National Semiconductor - High-Brightness LED Driver with Temperature Management Control

Thermal foldback combats high temperatures LEDs can reach as a result of environmental conditions. When LED temperatures rise above a safe threshold, the lifetime and efficacy of the LED decreases. With the LM3424, lighting designers can program temperature and slope breakpoints within which the LEDs operate safely. When an over-temperature condition occurs, the LM3424’s thermal foldback circuitry reduces the regulated current through the LEDs. The reduced current dims the LED to a range programmed by the designer and the LED remains within the range until it returns to a safe operating temperature.

This method of thermal foldback maintains the lifetime and efficacy of the LED and ensures the warranty period of fixtures in applications such as automotive headlights, high bay warehouse lighting and street lighting.

National’s WEBENCH LED Designer helps lighting designers identify the ideal temperature threshold or foldback breakpoint of the LEDs. Traditionally, resistor values are calculated by hand to identify temperature foldback breakpoints. The WEBENCH online design environment allows the designer to easily enter the temperature foldback and slope points and then visualize the behavior of the design at different LED temperatures. This requires no hand calculations. An interactive temperature slider shows the results graphically. The LED driver design automatically updates with entry of temperature breakpoint requirements.

Technical Features of National’s LM3424 LED Driver

Offered in a 20-pin thermally enhanced TSSOP package, the LM3424 drives up to 18 high-brightness LEDs in series with output current above 2A in a typical application. It contains all of the features to regulate currents based on buck, boost, SEPIC, flyback and buck-boost topologies.

The LM3424 operates over a wide input range of 4.5V to 75V. The pulse-width modulation (PWM) controller is designed for high-speed capability including an oscillator frequency range that can be synchronized up to 2.0 MHz. Additional features include PWM and analog dimming, programmable softstart and protection features such as low-power and thermal shutdown, and cycle-by-cycle current limit.