Common headlight driver module with low EMI: Page 4 of 7

August 26, 2018 //By Keith Szolusha, Analog Devices Inc.
Common headlight driver module with low EMI
Single 2MHz buck-boost controller drives entire LED headlight cluster, meets CISPR 25 class 5 EMI

The multi-beam LT8391A buck-boost LED driver in Figure 5 can drive LED string voltages ranging from 3V to 34V. This enables it to drive both a low beam string and create a high beam by adding LEDs to the low beam string. The same driver switches over and drives a higher voltage, yet lower current, DRL. Switching from low-beam-only LEDs to a low/high beam combo string generates no spike on the output voltage or LED current as shown in Figure 6a. The LT8391A can transition between boost, 4-switch buck-boost, and buck regions of operation smoothly. Changing from a small number of LEDs to a high number of LEDs without an LED spike can be challenging for a converter, but this multi-beam circuit does this with ease. Switching back from high and low beams to just low beams is also very clean, without any harmful LED spikes, as shown in Figure 6b.

Figure 5: LT8391A multi-beam LED headlight
cluster solution for low, high, and DRL lights


The same is true when switching to and from the DRL string. Figure 6c demonstrates how the low beam is turned off and the DRL is smoothly connected to the output capacitor. Even the LED current is changed from 1A (high and low beams) to 700mA (8 LEDs DRL) without any issues. Other trim or signal LEDs can be added in as well, and the DRL can be blinked as a signal light. Figure 6d shows how the DRL can be PWM dimmed with the internally set PWM generator and then switched over smoothly to low beams when darkness falls.

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