- National Semiconductor -

For 50 years, National has been known for its reliable, energy-efficient power management products. The company continues to bring this knowledge, experience, and manufacturing capability to help customers create better lighting designs. National’s LED drivers incorporate the
intelligence that systems need to deliver high-performance, reliable, and robust LED lighting solutions to the market.

National Semiconductor’s broad portfolio of PowerWise® energy-efficient power management ICs provide constant current for driving both low-power and high-brightness LEDs, enabling color and brightness matching over a wide temperature range. These LED drivers enable greater energy efficiency and flexibility in lighting designs. Driving numerous LEDs in one string, they provide greater than 90 percent efficiency and
accurate current regulation with less power and heat dissipation.

Solving Customer Design Challenges
To enable customers to build differentiated products more quickly, National’s easy-to-use solutions address a number of lighting design challenges through features such as:
? Dynamic headroom control to maximize system efficiency
? Multiple outputs maximize LED strings per driver and reduce system solution size, cost, and complexity
? Thermal foldback ensures LEDs operate reliably over varying temperature conditions

   (for a robust solution that doesn’t require complex external temperature sensing circuitry)
? Patent-pending architectures enable state-of-the-art

TRIAC/ phase dimming compatibility in LED retrofit lamps and new installation fixtures for a seamless transition to LED lighting technology
? Analog and Pulse-Width Modulation (PWM) dimming capabilities for design flexibility
? Small driver solutions for space-constrained applications
? Tools such as WEBENCH® LED Designer for easier design

Award-Winning Design Tools
National’s unique WEBENCH LED Designer online tool allows for quick and easy selection and simulation of a complete LED and LED driver solution. The tool provides lighting designers a competitive advantage and faster time to market. National’s on-demand tools make it easy for designers to explore and learn, compare and select products, and then design and build their system online.

Wide Range of Applications
National has products that fit into a wide range of applications including:
? Downlighting
? Replacement lamps
? Outdoor area fixtures such as street lamps and parking garage lights
? Industrial/commercial such as high bay and low bay fixtures for warehouse lighting
? Portable consumer: flashlights and sports equipment
? Entertainment and projection
? Architectural/decorative fixtures
? LED backlit displays
? Automotive headlamps
... and more

High-Brightness LED Lighting
Overview
Regardless of type, color, size, or power, all LEDs work best when driven with a constant current. LED manufacturers specify the characteristics (such as lumens, beam pattern, color) of their devices at a specified forward current (IF), not at a specific forward voltage (VF).

Most power supply ICs are designed to provide constant voltage outputs over a range of currents (see below); hence, it can be difficult to ascertain which parts will work for a given application from the device datasheet alone.

With an array of LEDs, the main challenge is to ensure every LED in the array is driven with the same current. Placing all the LEDs in a series string ensures that exactly the same current flows through each device.

High-Brightness LEDs: Input Voltage and Forward Voltage
Sources of input voltage for LED arrays come from batteries or power supplies that have a certain tolerance. An automotive battery, for example, may supply 8V to 16V depending on the load and the age of the battery. The "silver box" power supply inside a desktop CPU may supply 12V ±10%. High-brightness (HB) LEDs also give a range of forward voltage.

A typical HB LED might be characterized at a forward current of 350 mA. The forward voltage of the LED when IF = 350 mA is specified with a range that includes a typical value as well as over-temperature maximum and minimum values. To ensure that a true constant current is delivered to each LED in an array, the power topology must be able to deliver an output voltage equal to the sum of the maximum forward voltages of every device placed in the string.

Manufacturers bin their devices for color, brightness, and forward voltage. Binning for all three characteristics is expensive, and forward voltage is often the specification that is allowed to vary the most. Adding this to the shift in forward voltage as the LED die temperature changes gives rise to the need for constant-current regulators that have a wide range of output voltage.