Bright Future LEDs

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

The adoption of LEDs in a wide array of lighting applications continues to gain momentum and this is good business for the analogue IC supplier of LED drivers. Over the past 12 months, key metrics have been met by high brightness LEDs and that has meant a significant increase in the demand for LED driver ICs 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 are quickly becoming a mainstream lighting source. Some of these main drivers are in automotive, large panel HD LCD TVs, handheld devices and LED light output, as well as consideration in LED cost factors and their potential use as replacements for incandescent light bulbs.

Audi was first up among automotive manufacturers to use LED headlights back in 2008. The respective lighting assemblies contain 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 and their task is controlling the bright/dark boundary and the range of the headlights.

For the high beam headlight, a four LED array sits adjacent to the low beam arrays and near the lower edge of the assembly, a row of 24 LEDs forms the daytime running lights, DRL.
But it was the DRLs that gave Audi’s cars a new and exciting look, one that became popular with the buying public. Soon, many other manufacturers followed suit, with Mercedes, Jaguar, Lexus and Porsche having similar DRLs.

However, it was the LED headlamp assembly that lagged behind in the adoption stakes, principally because of cost. But early indications in 2011 seem to indicate that this is beginning to change with LEDs allowing more flexible styling choice while their improved efficacy delivers a more favourable cost/performance trade-off.

As a result, we now have Cadillac, Toyota and Lexus all featuring LED headlamps as options on their marques: 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 operate 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 inductor based dc/dc switching, 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 colour reproduction. With the current generation of LCD HDTVs, true blacks cannot be attained, providing a lower dynamic range of all colours. Conventional HDTVs are backlit with CCFL tubes and can only offer contrast ratios between 450~650cd/m2. And 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 1600 for a 46in display, for example – can be dimmed or turned off locally in backlighting clusters: contrast ratios here are almost an order of magnitude higher at >4,000cd/m2 than CCFL designs. Additionally, by adjusting LED backlighting cluster brightness, more colour mid-tones can be replicated for more vivid pictures.
Another benefit is being able to completely turn off the LEDs locally, thus reducing motion blur. By turning the LEDs completely off between frames, the blur associated with fast moving objects in virtually eliminated. Furthermore, an LED’s speedy response is critical in resolving fast motion blur which is 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 colour active matrix LCD to display the different types of information and data needed by the user. However, manufacturers have to ensure that users can read the information from these displays in any type of environment. To achieve this, they must provide the colour LCD with correct backlighting which is normally provided by white LEDs in various combinations depending on screen size. This in turn, creates the demand for compact, efficient and low noise LED driver ICs to power them.

LED growth factors
A high power or HB LED’s light output has already surpassed the critical milestone of 100lm/W, with some manufactures claiming 200lm/W in the laboratory. This means that the LED has now surpassed the CFL’s 80lm/W in terms of energy efficiency. Nevertheless, it is further projected that by next year, the LED will attain 150lm/W output.
Another added benefit is LED lifetime. Depending on how it is calculated, a white LED bulb has at least a 10,000hr lifetime and some even claim 50,000hr. Furthermore, LEDs are heralded as green, since they do not contain hazardous materials.
These advancements are significant because, for example, the US 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 US annual energy use by the equivalent of 500 million barrels of oil, with the attendant reduction in emissions of CO2.
LED lighting costs have come down 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 in the past year. Many LED industry analysts predict that over the course of the next year, 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 an LED bulb producing light comparable to 75W incandescent bulbs commonly used in the home. This type of LED chip usually requires 12~15W of power to output this amount of light.

How do high brightness LEDs challenge driver ICs?
A key performance feature required by today’s LED driver ICs is the ability to adequately dim LEDs. Since LEDs are driven by constant current, where the dc current level is proportional to LED brightness, to vary LED brightness there are two methods of dimming the light by controlling LED current. The first method is analogue 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 colour or inaccurate control of the LED current.
The second method is digital or pulse width modulation 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 colour during high dimming ratios. This method of PWM dimming can be used with ratios as high as 3000:1 in certain applications.
Specifically in the case of driving high brightness LEDs, Linear Technology’s LED driver ICs are capable of delivering sufficient current and voltage for many different types of LED configurations: its conversion topology satisfies input voltage range and required output voltage and current requirements. Thus, Linear’s high brightness LED driver ICs typically boast wide input and output voltage ranges, high efficiency conversion, tightly regulated LED current matching, low noise, constant frequency operation, independent current and dimming control, wide dimming range ratios and a
Small, compact footprint with minimal external components.
The company has a wide variety of products to address the design needs of LED driving. Included are the LT3754 and LT3956.

LT3754 is new from Linear Technology and addresses the design problems associated with driving white LEDs when they are used for backlighting in large panels. The device is an innovative boost mode LED driver IC that can be used for HDTVs with panels of 26in and larger. It has 16 individual channels, each of which can drive a string of up to 15, 50mA LEDs with a Vf of around 3.2V. Thus, each LT3754 can drive up to 240 50mA white LEDs. So a 26in LCD HDTV would require only one LT3754 to provide the necessary backlighting, with all 16 channels controlled via a single PWM input capable of up to 3000:1 PWM dimming ratio.

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 shown in figure 1. Each channel follows a master programmable current to allow 10~50mA 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 device comes in a compact, 32 pin, 5×5mm QFN package.


Figure 1: 38W LED driver for 16 strings of 15 LEDs at 50mA/string.
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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 Linear’s the recently introduced LT3956 monolithic LED driver. This dc/dc converter is designed to operate as a constant current, constant voltage regulator and is ideal for driving high current, high brightness LEDs as shown in Figure 2.

Figure 2: 94% efficient 25W white LED headlamp driver.
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LT3956 features an internal low side n-channel power MOSFET rated for 84V at 3.3A and 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 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 adjustment pin enables users to program the frequency over 100kHz~1MHz to optimise efficiency, performance or external component size.
The device senses output current at the high side of the LED string as 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 analogue dimming capabilities.

Tony Armstrong Is Linear Technology’s Director of Power Product Marketing