LED Driver Offers Good Phase-Dimmed Performance

National Semiconductor has introduced the LM3450 LED driver, which is claimed to feature the industry's best phase-dimmed performance for high-brightness light-emitting diode (LED) applications. The LM3450 LED driver integrates active power factor correction (PFC) and phase dimming decoding to provide uniform, flicker-free illumination across a wide, programmable dimming range. Part of the company's Powerwise energy efficient product family, the LM3450 is said to be suitable for use in high-performance, phase-dimmable, 10W to 100W 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 TRIAC-based dimmers, causing the dimmer to shut off or 'misfire' during operation. This causes inaccurate decoding of the phase angle resulting in flicker, particularly at very low dimming levels.

To compensate, today's drivers reduce the LED dimming range or continuously burn power to keep the dimmer from misfiring. National Semiconductor's 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 performance. The LM3450 LED driver also provides active power factor correction, ensuring a high power factor that complies with residential and commercial Energy Star standards.

It regulates an accurate output while maintaining good power factor at the input. The integrated phase dimming decoder interprets the phase angle and remaps it to a 500Hz PWM output to dim the LEDs. These features are suitable for implementing a phase-dimmable off-line LED driver for 10W to 100W loads. The phase-dimming decoder uses programmable mapping from input to output to increase design flexibility. It also features a dynamic filter and variable sampling rate for a fast, smooth response to dimmer movement. An integrated dynamic hold circuit ensures that the phase dimmer angle is decoded properly while minimising the extra power losses associated with holding current.

Other benefits include an analogue adjust input to allow for differentiation such as thermal fold-back, interface to sensors or dimmer range adjustability. The LM3450 can be implemented in either single-stage or two-stage configurations. The single-stage configuration yields the smallest footprint and allows for high efficiency when converting from the AC mains voltage. The two-stage configuration provides improved dimming performance and better noise immunity. National's LM3450 is offered in a 16-pin TSSOP package.

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