Dimming LEDs remained a hot topic at the just-concluded Strategies in Light (SIL) conference in Santa Clara, CA, and Marvell led the way with the new 88EM8183 single-stage, solid-state lighting (SSL) driver IC that the company says can be smoothly dimmed to 1% of drive current. Designed for retrofit-lamp applications, the new IC relies on a digital signal processing (DSP) core to detect and adapt to different phase-cut dimmers.
The iWatt exhibit also stressed dimming while NXP focused on smart lighting and On Semiconductor had an innovative automotive-centric demonstration. Texas Instruments (TI) hosted dimming demonstrations and highlighted its LM3466 linear driver IC designed for current matching with multiple LED strings. Other companies with IC demos included STMicroelectronics and Monolithic Power Systems.
Marvell's new IC can deliver 90% efficiency and a power factor above 0.95. Moreover the IC relies on a primary-side, current-control scheme that reduces the number of external components required in a design. For example, the design eliminates the need for an opto-coupler and other components that comprise the feedback circuit in secondary-side control.
|Marvell's 88EM8183 LED driver IC|
According to Lance Zheng, Marvell senior manager of technical marketing, the IC can result in a 20-50% reduction in external components compared with competitive ICs. Zheng showed driver circuit boards from some widely available retrofit lamps, one of which was easily recognizable as a Philips design, and the difference in component count relative to Marvell's reference design was clearly noticeable.
Still, dimming remained the main focus of the announcement. Indeed Marvell enlisted lighting-controls-specialist Lutron to demonstrate the extent of the dimming support.
"A major complaint we receive from retailers regarding the transition from incandescent to LED lighting solutions is consumer dissatisfaction with high-performance dimming capabilities in retrofitted bulbs," said Ed Blair, vice president and general manager of Lutron. "For excellent dimming performance, and a great customer experience, we recommend the use of the Lutron C•L dimmer family, which has shown good performance with Marvell’s 88EM8183 solution. While finished designs still need to be tested against applicable safety and performance criteria, LED lighting products which use the 88EM8183 driver chip should yield excellent dimming performance on approved Lutron controls."
Marvell said it has tested the dimming capability with more than 100 Triac phase-cut dimmers. And rather than trying to design a circuit that can adapt blindly to any such a dimmer, the IC designers use the DSP core to detect the specific dimmer on a circuit based on a table of dimmer characteristics collected via its testing program. If the IC can recognize the dimmer, flawless operation is guaranteed although the design can also operate with many untested dimmers.
Marvell's exhibit included a box equipped with a dozen or so dimmers. The demo offered visitors a chance to experience the dimming operation of the new IC with each dimmer.
iWatt's approach to dimming
Of course Marvell's approach to recognizing a dimmer isn't unique. Indeed iWatt relies on the same idea, although the actual implementation is quite different.
Rather than using a processor core and memory, iWatt uses a digital-state machine to detect the dimmer type and adapt for smooth operation. The company says it has also tested more than 100 dimmers.
At SIL, iWatt demonstrated the iW3614 IC that is based on two-stage power converter. The first stage is a boost converter that handles power factor correction, and supports power factor as high as 0.94. A pulse width modulation (PWM) stage minimizes ripple current and eliminates flicker even with smaller output capacitors according to iWatt.
|The LM3466 controls multiple LED strings|
The two-stage approach does require more external components than do one-stage designs. But Scott Brown, vice president of marketing at iWatt, said that its design eliminates flicker problems that are associated with many one-stage driver ICs. Brown cited the IEEE PAR1789 working group that is currently studying the health impact of low-frequency flicker as a concern for all lamp designers. We will be covering the work of the group in more detail in an upcoming article.
Texas Instruments' driver ICs
TI has what's probably the broadest selection of LED driver ICs in the industry – in part due to recently completed acquisition of National Semiconductor. At SIL, the company had demonstrations of its two most recent LED driver IC announcements – the LM3448 (National Semiconductor heritage) and the TPS92070 (TI heritage).
The LM3448 demonstration focused on dimmer compatibility, while the TPS92070 demonstration focused on the current accuracy at given dim levels. The video embedded below was made at SIL and includes an explanation of each demo.
TI marketing manager Dan Slupik also touted the benefits of the previously mentioned LM3466 IC. The IC is based on a linear architecture and is designed for multi-string applications such as street lights where a single AC/DC converter can supply power to multiple LM3466 ICs – each supplying constant current to an LED string.
Slupik pointed out that the ICs can be connected to one another to ensure that the output current of each IC is identical. The concept allows a luminaire design to recover from a failed string. For example consider a five string design with a conservative drive current specification. Were one string to fail open, the remaining converters could automatically increase the drive current to the remaining strings, thereby boosting light output to accommodate the missing string.
Smart lighting and automotive
NXP demonstrated its smart-lighting technology that is based on the IPv6 over Low power Wireless Personal Area Network (6LoWPAN) protocol. We covered that technology in detail in the most recent issue of LEDs Magazine in an article on lighting controls.
NXP also demonstrated its new SSL2108X driver IC family that targets non-dimmable retrofit lamps. The company says that the ICs can deliver 95% efficiency and require as few as 14 discrete components.
In the automotive area, we've seen broad adoption of LEDs in both head and tail lights – just consider the much-discussed LED-centric Audi Super Bowl ad. But many prognosticators expect LEDs to find broad use for ambient in-car lighting. On Semiconductor demonstrated a model of an auto with trios of red, green, and blue LEDs located at three different locations in the cabin. Drivers would presumably select ambient light colors based on their moods.
Other recent driver news
There have been a number of other recent LED driver IC announcements that you may find interesting as well, that span a breadth of application segments. Again in the automotive segment, Diodes Inc. introduced the ZXLD1371 driver IC that can power 20 LEDs using a choice of buck, boost, or buck-boost control topologies. Supertex announced the HV9989 three-channel IC for backlight applications. The IC can be used with RGB LEDs or to control multiple strings of white LEDs.
Power Integrations just announced a pair of new driver IC reference designs. The DER-286 reference design documents a 30W fluorescent tube LED retrofit that is based on the company's LNK419EG driver IC. Meanwhile the DER-314 focuses on dimmable PAR30/38 LED retrofit lamps.