Electronic Design

TRIAC LED Driver Enables Full-Range And Flicker-Free Dimming

The LM3445 from National Semiconductor enables a full 100:1 range of offline, uniform, flicker-free dimming for high-brightness LEDs with a conventional triode for alternating current (TRIAC) forward or reverse phase-control wall dimmer. It also can maintain more than 1 A of constant current for large strings of LEDs in a variety of residential, architectural, commercial, and industrial applications. According to National Semiconductor, it maximizes the light output for systems while maintaining Energy Star power factor requirements in typical applications, positioning it among the company’s PowerWise family of energy-efficient products.

TRIAC wall dimmers are designed to interface with a resistive load such as incandescent or halogen light bulbs. LED bulbs don’t appear as a resistive load to the TRIAC wall dimmer, though, so dimming an LED bulb using a conventional TRIAC wall dimmer doesn’t yield optimal dimming performance. LED drivers available today either give LEDs a 120-Hz flicker or they don’t enable the full 100:1 dimming range. The LM3445 overcomes this challenge by translating the TRIAC-chopped waveform to a DIM signal and decoding it for a full range of uniform, flicker-free dimming. The driver’s patent-pending control architecture maintains constant ripple through the LEDs, extending their life.

Also, the LM3445 enables direct LED bulb replacement of existing incandescent or halogen bulb systems connected to standard TRIAC wall dimmers. In addition, it allows master-slave operation, enabling the control of multiple strings of LED bulbs. Offered in a 10-pin mini small-outline IC (SOIC) package, the device is an adaptive constant off-time ac-dc buck (step-down) constant current controller that includes a TRIAC dimming decoder, which permits wide-range LED dimming using standard TRIAC dimmers. Its high-frequency-capable architecture allows the use of small external passive components.

The LM3445’s bleeder circuit ensures proper TRIAC operation by permitting current flow while the line voltage is low to enable the proper firing of the TRIAC. A passive power factor correction circuit ensures good power factor by drawing current directly from the line for most of the cycle and provides a constant positive voltage to the buck regulator. Additional features include thermal shutdown, current limit, and undervoltage lockout. Available now, the LM3445 costs $1.75 in 1000-unit quantities.

National Semiconductor


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