by Tony Armstrong, Linear Technology
ICs that drive LEDs handle the complexities of illumination that include management of short circuits and squelching electrical noise.
It’s no secret that LED lighting consumes much less energy than alternative forms of illumination and that the cost of LED lights is dropping fast. Consequently, the U.S. Dept. of Energy says LED lighting could account for 84% of lumen-hour sales in the general illumination market 15 years from now.
But look beneath the market projections and you’ll find that the general LED lighting market resembles the analog IC market insofar as it is fragmented. For example, LED sub-segments consist of replacement lamps, strips and strings, outdoor area, industrial, commercial, residential, consumer portable, entertainment, retail display, off-grid and safety/security.
General LED lighting is gaining traction in both commercial and residential applications. It represents ~57% of the total LED market of $25.9 billion that analysts project in three years. (The other segments are signage, automotive lighting, mobile devices, back lighting in displays, monitors and other.) Though many consumers still see LED lighting as too expensive, its long-term energy-savings and environment-friendliness, and its associated tax reductions, are expected to boost its use in commercial spaces such as parking lots, offices, factory facilities and warehouses. That’s because LED lights are attractive replacements not only for high-pressure sodium lamps, halogen lights and incandescent bulbs, but also for CFL and fluorescent lights in some areas.
Lighting generally represents 25 to 40% of total energy use in commercial buildings, so it is no surprise that commercial/industrial applications are leading the transition to LEDs. The long hours of high intensity light these applications require shorten the economic payback of LEDs. And the long life of LED fixtures dramatically reduces the replacement costs of lighting. These costs can be significant in such applications as high bay lighting where bulb replacement may entail renting a scissor or boom lift.
The primary driver behind the high growth of LED lighting is its dramatic reduction in power consumption over traditional light sources. LEDs marketed as incandescent equivalents give the same level of light (in lumens) as the bulbs they replace while consuming less than 20% of the electrical power. Other LED advantages include a lifetime that is orders of magnitude longer than incandescent bulbs. The ability to dim LEDs using existing triac dimmers is also a major benefit, especially in residential lighting.
LEDs turn on instantly and don’t experience the warm-up period associated with CFLs, nor are LEDs sensitive to power cycling like their CFL counterparts. Additionally, LED lighting fixtures do not contain any toxic materials, whereas CFLs contain small amounts of toxic mercury. Finally, LEDs enable new low-profile form factors that other technologies can not.
While consumers can be fickle, there is clearly a point at which they will pay for an LED 60-W incandescent bulb replacement. Cree recently lowered the price of its 60-W LED equivalent to $7.97 in the U.S. This LED bulb is designed to last 25,000 hours and consumes 14% of the electrical energy consumed by its incandescent counterpart. Cree claims that by replacing a home’s five most frequently used incandescent bulbs with their LED equivalents, consumers can shave an average of about $60 annually from their electric bills.
Comparing LEDs, CFLs and incandescent light sources
|Efficacy (lumens/Watt)||80 to 180
|40 to 70||10 to 15|
|Watts consumed (60W bulb equiv.)||8-10||13-15||60|
|Lifespan (hours)||>25K||2K to 10K||1K to 2K|
|Driver power||DC||AC||Offline AC|
|Power factor||0.5 without PFC
>0.90 with PFC
|Sensitive to power cycling?||No||Yes||Yes|
|Failure modes||None||Yes, may catch on fire, smoke, or emit an odor||Some|
|Cost of 60-W (or equiv.) bulb||$8||$3||$1|
Although LED replacement fixtures look relatively simple, they place tough requirements on the LED driver ICs. LEDs require a well-regulated constant-current source to deliver a constant level of light output, and powering them from an ac source entails special design techniques. Clearly there’s a great opportunity for makers of LED driver ICs in this growing market.
What LEDs want
LEDs are not “heaters” like incandescent bulbs, which electrically behave as resistors. Because LEDs are diodes, they need drivers that provide a tightly regulated current and voltage. LED-based light sources typically employ a string of medium-power LEDs rather than one or two higher-power devices. For example, LED bulbs meant to replace 60-W incandescent bulbs usually contain a dozen or more LEDs in series. The use of LED strings makes drivers more susceptible to open or short circuits.
Furthermore, high temperatures degrade an LED’s useful light output, so thermal management is a big consideration in both the driver circuit and its housing. It can be tough to protect LEDs from thermal overstress. Often the LED sits in a small fixture with little opportunity for heat sinking. So most heat must be dissipated by conduction. This is also why an LED driver circuit that operates with high conversion efficiency is a great help since it produces less heat. State-of-the-art LED drivers now sport low-to-mid-90% efficiencies to help ensure good thermal design.
Of course, an LED only emits light when it conducts, so it needs a regulated dc voltage and current. When working from ac mains, LEDs need ac-to-dc conversion. Regardless of the voltage source, catastrophic events can harm the LED driver; so it is helpful if the driver ICs have protection mechanisms built in.
Over-voltages, over-currents and many other factors can degrade LED operation. Because an LED is current-driven, supplying it with the right amount of current will let it attain its rated light output. This might seem like a simple task for a single LED; however, it’s more challenging when many LEDs are strung in series.
One example of an LED driver IC optimized for handling complexities that arise in LED strings is the LT3795, employing a boost topology. It also provides several other noteworthy operating features. The first is called spread-spectrum frequency modulation, a technique that dithers the system clock in a way that lowers the radiated noise. The LT3795, like all LED drivers, is basically a switching power supply whose switching frequency lies in the same band as AM radio. Thus, in the absence of spread-spectrum frequency modulation, it emits RF noise at the power supply switching frequency and its harmonics. Modulating the switching frequency using spread-spectrum techniques reduces the radiated energy peaks such that they don’t interfere with other electronics.
The second feature is short-circuit protection, not easily implemented in a boost converter. Short circuit protection comes in handy when the LED string lies some distance away from the driver electronics. This scenario could arise in automotive wiring, where LED tail lights might be at the end of a wiring harness. A boost converter provides short-circuit protection through the addition of a disconnect FET and sense resistor in series with and above the LED string. The P-channel MOSFET and resistor serves as a way to monitor the current flowing through it and the LEDs in the string. (It can also serve as a means of dimming the LED string.)
In the event of a short circuit, the current flowing through the disconnect FET would rise quickly. This current can potentially harm the LEDs in the string. To stop this from happening, the driver IC senses the voltage rising across the sense resistor. The driver chip must then turn off the disconnect FET quickly. This is not easy; it must take place in less than one microsecond. The LT3795 is one of the few chips that can do it.
Another LED driver IC is the LT3797, a multi-topology triple-output LED driver. It has an integrated rail-to-rail current-sense amplifier with an output voltage range of 0 to 100 V. Each of its three channels can be configured for a buck, boost or SEPIC mode of operation, and each output can be operated autonomously from one another.
This IC incorporates other protection features, including short-circuit protection when a channel is operated in boost mode. Also implemented is open LED protection, which is afforded by a resistor network on the disconnect FET, which the chip monitors. If the voltage rises above a certain point on these sensing resistors, current through the LEDs has ceased to flow, signaling an open circuit.
The LT3797 is particularly suited to situations that demand LED dimming. Buck LED drivers give the highest PWM dimming ratios. But buck regulators need relatively high input voltages that might not be available in scenarios such as vehicular electrical systems. The electrical system voltage must first be boosted with a preregulator. The boosted output voltage can then be applied as input to buck-mode LED drivers. To get both the necessary voltage boost and LED drive, the LT3797 can be configured with one of its channels as the boost preregulator feeding the other two channels, which both act as buck-mode LED drivers.