PI tunes dimmable LED drivers for cost and market appeal

July 20, 2016 // By Graham Prophet
The latest generation of Power Integrations’ non-isolated driver ICs for LED light bulbs, LYTSwitch-7, is dimmable by traditional TRIAC dimmers, offers a much-reduced component count and bill-of-materials, with good power factor.

LYTSwitch-7 is a single-stage, non-isolated, TRIAC-dimmable, buck topology LED driver IC family. Capable of delivering up to 22W without a heatsink from an SO-8 footprint, these high-efficiency devices are suitable for bulbs, tubes and fixtures. LYTSwitch-7 designs do not require bleeders; employing simple, passive damping for TRIAC management and an off-the-shelf, single-winding inductor, reducing component count to just 20, as compared to approximately 35 parts for typical dimmable LED driver boards.

PI explains that LED lights take so much less current than the incandescent lamps that TRIAC dimmers were designed for, the dimmers are not sufficiently sensitive to detect the current, hence bleed resistors have been required to artificially boost the current (wasting power). The new chip, by waveform shaping, compensates without the need for a bleed resistor. Dimming response is tagged ‘monotonic’ - the eye’s response to brightness levels is closer to logarithmic, but PI says it is not necessary to mimic that, close-to-linear relationship of drive-to-control-knob position will do as long as any alteration to the control produces a perceived brightness change. This, PI says, the -7 chip achieves – the user closes the control loop. Dimming is only available on low-line (110V nominal) supplies in the first version of the chip; a subsequent release will dim at high-line (230V nominal).

The LED drivers enable efficiency of greater than 86% - around 2% higher than conventional dimmable products – with high PF of 0.9, accurate regulation and comprehensive protection. They suit low- or high-line input as well as wide-range universal-input designs for U.S. commercial lighting applications, which operate from 90 VAC to 305 VAC with TRIAC dimming enabled in low-line installations.