Short-circuit protection is a concern for most headlamp manufacturers. The failure mode of each LED might be either short or open, and an outside factor such as removal of the LEDs while the lights are on or improper connection of the headlamp may lead to an open or short circuit at the output of the LED+ and LED- (or GND/chassis) terminals.

The SEPIC topology uses a second inductor and a coupling capacitor to provide a DC block for short-circuit protection from LED+ to LED- or LED+ to GND as an improvement to the boost topology in Figure 1. Although the boost topology is simpler, the LT3755 SEPIC shown in Figure 5 has similar efficiency as the boost and has the addition of short-circuit protection. The efficiency for the SEPIC ranges from about 88 percent to 91 percent over an input voltage range of 10 V to 40 V.

The short-circuit waveform shows how the converter maintains control of the inductor current, and thus switch current, during a shorted output. The switch current is the sum of the inductor currents during switch on time and the catch diode current is the sum of the inductor currents during switch off time. The ability to survive the harsh short-circuit condition makes the SEPIC topology particularly robust. The unique short-circuit detect circuitry inside the controller IC is able to distinguish between collapsed output voltage due to short circuit and that due to startup.

Similar to the boost, conducted EMI of the SEPIC is well controlled with a simple filter on the front end in Figure 6. The conducted EMI measurements are similar to the boost measurements and they also meet the CISPR 25 Class 5 standard.

Although the switching frequency of the controller IC is adjustable from 100 kHz to 1 MHz, 300 kHz to 400 kHz is the frequency range of choice for automotive applications, set to be as high as possible while remaining outside of the AM band. The main spike in the conducted EMI spectrum is understandably at the switching frequency of 350 kHz in this application.

Switching frequency also affects solution size, efficiency, ripple current, and thermals. Higher switching frequency results in smaller components and a lower-cost solution, but increases AC switching losses in the switch (M1) and catch diode (D1). Lower switching frequency returns more ripple on the inductor current and can increase the heat rise of the inductor if a larger inductor is not chosen to reduce ripple.

In this application, 350 kHz provides a balance of thermal management, efficiency, and small solution size. The choice of MOSFET is optimized for the 350-kHz application with a combination of low RDS(ON), a 100-V drain-to-source rating, high rise and fall times at 7-V, 1-A gate drive and low gate charge.

In a 350-kHz, high-voltage and high current (8-A+ peak switch current limit) switcher, the rise and fall time of the main power switch is just as important as the low RDS(ON) rating. The high power gate driver of the LT3755 and the low gate charge of the Si7454DP 100-V MOSFET (Si7850DP 60-V MOSFET for the boost) are a nice match for automotive headlight applications.

The boost and SEPIC 50-V/1-A LED drivers described here are high performance and robust solutions for powering automotive headlights. Both are small, easy to apply and include many built-in features required in headlamp applications. These two ap proaches give designers alternatives for optimizing their LED driver designs. While the boost circuit offers the smallest solution size, the SEPIC provides short-circuit protection.

Keith Szolusha is a senior applications engineer at Linear Technology in Milpitas, CA, with a BSEE and MSEE from MIT.