LED Technology


Single 2 MHz Buck-Boost controller drives entire LED headlight cluster, meets CISPR 25 Class 5 EMI


By Keith Szolusha, LED drivers applications manager at Analog Devices


A


utomobile 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 this 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 30 W 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 it must 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 2 MHz 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 2 MHz Synchronous Controller with Low EMI The LT8391A is the first of its kind—a 2 MHz buck-boost controller for LED current regulation. The very high 2 MHz 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 10 A. Such peak currents would burn up the small IC packages of typical integrated converters. In contrast, a controller with external 3 mm × 3 mm 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. The 2 MHz LT8391A 16 V, 1.5 A (24 W) buck-boost LED driver in Figure 1 boasts as


Figure 2. Efficiency of LED driver solution in Figure 1. Measurements made using 16 V, 1.5 A, demonstration circuit DC2575A LED driver with and without optional EMI components


high as 93 percent efficiency with EMI filters and gate resistors, as shown in Figure 2. Efficiency is 1 per cent higher with the optional EMI components removed. With small 3 mm × 3 mm MOSFETs and a single high power inductor, the temperature rise for this converter is low, even at 24 W. At 12 V input, no component rises more than 25ºC above room temperature. At 6 V input, the hottest component rises less than 50ºC with a standard 4-layer PCB and no heat sink or airflow. It continues to run at full 24 W load in the face of input transients down to 4.3 V; or reduced load current via analog or PWM dimming when the input drops for long periods. The 8 A to 10 A sense resistor makes this high power at low VIN possible. The LT8391A includes the latest PWM dimming features and open LED fault protection. This synchronous buck-boost regulates current through a string of LEDs with a voltage that may or may not lie within the input voltage range, such as a 9 V to 16 V car battery or a truck battery (18 V to 32 V). It can run down to 4.0 V cold crank input and can withstand up to 60 V input transients. The LT8391A provides up to 2000:1 PWM dimming ratio at 120 Hz and can use its internal PWM dimming generator for up to 128:1 accurate dimming ratio without the need for an externally supplied PWM clock.


Figure 1. LT8391A 2 MHz 16 V, 1.5 A automotive buck-boost LED driver passes CISPR 25 Class 5 EMI www.cieonline.co.uk


CISPR 25 EMI for automotive applications The 2 MHz 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.


Components in Electronics February 2019 35


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