Given their excellent lighting characteristics as well as energy efficiency, high-power LEDs are becoming quite common for automotive exterior lighting applications. You could say that they are playing an integral role in safety, illuminating roadways with greater clarity and brightness and lighting up brake lights faster than their incandescent counterparts.
We are, indeed, seeing the use of LEDs in a variety of configurations in vehicles. There are single LEDs, LED strings, and LED matrixes, for instance. From an electronics standpoint, the components supporting the LEDs have to be fast, accurate, and efficient in order to control factors such as light intensity, direction, and focus. LED controllers, in particular, are integral in preserving and also enhancing the clarity, efficiency, and speed of LEDs.
When selecting the right LED controller for your automotive design, there are a variety of considerations to factor in. High-brightness (HB) LEDs, which need constant current for optimal performance, cannot be driven by voltage. The current correlates with junction temperature and, as a result, color. The power source for these LEDs can be anything from a 12V car battery to a 60V boost converter to support a long string. When relying on the car battery, however, keep in mind that vehicles with start/stop technology experience big battery voltage dips when the engine starts, so the battery voltage can drop to even 6V or lower.
An important safety feature for LED headlights is dimming. For example, Adaptive Front-Lighting Systems (AFS) that use LED matrixes can automatically dim the lights in response to the beams of an incoming car, so that the oncoming driver doesn’t get blinded by harsh lighting. The best dimming approach for LEDs is pulse-width modulation (PWM), where light intensity is modulated by time-slicing the current rather than changing the amplitude. This keeps the current constant to preserve color. To prevent the LED from flickering, the PWM frequency must be maintained above 200Hz. In PWM dimming, the limit to the minimum LED on/off time is the time required to ramp up or down the current in the switching regulator inductor. This could add up to tens of microseconds of response time—too slow for LED headlight cluster applications that need fast, complex dimming patterns. In this situation, dimming is only possible by individually switching on/off each LED in a string using dedicated MOSFET switches. The current control loop has to be fast enough, however, to recover quickly from the output voltage transient that stems from the switching in and out of the diodes.
LED controllers for these applications should support a wide input voltage range and provide a fast transient response. To reduce radio frequency interference and meet electromagnetic interference (EMI) standards, a high, well-controlled switching frequency outside of the AM band is required. A solution with high efficiency would reduce heat generation, which would enhance reliability of the LED light system.
Figure 2. Diagram of an advanced LED lighting system.
Sophisticated headlight systems rely on a boost converter to manage input voltage variabilities and EMI emissions. Typically, this is a boost converter that would provide a well-regulated, sufficiently high-output voltage. Using this stable input supply, dedicated buck converters can then manage the complexities of controlling the lamp’s intensity and position by allowing each buck converter to control a single function (high or low beams, daytime running lights, etc.). In the advanced LED lighting application depicted in Figure 2, there are two buck converters, and each of their main control loops sets the current in the LED string. There are also two secondary loops that implement the overvoltage and overcurrent protection. Maxim’s MAX20078 synchronous high-power buck LED driver is an example of a solution that meets the criteria we’ve highlighted for automotive lighting LED controllers. To learn more about why a synchronous solution is superior to a non-synchronous one and how the MAX20078 can support your automotive lighting designs, read the Design Solution, “Achieve Superior Automotive Exterior Lighting with a High-Power Buck LED Controller.”
Nazzareno (Reno) Rossetti, PhD EE at Maxim Integrated, is a seasoned analog and power management professional and a published author. He holds several patents in this field. He has a doctorate in electrical engineering from Politecnico di Torino, Italy.
Yin Wu, MBA, MSEE at Maxim Integrated, is a semiconductor business professional. He holds a master’s in business administration from Santa Clara University and a master’s in electrical engineering from San Jose State University.