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 greenpowerZONE Products for the week of December 17, 2007
Linear Technology Says…
LT3517/18: 45 V, 1.3 A/2.3 A LED Drivers for Boost, Buck or Buck-Boost High Current LED Applications
Linear Technology announced the LT3517 and LT3518. They are both 45V, high side current sense DC/DC converters designed to drive high current LEDs. Their 3V to 30V input voltage range (with transient protection to 40V) makes them ideal for a wide variety of applications including automotive, industrial and architectural lighting. The LT3517 and LT3518 are identical except for switch current limits. The LT3517 has an internal 1.3A switch, whereas the LT3518 has an internal 2.3A switch.
The LT3518 can drive up to ten 350mA white LEDs from a 12V input, whereas the LT3517 can drive five, making them ideal for a variety of applications such as automotive display backlighting. Both parts can deliver efficiencies up to 90% in boost mode from a compact 4mm x 4mm QFN package. They utilize True Color PWM dimming to deliver constant LED color with dimming ranges of more than 3,000:1. For less demanding dimming requirements, the CTRL pin can be used to offer 10:1 analog dimming. Their fixed frequency, current mode architecture offers stable operation over a wide range of supply and output voltages. A frequency adjust pin enables the user to program the frequency between 250kHz and 2.5MHz to optimize efficiency while minimizing external component size.
The LT3517 and LT3518 sense output current at the high side of the LED, enabling buck, buck-boost or boost configurations. Other features include open LED protection, a gate driver for PMOS LED disconnect, and soft-start.
EN-Genius Says…
Linear’s versatile LED driver is well-suited to be the power behind the new generation of HBLEDs that are now delivering more lumens/W than the most efficient fluorescent lamps (Today’s HBLEDs typically deliver 60 - 100 lumens/W and as high as 129 lumens/W for Cree’s XR-E series. This compares exceptionally well to the 45 - 65 lumens/W produced by most compact fluorescent lamps, and 104 lumens/W for the best T5-style fluorescent tubes.). Equally important, their ability to work from the low-cost, readily-available 12 V and 24 V supplies found in most low-voltage halogen lighting systems should greatly accelerate adoption for architectural lighting in both new and retrofit applications. Their wide dimming range, high efficiency, and ability to support buck, boost, and buck-boost configurations also makes them an excellent choice for automotive display applications as well.
There are a couple of other companies that offer buck-mode LED drivers suitable for architectural applications but Linear has several value-add features in their design that make it especially attractive. For one thing, their PWM circuit provides a highly controllable current despite LED impedance that varies from device to device as well as over temperature and time. This more accurate current control enables better lighting and power accuracy as well as tighter fault regulation -- things which Linear says are greatly appreciated by their customers. Their soft-start feature protects against input current spikes that could shorten junction life and works better with transformers in existing halogen lighting supplies. Linear was hesitant to go into details but, as I understand it, the characteristics of the transformers in many of the commercially-available power supplies make it tough for most PWM regulators to implement a LED driver circuit that has a decent dimming range.
When used in its buck-mode configuration, the regulator uses a low-side switch (switch is tied to ground rail rather than Vin) that simplifies designs and makes higher efficiency (often 90%, or better) possible. Only a few other products on the market support this mode which cuts external component count as well as the waste heat that can be so tricky to get rid of in compact, unventilated housings. These characteristics lend themselves very nicely to either a controller running a small bank of lights or actually being embedded in the base of a lamp so it could be a plug-in replacement for a halogen bulb that drew 25% or less of the power.
 Giving HBLEDs this ability to tap into the technical resources developed for the halogen lighting industry, and even serve as direct replacements has tremendous potential to accelerate the general acceptance of solid-state lighting. Considering the fact that as much as 22% of America’s electricity is used for lighting, widespread adoption of the kinds of solid-state lighting this product makes possible could help take a large chunk out the nation’s carbon footprint. It also means a tremendous potential market for Linear -- provided they can hold their own against penny-shaving competitors that are certain to crop up as soon the market shows a hint of being lucrative.
But even if Linear feels a slight pinch in the solid-state lighting market, it should still do well in the monitors, TVs, and automotive displays where their TrueColor dimming capabilities give TFT/LCD backlighting applications (TVs, monitors, notebooks, and other displays) color-accurate light across a wide dimming range. As a bonus, moving to solid-state lighting eliminates the mercury-laden CCFL backlights that are most commonly used today.
The LT3517EUF and LT3518EUF are in production in QFN-16, priced starting at $2.70 and $2.95, respectively, in 1-k piece lots.
Data Sheet LT3517
Data Sheet LT3518
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