One low-EMI driver module for all headlight functions

August 29, 2018 // By Keith Szolusha, Analog Devices Inc.
Single 2MHz buck-boost controller drives entire LED headlight cluster, meets CISPR 25 class 5 EMI

Automobile LED headlight clusters combine high and low beams, daytime running lights, and sometimes signal and clearance lights into a single headlight cluster. The components of the cluster can have vastly different driver requirements, including voltage and current requirements, topologies, power levels or unique dimming functions. Meeting the range of requirements usually means employing separate driver solutions. Using multiple drivers not only complicates BOMs and production; it can make it difficult to meet EMI standards. Each additional driver adds its high frequency signals to the EMI mix, complicating EMI qualification, troubleshooting and mitigation.

Although the headlight cluster for each automobile make and model may be outfitted with a creative variety of LED currents and voltages, they commonly top out at 30W total. With that in mind, there should be a number of drivers that satisfy the power and feature requirements of every string in the cluster. There are not. Such a driver needs to take the relatively wide battery voltage range, and using a buck-boost topology, convert to the wide variety of string voltages. It needs to be small and versatile, to fit easily into the space constraints of the cluster, and produce little EMI, to minimize R&D efforts and eliminate the need for costly metal-shielded EMI cases. It should also be efficient. The Power by Linear LT8391A 2MHz buck-boost controller is unique in satisfying all of these requirements, making it possible to drive the entire headlight cluster, and more, with a single controller.

LT8391A 2MHz Synchronous Controller with Low EMI

The LT8391A is the first-of-its-kind 2MHz buck-boost controller for LED current regulation. The very high 2MHz switching speed enables the use of a single, small inductor and small overall solution size for high power LED applications. Unlike monolithic converters, whose power switches are contained within the IC package, controllers such as the LT8391A can drive external power switches with much higher peak currents, such as 10A. Such peak currents would burn up the small IC packages of typical integrated converters. In contrast, a controller with external 3mm × 3mm synchronous MOSFETs can deliver much higher power. These MOSFETs can be arranged in tight quarters with hot-loop capacitors for very low EMI. The unique peak switch current sense amplifier architecture places the sense resistor next to the power inductor, which is outside of the critical input and output hot loops—reducing EMI. Optional spread spectrum frequency modulation (SSFM) further reduces the controller’s EMI.

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