One low-EMI driver module for all headlight functions: Page 2 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

The 2MHz LT8391A 16V, 1.5A (24W) buck-boost LED driver in Figure 1 boasts as high as 93% efficiency with EMI filters and gate resistors as shown in Figure 2. Efficiency is 1%–2% higher with the optional EMI components removed. With small 3mm × 3mm MOSFETs and a single high power inductor, the temperature rise for this converter is low, even at 24W. At 12V input, no component rises more than 25ºC above room temperature. At 6V 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 24W load in the face of input transients down to 4.3V; or reduced load current via analog or PWM dimming when the input drops for long periods. The 8A–10A sense resistor makes this high power at low VIN possible.

Figure 1: LT8391A 2MHz 16V, 1.5A automotive buck-boost
LED driver passes CISPR 25 Class 5 EMI


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

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 the 9V–16V car battery or a truck battery (18V–32V). It can run down to 4.0V cold crank input and can withstand up to 60V input transients. The LT8391A provides up to 2000:1 PWM dimming ratio at 120Hz 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.

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