LED Drivers Get Smart

Time was when light-emitting diodes (LEDs) were merely indicator lights. Power was low, electrical current was lower, and heat was no problem.

Times have changed, though. Today's LEDs have graduated from milliamps to amps. Power has jumped 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 old days, LEDs were so underpowered that the waste heat was very low and they'd essentially never burn out," says Rick Zarr, technologist at National Semiconductor. "But the new LEDs generate so much heat that if you don't manage them correctly, their lifespan will be tremendously affected. Instead of getting 10, 20 or 30 years out of a bulb, you'll get two."

That's why project engineers now need to be concerned with LED drivers. At their simplest, drivers take the input current and input voltage and then reconfigure them for use by an LED. In that sense, they're a lot like the ballasts used for decades in fluorescent lights.

In another sense, though, drivers are growing a lot more sophisticated, enabling them to play a 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 a decade ago. By sensing current, for example, they can surmise an LED's brightness 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 heat from ever reaching the point where its longevity might be damaged.

Thanks to those kinds of innovations, the new breed of LEDs is finding a home in a multitude of unforeseen applications. Automobiles are using them for brake lights, "puddle lights" and even headlights. Televisions are employing them for backlighting; municipalities are using them in street lights; and consumer electronics manufacturers are incorporating them in netbooks, tablet computers and GPS systems.

"LEDs are getting better every year," says Peter Di Maso, marketing manager for lighting power products at Texas Instruments. "The improvements are making LEDs applicable to general lighting, which is why there's such a need for better drivers."

Growing Demand
Not all forms of lighting need drivers, of course. Incandescent bulbs, for example, have long been optimized to work on our common 110-V power supplies, and therefore need no extra circuits to adapt the input current to the output.

LEDs, however, are more complicated. "With an LED, you first need to understand your input power source and its variability," says Steve Bowling, applications manager in the 8-bit Microcontroller Product Group at Microchip Technology Inc. "And on the output side, you need to understand your optical requirements and the amount of lumens you want to generate. In the end, your ultimate goal is for the driver to supply a source of constant current to the LED."

Thanks to a phenomenon known as Haitz's Law, though, driving an LED has become more complex and much more necessary. Haitz's Law — named for Roland Haitz, a retired scientist at Agilent Technologies — states that LED cost per lumen falls by a factor of 10 every 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 means that the onus of new technology development flows downstream.

"People just want to screw their LED into place and forget it," says Zarr of National Semiconductor. "They don't want to have to think about it, so the burden moves to the manufacturer of the bulb, who moves it downstream to the driver manufacturer."

Heat Prevention
For driver manufacturers, the challenge lies in adding the necessary features in a small package. Driver chips are tiny, often measuring 10 x 10 mm, or less. Moreover, future generations will need to be even smaller, as LED manufacturers incorporate them in bulbs designed to replace incandescents.

As they migrate toward such applications, drivers will need is the ability to prevent LEDs from getting too hot. To be sure, plenty of suppliers are willing to sell heat sinks and blowers to help dissipate heat. Increasingly, though, users of LEDs want to minimize the heat before it needs to be dissipated, and the solution to that problem lies in the driver.

"LEDs can offer thousands of hours of lifetime," says Bowling. "But for that to be the case, it has to remain within its thermal operating limit. If you put an LED in a tight space, you're going to run into issues of what to do with the heat."

Driver manufacturers are helping engineers deal with such problems through the development of new and better features. National Semiconductor, for example, has incorporated a feature known as "thermal foldback" in a pair of devices, the LM3464 and the LM3424. By using the thermal foldback scheme, the new devices can tweak the current to enable LEDs to stay within their maximum operating range.

"Thermal foldback 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 their mission-critical temperature, the driver folds back the current and dims the bulb."

Drivers are also giving users of LEDs other reasons to limit the current that goes through the device. Analog Devices Inc., for example, can control the current in the high-brightness LEDs commonly employed in flash cameras with its ADP1650 driver.

"From a brightness perspective, LEDs are very controllable through current," says Jose Rodriguez, technical director for ADI's Power Management Group. "So we set the current inside our driver and control it very tightly."

Controlling Dimmers
In the past few years component makers have enabled drivers to do even more. Triode AC (TRIAC) dimming circuits, long considered a problem for semiconductor-based lighting, can now be used in conjunction with LEDs. The ability to use LEDs in such circuits opens up a multitude of new applications in home, commercial and industrial lighting, experts say.

"When people do switch to LED bulbs, they don't want to have to change their infrastructure," Zarr says. "They don't want to have to remove the dimmers from their walls. That's why we needed to have a solution for dimming applications."

One example of a dimming solution is National Semiconductor's LM3445. The LM3445 incorporates circuitry that reads the dimming signature of a standard TRIAC dimmer, and translates that information to a pulsewidth-modulated current to drive the LEDs.

Texas Instruments' TPS92010 solves a common power dissipation problem in the TRIAC-LED equation. The TPS92010 incorporates a circuit that doesn't dissipate power when the TRIAC is in use.

"It only draws current when the voltage 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 engineers looking to incorporate such abilities in their bulbs.

Adding Intelligence
As driver manufacturers look to add new features, they're also incorporating microcontrollers in their products. Onboard intelligence makes microcontroller-based drivers a strong candidate for functions such as dimming, because it enables them to monitor various functions and then make decisions about them.

"The driver can take inputs from a system — in this case from the TRIAC dimmer," says Bowling of Microchip. "It can read the line voltage. It can read the duty cycle from the TRIAC dimmer. It can monitor the operating voltage and the temperature of the LED. It can then make decisions to keep the system operating properly for the chosen dimming level."

Microchip offers an 8-bit microcontroller family and a 16-bit digital signal controller family for driver applications. The 8-bit PIC16F785 MCU incorporates analog peripherals, including op-amps, comparators and 12 channels of 10-bit A/D conversion. "The building blocks are right on the microcontroller," Bowling says. "You can use the analog components to regulate the driver and the microcontroller to assist the functionality."

Microchip's 16-bit dsPIC 33GS digital signal controller family goes a step further, incorporating very high-speed A/D converters that enable them to collect data on the fly. As a result, the dsPIC family allows developers to do more advanced applications, including dimming, thermal protection and color control.

Similarly, Texas Instruments offers its Piccolo family of microcontrollers for use in LED driver applications. Available in a dc/dc LED developers kit (see video of the kit), the technology employs an MCU to control strings of LEDs. The kit is targeted at customers who want to use LEDs in high-power applications with less bulb replacement. Applications include street lamps, airplane hangars, and industrial lighting, where users don't want to repeatedly replace burned-out bulbs.

"The Piccolo microcontroller can remotely receive commands to set certain strings to prescribed levels of brightness," says Charlie Ice, C2000 microcontroller marketing manager for TI. "It can sense the current and determine what the brightness level is, and it can compensate for it. When an LED starts to age or heat up, it can also compensate for that, too."

No "One-Size-Fits-All"
In almost all LED applications, experts say the motivating factor will be efficiency. In contrast to incandescent bulbs, many of which now use 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 outlast virtually any product, making it almost unnecessary to implement smart control.

That's why automakers have joined the charge toward implementing LEDs in tail lights, interior lighting and headlights. In some cases, LEDs can last for 300,000 miles, thus outlasting their vehicles. As a result, car designers are eagerly incorporating LED modules on vehicle exteriors, knowing that the modules may never have to be replaced. That, in turn, opens the door to greater creativity on the parts of designers, who find it easier to incorporate sealed modules.

Design engineers are also using LEDs in GPS systems, netbooks, tablet computers and television backlighting systems for the same reason. As brightness rises and cost drops, high-efficiency LEDs keep making more sense. "Sure, there are dynamic 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 more efficient in the first place."

That's why drivers will continue to be a key part of the LED equation. Drivers keep LEDs cool and they promote bulb longevity. Moreover, the lack of standardization in the LED world means that a multitude of driver solutions will continue to be needed, especially as the volume of new applications grows.

"The LED industry is very much in a state of evolution right now," says Bowling of Microchip. "We always need new solutions. There is no 'one-size-fits-all.'"