Electronic Design

Digging Out The Story -- One Case History

Sometimes it takes a little digging to pull a story out of a new product announcement. What’s rewarding is when that digging turns up a story that illustrates how the design process actually works.

Here’s an example. A week ago, Linear Technology revealed its LTC3783. According to the press materials, it’s a “current-mode multi-topology converter with constant-current PWM and dimming for driving high-power LED strings and clusters.” It could be used “as a boost, buck, buck-boost, SEPIC, or flyback converter, and as a constant-current/constant-voltage regulator.”

But wait a minute. Info on the part also says “ratios of 3000:1 can be achieved digitally as True Color PWM dimming guarantees color integrity of white and RGB LEDs. The LTC3783 allows an additional 100:1 dimming ratio using analog control.” Three-thousand-to-one dimming precision for LEDs? True-color PWM?

That seemed like a strange mix of functionalities. And, Linear doesn’t create products without strong market demand. So I arranged to talk to Tony Armstrong, the marketing guy for Linear’s power business unit, to find out who was asking for all those features.

The LTC3783 is an enhanced version of an earlier Linear chip, the LTC1871, which targeted telecommunications, networking, and industrial-control applications. The evolutionary processes that led to the LTC3783 began when a European customer looked at the LTC1871 and decided it wanted to use it for an LED fog lamp. Naturally, the customer wasn’t happy with the part exactly as it was.

The additional features the customer said it would like to see were the first indication of what automotive companies would like to see in follow-on products. As a result, while the LTC1871 is a boost-only controller, the new LTC3783 can operate as a boost controller, a buck controller, or a buck/boost controller. And it can operate in the buck/boost regime either using SEPIC topology or with only a single inductor thanks to some proprietary technology you can read about in the datasheet for another Linear part called the LTC3780.

The LTC3783 also incorporates soft-start and overvoltage protection, along with input current-limiting. In addition, the thermal resistance of the newer part’s package is three times better.

That initial interest by automotive companies in what had originally been a high-power regulator controller for telecom/industrial-control/robotics applications happened roughly three years ago. A year-and-a-half ago, it became apparent to Linear’s engineers that interior lighting also was a significant market, and that led to the 3000:1 dimming capability.

Such extremely precise LED-dimming capability, it turns out, was needed for dashboard instrument lighting. Long gone are the days of incandescent bulbs and rheostats. In many cars today, you set your desired dash-lighting intensity and the car maintains it automatically, compensating for ambient light conditions ranging from noonday July sunshine in a top-down convertible to a moonless night on a lonely secondary highway. At least that’s the goal.

Armstrong says cold-cathode fluorescents (CCFLs) primarily handle backlighting today. Yet by 2007 or 2008, most of the industry will have moved to LEDs.

Bright LEDs and judicious shading provide the ability to avoid washing out an electronic display under most circumstances short of somebody doing arc welding in the back seat. But maintaining the impression of a consistent dashboard-display brightness as the vehicle passes rapidly through changes in ambient conditions requires, yes, a good 3000:1 dimming capability. Achieving this is far more challenging than the 10:1 (or smaller) brightness ratio adjustments used in handheld devices. So for the automotive application, Linear’s LTC3783 is the follow-on for its LTC1768 and LTC1882, which were used with CCFLs.

“And what about the ‘True-Color PWM’ part?” I asked. (Linear has trademarked the term.)

“The part maintains constant current to the LEDs,” Armstrong explained. He says it’s particularly important in an RGB color matrix, where the eye interprets saturation levels differently at different luminance values.

So that’s the story behind the story, the things neither a press release nor a datasheet will tell you. But it’s the way things happen down in the trenches for electronic design engineers every day.

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