LED driver claims flickerless dimming

Responding to a significant number of requests from potential customers for a TRIAC dimmable light emitting diode (LED) driver, National Semiconductor has introduced the LM3445, a constant-current controller that enables uniform dimming of high brightness LEDs, without flicker.

Conventional TRIAC dimmers are designed to interface to a resistive load (for a halogen or incandescent bulb). A 300ohm bleeder resistor in National's LM3445 allows current flow while the line voltage is low, emulating the resistive load, thus enabling proper firing of the TRIAC.

An issue for current LED driver solution if interfaced to a standard wall dimmer is that they will produce 120Hz flicker and/or do not allow 100:1 dimming. This part includes an angle detector and a TRIAC dimming decoder that interfaces to any standard TRIAC dimmer and translates the TRIAC chopped waveform to a DIM signal. Meanwhile, a passive power factor correction circuit draws current directly from the line for most of the cycle, and feeds the buck regulator with a constant positive voltage. Constant off-time allows for constant ripple current, which extends the life of the LEDs.

The part, which was announced at the Applied Power Electronics Conference in Washington, US, maintains greater than 1A of constant current to large strings of LEDs. By supporting TRIAC dimming, National expects that it will suit those looking to retrofit existing lighting systems with TRIAC wall dimmers. “There will be no need to redesign the lighting distribution system,” noted Salvatore Napolitano, Director of Product Marketing, Europe.

The LM3445 is already sampling, and there is an evaluation board available. Designers can also use National's Webench LED Designer toolsuite to configure and create the bill of materials for a custom LED design.

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