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LED Drivers Get Smart
June 22, 2010
9 Min Read
Time was when light-emitting diodes (LEDs) were merelyindicator lights. Power was low, electrical current was lower, and heat was noproblem.
Times havechanged, though. Today's LEDs have graduated from milliamps to amps. Power hasjumped from milliwatts to more than 10W, in some cases. And heat - well,suffice to say, it's now an issue to be reckoned with.
"In the olddays, LEDs were so underpowered that the waste heat was very low and they'd essentiallynever burn out," says Rick Zarr, technologist at National Semiconductor. "But the new LEDsgenerate so much heat that if you don't manage them correctly, their lifespanwill be tremendously affected. Instead of getting 10, 20 or 30 years out of abulb, you'll get two."
That's whyproject engineers now need to be concerned with LED drivers. At their simplest,drivers take the input current and input voltage and then reconfigure them foruse by an LED. In that sense, they're a lot like the ballasts used for decades influorescent lights.
In anothersense, though, drivers are growing a lot more sophisticated, enabling them to playa bigger role in this new era of high-power, high-current, high-heat LEDs.Today's drivers now perform functions that would have been unimaginable adecade ago. By sensing current, for example, they can surmise an LED'sbrightness level and alter it accordingly. They can compensate for changes,such as heat or age, and even enable LEDs to serve in dimming applications.Moreover, by using a "thermal foldback" scheme, they can prevent an LED's heatfrom ever reaching the point where its longevity might be damaged.
Thanks tothose kinds of innovations, the new breed of LEDs is finding a home in amultitude of unforeseen applications. Automobiles are using them for brakelights, "puddle lights" and even headlights. Televisions are employing them forbacklighting; municipalities are using them in street lights; and consumerelectronics manufacturers are incorporating them in netbooks, tablet computersand GPS systems.
"LEDs aregetting better every year," says Peter Di Maso, marketing manager for lightingpower products at Texas Instruments. "Theimprovements are making LEDs applicable to general lighting, which is whythere's such a need for better drivers."
Not all forms of lighting need drivers, of course.Incandescent bulbs, for example, have long been optimized to work on our common110-V power supplies, and therefore need no extra circuits to adapt the inputcurrent to the output.
LEDs,however, are more complicated. "With an LED, you first need to understand yourinput power source and its variability," says Steve Bowling, applicationsmanager in the 8-bit Microcontroller Product Group at Microchip Technology Inc. "And on theoutput side, you need to understand your optical requirements and the amount oflumens you want to generate. In the end, your ultimate goal is for the driverto supply a source of constant current to the LED."
Thanks to aphenomenon known as Haitz'sLaw, though, driving an LED has become more complex and much morenecessary. Haitz's Law - named for Roland Haitz, a retired scientist atAgilent Technologies - states that LED cost per lumen falls by a factor of 10every decade, while the light generated per package rises by a factor of 20.For users of LEDs, Haitz's Law means that demand rises, which in turn meansthat the onus of new technology development flows downstream.
"Peoplejust want to screw their LED into place and forget it," says Zarr of NationalSemiconductor. "They don't want to have to think about it, so the burden movesto the manufacturer of the bulb, who moves it downstream to the driver manufacturer."
For driver manufacturers, the challenge lies in adding thenecessary features in a small package. Driver chips are tiny, often measuring 10x 10 mm, or less. Moreover, future generations will need to be even smaller, asLED manufacturers incorporate them in bulbs designed to replace incandescents.
As theymigrate toward such applications, drivers will need is the ability to preventLEDs from getting too hot. To be sure, plenty of suppliers are willing to sellheat sinks and blowers to help dissipate heat. Increasingly, though, users ofLEDs want to minimize the heat before it needs to be dissipated, and thesolution to that problem lies in the driver.
"LEDs canoffer thousands of hours of lifetime," says Bowling. "But for that to be thecase, it has to remain within its thermal operating limit. If you put an LED ina tight space, you're going to run into issues of what to do with the heat."
Drivermanufacturers are helping engineers deal with such problems through thedevelopment of new and better features. National Semiconductor, for example, hasincorporated a feature known as "thermal foldback" in a pair of devices, the LM3464 and the LM3424. By using thethermal foldback scheme, the new devices can tweak the current to enable LEDsto stay within their maximum operating range.
"Thermalfoldback is not designed so much to get rid of heat as to protect the bulb,"Zarr says. "If you get into a circumstance where the LEDs are reaching theirmission-critical temperature, the driver folds back the current and dims thebulb."
Drivers arealso giving users of LEDs other reasons to limit the current that goes throughthe device. Analog Devices Inc., forexample, can control the current in the high-brightness LEDs commonly employedin flash cameras with its ADP1650driver.
"From abrightness perspective, LEDs are very controllable through current," says JoseRodriguez, technical director for ADI's Power Management Group. "So we set thecurrent inside our driver and control it very tightly."
In the past few years component makers have enabled driversto do even more. Triode AC (TRIAC) dimming circuits, long considered a problemfor semiconductor-based lighting, can now be used in conjunction with LEDs. Theability to use LEDs in such circuits opens up a multitude of new applicationsin home, commercial and industrial lighting, experts say.
"Whenpeople do switch to LED bulbs, they don't want to have to change theirinfrastructure," Zarr says. "They don't want to have to remove the dimmers fromtheir walls. That's why we needed to have a solution for dimming applications."
One exampleof a dimming solution is National Semiconductor's LM3445. TheLM3445 incorporates circuitry that reads the dimming signature of a standardTRIAC dimmer, and translates that information to a pulsewidth-modulated currentto drive the LEDs.
TexasInstruments' TPS92010solves a common power dissipation problem in the TRIAC-LED equation. TheTPS92010 incorporates a circuit that doesn't dissipate power when the TRIAC isin use.
"It only draws current when thevoltage input to the light source is zero, so you're dissipating zero watts,"says Di Maso of Texas Instruments. TI offers an evaluation module for engineerslooking to incorporate such abilities in their bulbs.
As driver manufacturers look to add new features, they'realso incorporating microcontrollers in their products. Onboard intelligencemakes microcontroller-based drivers a strong candidate for functions such asdimming, because it enables them to monitor various functions and then makedecisions about them.
"The drivercan take inputs from a system - in this case from the TRIAC dimmer," saysBowling of Microchip. "It can read the line voltage. It can read the duty cyclefrom the TRIAC dimmer. It can monitor the operating voltage and the temperatureof the LED. It can then make decisions to keep the system operating properly forthe chosen dimming level."
Microchipoffers an 8-bit microcontroller family and a 16-bit digital signal controllerfamily for driver applications. The 8-bit PIC16F785 MCU incorporatesanalog peripherals, including op-amps, comparators and 12 channels of 10-bitA/D conversion. "The building blocks are right on the microcontroller," Bowlingsays. "You can use the analog components to regulate the driver and themicrocontroller to assist the functionality."
Microchip's16-bit dsPIC33GS digital signal controller family goes a step further, incorporatingvery high-speed A/D converters that enable them to collect data on the fly. Asa result, the dsPIC family allows developers to do more advanced applications,including dimming, thermal protection and color control.
Similarly,Texas Instruments offers its Piccolofamily of microcontrollers for use in LED driver applications. Available in adc/dc LED developers kit (see video of thekit), the technology employs an MCU to control strings of LEDs. The kit istargeted at customers who want to use LEDs in high-power applications with lessbulb replacement. Applications include street lamps, airplane hangars, andindustrial lighting, where users don't want to repeatedly replace burned-outbulbs.
"ThePiccolo microcontroller can remotely receive commands to set certain strings toprescribed levels of brightness," says Charlie Ice, C2000 microcontrollermarketing manager for TI. "It can sense the current and determine what thebrightness level is, and it can compensate for it. When an LED starts to age orheat up, it can also compensate for that, too."
In almost all LED applications, experts say the motivatingfactor will be efficiency. In contrast to incandescent bulbs, many of which nowuse digital control to turn on and off at certain times so they can save power,LEDs don't necessarily need smart control schemes. They're so efficient they outlastvirtually any product, making it almost unnecessary to implement smart control.
That's whyautomakers have joined the charge toward implementing LEDs in tail lights,interior lighting and headlights. In some cases, LEDs can last for 300,000miles, thus outlasting their vehicles. As a result, car designers are eagerlyincorporating LED modules on vehicle exteriors, knowing that the modules may neverhave to be replaced. That, in turn, opens the door to greater creativity on theparts of designers, who find it easier to incorporate sealed modules.
Designengineers are also using LEDs in GPS systems, netbooks, tablet computers andtelevision backlighting systems for the same reason. As brightness rises andcost drops, high-efficiency LEDs keep making more sense. "Sure, there aredynamic mechanisms to turn lighting systems on and off," says Zarr of National Instruments."But the simplest mechanism is to replace bulbs with something that's moreefficient in the first place."
That's whydrivers will continue to be a key part of the LED equation. Drivers keep LEDscool and they promote bulb longevity. Moreover, the lack of standardization inthe LED world means that a multitude of driver solutions will continue to beneeded, especially as the volume of new applications grows.
"The LEDindustry is very much in a state of evolution right now," says Bowling ofMicrochip. "We always need new solutions. There is no 'one-size-fits-all.'"
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