LM3450 LED Driver

National Semiconductor

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Integrating active power factor correction and phase-dimming decoding across a wide programmable dimming range to provide flicker-free uniform dimming, National Semiconductor Corp.’s LM3450 LED driver is suitable for use in high-brightness LED applications. A member of the company’s PowerWise energy-efficient product family, it is well suited for use in high-performance, phase-dimmable, 10- to 100-W LED fixtures.

Residential and commercial lighting applications that interface with phase-based dimmers require circuitry that can correctly decode a phase-chopped waveform to provide proper dimming of the LEDs. Dimmable LED drivers available today do not always meet the minimum current requirement for forward phase or triode alternating current switch-based dimmers, causing the dimmer to shut off or misfire during operation. This causes inaccurate decoding of the phase angle and results in flicker, particularly at low dimming levels. To compensate, today’s drivers reduce the LED dimming range or continuously burn power to keep the dimmer from misfiring.

The LM3450 LED driver integrates hold circuitry to dynamically adjust the current through the phase dimmer. The dynamic hold circuitry, coupled with a programmable, intelligent dimming decoder, ensures smooth, consistent illumination across the full dimming range. This combination of features provides efficient and uniform dimming. The active power factor correction ensures a high power factor that complies with residential and commercial Energy Star standards.

The driver regulates an accurate output while maintaining a good power factor at the input. The integrated phase dimming decoder interprets the phase angle and remaps it to a 500-Hz pulse width modulation output to dim the LEDs. This combination of features is suitable for implementing a phase-dimmable off-line LED driver for 10- to 100-W loads.

The phase dimming decoder uses programmable mapping from input to output to increase design flexibility. It also features a dynamic filter and a variable sampling rate for a fast, smooth response to dimmer movement. The integrated dynamic hold circuit ensures that the phase dimmer angle is decoded properly, while minimizing the extra power losses associated with holding current.

Other benefits include an analog adjust input to allow for differentiation such as thermal fold back, interface to sensors or dimmer range adjustability. The LED driver can be implemented in either single- or two-stage configurations. The former yields the smallest footprint and allows for the highest efficiency when converting from the AC mains voltage. The latter provides improved dimming performance and better noise immunity. Both are offered in a 16-pin thin shrink small outline package.

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Published: December 2010