One low-EMI driver module for all headlight functions: Page 3 of 7

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

CISPR 25 EMI for Automotive Applications

The 2MHz LT8391A LED driver in Figure 1 is designed for automotive headlights. It uses AEC-Q100 components and meets CISPR 25 Class 5 radiated EMI standards. Spread spectrum frequency modulation (SSFM) reduces EMI, and also runs flicker-free simultaneously with PWM dimming as shown in Figure 7. Its small size is highlighted by its small inductor and especially small input and output EMI filters. Large LC filters are not needed for 2MHz converters and only small ferrite beads are used for high frequency EMI reduction.

Automotive EMI requirements are not easily met by high power converters. High power switches and inductors, placed on large PCBs next to large capacitors can create undesirable hot loops, especially when a large sense resistor is included. The unique LT8391A buck-boost architecture removes the sense resistor from both the buck and boost switch-pair hot loops, enabling low EMI.

Figures 3 and 4 show measured EMI of the 24W LED driver of Figure 1. Despite this controller’s 2MHz operating frequency and 24W of power, this buck-boost passes CISPR 25 Class 5 radiated and conducted EMI. Class 5 is the most stringent requirement and the goal of most automotive EMI testing. Converters that cannot pass Class 5 EMI either get designed out of automotive circuits or must be encased in large metallic EMI shields. Even if the bulkiness of the shield does not create assembly issues, adding them is costly.


Figure 3: LT8391A demonstration circuit DC2575A passes
CISPR 25 Class 5 automotive radiated EMI

Figure 4: LT8391A demonstration circuit DC2575A passes
CISPR 25 Class 5 automotive conducted EMI


Buck-Boost for Multi-Beam Applications

LED headlight clusters can be both innovative and artistically creative. High beams and low beams can be wrapped up with nifty and distinctive daytime running lights (DRL). Because the daytime running lights are only needed when high and low beams are off, a single LED driver can be used to power either the high and low beam LEDs or the daytime running lights. This only works if the LED driver has a flexible input-to-output ratio and can both step-up and step-down the input-to-output voltage. A buck-boost design satisfies this requirement.

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