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