White LED Applications Benefit from Highly Integrated IC Drivers

Combined with improved LEDs, these ICs enhance several products

Randy Frank, Contributing Editor -- Design News, October 22, 2007

Based on recent advancements, high brightness (HB) white LEDs are replacing incandescent bulbs in flashlights, 115V light bulbs, night lights and even automotive headlamps. Each of these applications has a different voltage source, 3V dc for flashlights, 115V ac for household use and 12V dc for automotive.

To simplify the control circuitry design, several semiconductor companies make LED drivers for these and other applications. The driving techniques vary to ensure uniform brightness from multiple LEDs and the integrated circuits (ICs) include a variety of additional features. Here are five examples of recent drivers that employ different approaches for several applications.

1.2A LED DRIVER WITH I2C INTERFACE
The I2C-compatible interface in Texas Instruments’ TPS61050 simplifies its use to drive single, high-brightness white light emitting diodes (LEDs) in applications such as camera phone flash units. Using a high-frequency synchronous-boost topology, the unit provides a constant current sink with a maximum of 1.2A. The interface allows programming of the output voltage, as well as enabling a shutdown mode to reduce the input current to 0.3 µA when not in use. In addition to operating as a regulated current source, the device functions as a standard voltage-boost regulator to power other systems. The TPS61050 is offered in a 2 x 1.5-mm, 12-pin wafer chip scale and 10-pin QFN packaging.

AUTOMOTIVE LED DRIVER
In May 2004, Supertex introduced the HV9910 to convert high voltage supplies such as rectified 85 to 265V ac or 8 to 450V dc to a constant current source for powering a string or a combination of strings of HB LEDs. This product addressed a variety of markets including signage and decorative LED lighting in industrial applications, as well as automotive. More recently the company’s AT9933 specifically targets automotive applications. Rated for ambient temperatures up to 125C and Automotive Electronics Council (AEC) Q100-compliant, the variable frequency PWM controller IC uses an external power MOSFET and operates from 8 to 100V dc. The unit is available in an 8-lead SOIC package.

DUAL LED DRIVER
Linear Technology’s LT3497 is a dual driver with integrated Schottky diodes that directly drive up to 12 LEDs (six white LEDs in series per converter). Operating from a single Li-Ion cell, each step up dc to dc converter independently boosts the voltage to each bank. This allows asymmetric LED strings and separate LED dimming and shutdown. Other features include open-LED protection and a one-wire current source capability enabled by a unique high-side LED current sense. The LT3497 is offered in a 3 x 2-mm DFN-10 package. The high level of integrating, single Li-Ion cell operation and small packaging size target applications in cell phones.

LED DRIVER WITH INTEGRATED MOSFET
An integrated high-side MOSFET allows National Semiconductor’s LM3404HV to directly switch up to 1.2A output current over temperature. With an input voltage range of 6 to 75V, the monolithic switching regulators deliver constant currents to high power LEDs. Based on hysteretic controlled on-time and a single external resistor, the converter’s output voltage adjusts as needed to deliver a constant current to a varying number and type of series and series-parallel connected LEDs. Other features include pulse width modulation (PWM) LED dimming, broken/open LED protection, low-power shutdown and thermal shutdown. Available in SO-8 and PSOP-8 packages, the units target automotive, industrial, and general lighting applications.

AUTOMOTIVE AND GENERAL LIGHTING LED DRIVER
Maxim Integrated Products’ MAX16831 is a high-voltage, high-power, constant-current LED driver with analog and PWM dimming. Operating from a 5.4 to 76V input-voltage range, the units can handle the low voltages of cold-start and high load-dump (up to 80V) voltages commonly found in automotive applications. Specified for the -40 +125C automotive temperature range, the units target applications such as high and low-beam assemblies, adaptive front-light systems, daytime-running lights (DRLs) and fog lamps. With its wide operating voltage range and ability to withstand the harsh automotive environment, the MAX16831 also addresses several general lighting applications. Packaging is a 5 x 5-mm, 32-pin TQFN package with exposed pad.

White LEDs Bright Future

The high efficiency and low environmental impact of high brightness LEDs make them the technology of choice in many applications. The Strategies Unlimited report The Market for High-Brightness LEDs in Lighting: Application Analysis and Forecast — 2007 claims that the introduction of high-performance white LEDs with efficiencies above 100 lumens per watt (lm/W) in the next two years will drive growth in several markets. As a result, by 2011 LED use in residential, retail and off-grid applications as well as outdoor area applications will approach $1 billion. LEDS will even start to replace high-efficiency fluorescent lamps that have hazardous disposal concerns.
Christopher James, vice-president of marketing, Cree Inc. claims a lighting revolution has begun. “Three things are really converging to make this all possible: The technology itself has gotten to the point that it is bright enough and efficient enough, the economics and the energy crisis,” says James.
In June, Cree announced the volume quantity availability of its 100-lumen (at 350 mA) XLamp LEDs. With a 100 percent improvement in performance over the past 17 months, the latest units deliver either 25 percent greater brightness with improved efficacy or up to 55 percent greater brightness at the same efficacy, compared to the previous generation.
Cree has company in its revolution. Among them is Seoul Semiconductor, whose Z-Power LED (P4 series) emits 240 lm at 1A of light with a luminous efficacy of 100 lm/W at 350 mA (maximum) with a single die.

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White LED Applications Benefit from Highly Integrated IC Drivers

Combined with improved LEDs, these ICs enhance several products

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