LEDs See Bright Future

Background

The adoption of LEDs for use in a wide array of lighting applications continues to gain momentum. This invariably means good business for the analog IC supplier of LED driver ICs. Over the course of the last 12 months, some key metrics were met by high brightness (HB) LEDs, which will translate into a significant increase in the demand for the LED driver ICs necessary to power them in all end market applications.

By examining a few of the catalysts that will precipitate the escalation of demand for LED driver ICs from their current post-embryonic stage into an accelerated growth stage, it is evident that LEDs will quickly be a mainstream lighting source. Some of these main drivers are automotive lighting, large panel high definition (HD) LCD TVs, handheld devices, LED light output, LED cost factors and their potential use as a replacement for incandescent light bulbs.

Audi was the first automotive manufacturer to use LED headlights in its car models, way back in 2008. Their lighting assembly contains two low-beam headlamps as the main function, consisting of two LED arrays with four active elements each. Three additional LED arrays with two LED ICs each are located behind the optical lens; their task is controlling the bright/dark boundary and the range of the headlights. For the high-beam headlight, a four-LED array is located adjacent to the low-beam arrays. Near the lower edge of the assembly, a row of 24 LEDs forms the daytime running lights (DRL). But it was the DRLs which gave Audi’s cars a new and exciting look – one which became very popular with the “buying” public. Soon, many other manufacturers followed suit, with Mercedes, Jaguar, Lexus and Porsche having similar DRLs to name but a few.

However, it was the LED headlamp assembly that lagged adoption, principally due to cost reasons. But early indications in 2011 would seem to indicate that this is beginning to change due to the fact that LEDs allowing more flexible styling choices, while their improved efficacy offers a more favorable cost/performance trade off. As a result, we now have Cadillac, Toyota and Lexus all offering LED headlamps as option on their models. For example, the Cadillac Escalade, the Lexus LS600H and the Toyota Prius.

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) 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. Furthermore, a LEDs very fast response rate is critical in resolving this fast motion blur routinely encountered by CCFL backlit LCD TVs.

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.

Similarly, 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 in various combinations depending on the screen size. This in turn, creates the demand for compact, efficient and low noise LED driver ICs to power them.

Important LED Growth Factors

A high-power or HB LED’s light output has already surpassed the critical milestone of 100 lumens per Watt (lm/W), with some manufactures claiming 200 lm/W in the laboratory. This means that the LED has now surpassed the CFL (80 lm/W) in terms of energy efficiency. Nevertheless, it is further projected that by 2012, the LED will attain150 lm/W output. Another added benefit is LED lifetime. Depending on how it is calculated, a white LED bulb has at least a 10,000-hour lifetime and some even claim 50,000 hours. Furthermore, LEDs are “green,” since they do not contain any hazardous materials.

These advancements are significant because the U.S. Department of Energy has stated that lighting consumes 22% of the electricity produced in the United States. Widespread use of LED lighting could cut this consumption in half. To put this into perspective, by 2027, LED lighting could cut the annual energy use by the equivalent of 500 million barrels of oil, with the attendant reduction in emissions of carbon dioxide.

The cost of LED lighting has been come down very quickly. The cost of individual white LEDs, several of which go into an LED bulb and make up much of the cost, have come down in price from about $5 a few years ago to under $1.00 in the last twelve months. Many LED industry analysts predict that over the course of the next twelve months LED bulb replacements for the incandescent light bulb will be priced at a level that will be acceptable for the consumer. Some LED manufacturers have already claimed that they have designed light-emitting chips that could power a LED bulb producing light comparable to the 75-Watt incandescent bulbs so commonly used in homes. This type of LED chip usually requires 12W to 15W of power in order to be able to output this amount of light.

How Do High Brightness (HB) LEDs challenge Driver ICs?

One key performance feature that an LED driver IC must have today is the ability to adequately dim LEDs. Since LEDs are driven with a constant current, where the DC current level is proportional to LED brightness, to vary the LED brightness, there are two methods of dimming the light by controlling the LED current. The first method is analog dimming, in which the LED DC current level is reduced proportionally by reducing the constant LED current level. Reducing the LED current can result in a change in LED color or inaccurate control of the LED current. The second method is digital or pulse-width-modulation (PWM) dimming. PWM dimming switches the LED on and off at a frequency at, or above 100Hz, which is not perceivable to the human eye. The PWM dimming duty cycle is proportional to LED brightness, while the on-time LED current remains at the same level (as set by an LED driver IC), maintaining constant LED color during high dimming ratios. This method of PWM dimming can be used with ratios as high as 3,000:1 in certain applications.

Specifically in the case of driving HB LEDs, Linear Technology’s LED driver ICs are capable of delivering sufficient current and voltage for many different types of LED configurations -in a conversion topology which satisfies both the input voltage range and required output voltage and current requirements. Thus, Linear’s HB LED driver ICs typically have the following features:

(A) Wide input voltage range
(B) Wide output voltage range
(C) High efficiency conversion
(D) Tightly regulated LED current matching
(E) Low noise, constant frequency operation
(F) Independent current and dimming control
(G) Wide dimming range ratios
(H) Small compact footprint with minimal external components

Linear Technology has a wide variety of products to address the design needs of LED driving. Two examples are the LT3754 and LT3956.

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A new product introduction from Linear Technology address the design problems associated with driving white LEDs when they are used for the backlighting of large panels. The LT3754 is an innovative LED driver IC that can be used for HDTVs with panels of 26” and larger. This boost mode LED driver has 16 individual channels—each one can drive a string of up to fifteen 50mA LEDs with a Vf of approximately 3.2V. Thus, each LT3754 can drive up to 240 50mA white LEDs. As a result, a 26” LCD HDTV would require only one LT3754 to provide the necessary backlighting. All of the 16 channels are controlled via a single PWM input that is capable of up to a 3000:1 PWM dimming ratio.

The LT3754 uses one small inductor and even tinier ceramic output capacitors. The only other required components are a single input capacitor, MOSFET and a current setting resistor, as seen in figure 1. Each channel follows a master programmable current to allow between 10mA to 50mA of LED current per string. Channels can also be paralleled for higher LED current. Output voltage adapts to variations in LED Vf for optimum efficiency and open LED faults do not affect operation of connected LED string. The LT3754 is housed in a compact 32-pin, 5mm × 5mm QFN package.

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Figure 1: 38W LED Driver for 16 Strings of 15 LEDs at 50mA per String.

A 25W white LED headlamp can now be configured using an array of 18 LEDs in series with 350mA of current passing through them to produce the necessary light output. However, a major obstacle is how to efficiently and simply drive such a configuration. One possible solution is to use the recently introduced LT3956 monolithic LED driver from Linear Technology. The LT3956 is a DC/DC converter designed to operate as a constant-current and constant-voltage regulator. It is ideally suited for driving high current, high brightness LEDs (see Figure 2).

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Figure 2: 94% Efficient 25W White LED Headlamp Driver

The LT3956 features an internal low side N-channel power MOSFET rated for 84V at 3.3A and is driven from an internal regulated 7.15V supply. The fixed frequency, current-mode architecture results in stable operation over a wide range of supply and output voltages. A ground based referenced voltage feedback (FB) pin serves as the input for several LED protection features and also makes it possible for the converter to operate as a constant-voltage source. A frequency adjust pin allows the user to program the frequency from 100kHz to 1MHz to optimize efficiency, performance or external component size.

The LT3956 senses the output current at the high side of the LED string. High side sensing is the most flexible scheme for driving LEDs, allowing boost, buck mode or buck-boost mode configurations. The PWM input provides LED dimming ratios of up to 3000:1 and the CTRL input provides additional analog dimming capability.

Conclusion

Any LED being driven by the LED driver has to be capable of delivering the necessary lumens of light output from the lowest possible level of power without causing significant thermal design constraints in the end system. Fortunately for lighting designers there exists both the high efficacy LEDs and the high performance LED drivers to deliver what they want the most: high value light output from modest power levels and at a reasonable cost.