDs very fast response rate is critical in resolving this fast motion blur routinely encountered by CCFL backlit LCD TVs.
Regardless of the three lighting power ranges of less than 25W, 25W-100W and greater than 100W, the choice of LED driver circuit will largely depend upon the input voltage range, the cumulative voltage drop of the LED string itself and the current required to adequately drive the LEDs. This leads to number of different possible LED driver topologies, such as buck (or step-down), boost (or step-up), buck-boost and SEPIC. Clearly, each topology has its advantages and disadvantages, with a standard buck converter being the simplest and easiest to implement, all the way to a SPEIC converter with its complex magnetic design elements and the need for high degree of switch-mode power supply design expertise.
Nevertheless, many analog IC LED driver providers offer a portfolio of products with wide input and output voltage ranges, some with integrated switches on-chip, and in most conversion topologies. Furthermore, these drivers usually incorporate wide dimming ranges and all the necessary protection features to enable cost-effective and compact solution footprints, thereby ensuing that they will have a bright future.
Director of Product Marketing
Linear Technology Corporation