Originally fueled by environmental concerns and the steadily rising cost of electricity, the market for energy- efficient lighting has been dramatically accelerated by regulations that will restrict the sale of most incandescent bulbs in North America, Europe, and Asia.

The CRD1611 reference design demonstrates the performance of the CS1611 resonant mode AC/DC dimmable LED driver IC with a 550mA output driving 4xLEDs in series. The form factor is designed to fit into many LED bulb applications.

Over the next few years, billions of light sockets currently occupied by inefficient, filament-based incandescent lamps will be replaced by solid-state lighting (SSL), compact fluorescent lighting (CFL) and other energy-saving technologies. In combination with rapidly falling LED prices, research analyst firm McKinsey & Company predicts that this market segment will grow to one billion units by 2015.

Although a relative newcomer to the mainstream lighting market, LED-based technologies have already enjoyed considerable success in selected applications. Now that SSL is poised to enter the mainstream general lighting market, the efficiency, versatility and longevity it offers also gives SSL products the potential to win a majority of the larger market from incumbent lamps, both incandescent and CFL.

These virtues notwithstanding, however, to date most LED retrofit products available to consumers and retailers have shown some significant limitations that have slowed their march toward the ultimate destination – supplanting the long-lived and much-loved incandescent lamp.

Compatibility Problems

Figure 1. Cirrus Logic’s CS161X family of digital LED controller ICs were designed with dimmer compatibility in mind.

Chief among the problems is the unreliable nature of most current-generation dimmable LED lamps to provide dimmer compatibility with the hundreds of existing different dimmer systems installed in homes and retailers globally. To become a true, market-viable incandescent replacement product, LED lamps must be near 100 percent compatible with these dimmers.

Many SSL products currently on the market use analog LED driver ICs that claim to be “dimmer compatible.” In reality, however, what consumers get is “lucky dimming” – meaning, you’re lucky if the LED lamp that you have actually dims (and dims well!) with your existing dimmer system. The actual metric of most of today’s LED lamps is 50 to 60 percent compatibility in the high-volume consumer market.

With this wide gap of incompatibility, the result is a significantly high rate of return back to the retailers. This problem cuts deeply into manufacturer/retailer profit margins and can damage a manufacturer’s reputation and brand image. In addition, customer returns drive customer dissatisfaction and will severely limit the entire SSL industry’s chances for rapid market acceptance.

The market potential for a solution to this dilemma is one of the key reasons Cirrus Logic selected dimmer compatibility as a product feature priority when it developed its first line of digital LED controller ICs, the CS161X family.

As Cirrus Logic engineers drilled into the problem of dimmer compatibility, it became apparent that the majority of dimming issues encountered by earlier LED driver solutions arose from three commonly used dimming circuit architectures, each with a different type of waveform and its own unique set of electrical characteristics.

This problem was further compounded by the fact that there are no standards that govern how a dimmer controls power to its load. Even products from different manufacturers that use the same basic dimming architecture can exhibit widely varying characteristics. In order to achieve the widest level of compatibility, the design team determined that an SSL driver circuit must be able to correctly interpret signals from all types of dimmers — from multiple manufacturers — in order to control the light output of the bulb’s LEDs in an accurate and consistent manner.

Failure to do so causes the SSL bulb to have an annoying “flicker” during steady-state operation. The bulb also will exhibit a limited dimming range, “choppy” dimming during adjustment, and in some cases, fail to work at all.

The key to solving the dimmer compatibility problem was the creation of a digital architecture that avoids the inherent pitfalls of the inflexible analog approaches by using an “intelligent” digital controller to detect each dimmer’s unique characteristics and operating requirements. The LED driver changes its behavior and characteristics depending on the type of dimmer connected on the other end of the power line.

Figure 2. Results of dimmer IC compatibility tests.

This information is used to control the circuit’s analog section, which takes input power from the AC line, filters it and tailors it to optimally drive the SSL bulb’s LEDs. This approach allows the controller to adapt to the type of dimmer to which it is connected while setting the light level as requested by the user. LED lamps that feature analog solutions are non-adaptive and cannot address the challenges of the dimmer compatibility problem.

The first commercially available LED lamps from a major LED lamp OEM to feature this new IC approach to dimmer compatibility began appearing on retail shelves in Europe in the spring of 2012, with expected availability in North America this fall.

Testing Dimmer Compatibility

To test this new digital approach to LED driver ICs, a rigorous series of standardized tests was developed, evaluating the compatibility and performance of more than 200 known dimmer systems and application environments against market- leading products that feature analog ICs.

The study examined eight different commercially available LED bulbs from leading OEMs, which are based on LED driver products from six semiconductor makers. Each bulb was tested for approximately two days in a laboratory environment. The dimmer compatibility test protocol included a set of scoring criteria based on the characteristics believed to be of concern to consumers:

  • Flicker: Scoring was based both on visually detectable flicker and the more subtle low-level flicker that can cause long-term discomfort. This criterion was heavily weighted in the overall scoring formula.
  • Smoothness: Scoring is based on the driver’s ability to control lamp intensity in a smooth, continuous manner across the entire dimming curve.
  • Maximum light level: To receive a full score, the bulb must be within two percent of the brightness of a bulb running on an un-dimmed circuit.
  • Minimum light level available: To receive a full score, the bulb must be able to dim to two percent of its maximum output.

Since many other dimming solutions experience a narrower dimming range and/or increased flicker as more lamps are added to the circuit, the suite of application environments included tests for single lamp and multi-lamp (5 and 10 bulbs) performance. Each type of SSL bulb was evaluated for its ability to meet all these characteristics for each of the study’s application environments at both 110V and 230V line voltages and both 50Hz and 60Hz line frequencies.

All evaluations were run across a range of voltage and frequency variations reflecting maximum and minimum allowable levels defined by the generally accepted tolerances for power grids in various regions of the world.

The final test results showed that the new digital controller IC approach provided 98 percent compatibility (judged on the criteria listed above) with the vast majority of the world’s dimmer systems. Most LED lamps today that are based on analog controller ICs provided between 47 to 58 percent compatibility, meaning that they performed well in some test areas but still proved measurably incompatible with a number of the defined test criteria (Figure 2).

Solving the problem of dimmer compatibility is a major step forward for LED lamps to propagate into homes and retailers worldwide. Consumers are migrating to energy-efficient solutions, provided that these solutions meet their expectations for performance and cost. With the dimmer problem now effectively solved, LED lamps are on their way to fulfilling their promise to be the new standard for the incandescent replacement market.

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