Feature-rich LED driver IC enables high-power automotive LED headlights

The market for high brightness (HB) LEDs is expected to reach $20.2 billion by 2015 (Source: Strategies Unlimited). One of the key application areas driving this growth is LEDs used in automotive designs. Applications include headlights, daytime running and brake lights, instrument panel backlighting, various interior vanity lighting… and more. This amazing growth rate is not only due to the high reliability, low power consumption and more compact form factors of LEDs, but also because LEDs can be used to achieve innovative designs, such as steerable headlights and anti-glare dimming. Furthermore, in the automotive environment, all of these improvements must be optimized while withstanding the relatively harsh electrical and physical environment of the vehicle. Needless to say, these solutions must also be very flat, have a very compact footprint, and improve overall cost performance.

Although LEDs have been used for daytime running lights, brake lights, turn indicators, and interior lighting for several years, a few headlight-specific applications have only recently emerged, such as the Audi A8 and R8 and the Lexus LS600h. However, in the past year, several automakers have introduced LED headlights in their most popular models. In 2014, Toyota introduced LED headlights for its best-selling Toyota Corolla, as shown in Figure 1. In addition, Toyota has adopted LED headlights in several other models, and Honda, Nissan, Porsche, and Audi have also introduced LED headlights in most of their upcoming models. Many estimates put the LED headlight market size in 2014 at more than $3 billion.

Figure 1: 2014 Toyota Corolla LED headlights

One of the biggest challenges facing automotive lighting system designers is how to fully exploit all the advantages of the latest generation of HB LEDs. HB LEDs generally require an accurate and efficient DC current source and a dimming method. LED driver ICs must be designed to meet these requirements under a variety of conditions. Therefore, the power conversion solution must be very efficient and functionally reliable, while being very compact and cost-effective. Arguably, one of the most demanding applications for driving HB LEDs is automotive headlights, as these applications must withstand the harsh automotive electrical environment, must provide a lot of power (typically 50W to 75W), must be able to fit into thermally and space-constrained enclosures, and at the same time must maintain an attractive cost structure.

Car LED Headlights

Small size, extremely long life, low power consumption, strong dimming capabilities, etc. are the catalysts for the widespread adoption of HB LED headlights. A few years ago, most car manufacturers such as Audi, Daimler, Lexus, etc. began to use LEDs to design very distinctive running lights or "eyebrows" of headlights to highlight their brand differences. LED headlight designs reflect similar trends in Acura's jewel eye design, Porsche's 4-quadrant design, and countless new designs.

There are several positive effects of using LEDs for automotive headlights. First, such headlights never need to be replaced because their solid-state devices last more than 100,000 hours (11.5 years of use), which exceeds the life of the vehicle. Therefore, automakers can embed LEDs into headlight designs permanently without having to consider replacement. This allows for a wide range of headlight styles because LED lighting systems do not require the depth or area of ​​HID or halogen lamps. HB LEDs are also more efficient than halogen lamps (and will soon be higher than HID) in providing light output (in lumens) for input electrical power. This has two positive effects. First, LED lighting systems consume less electrical power from the vehicle bus, which is particularly important in electric and hybrid vehicles, and equally important, LED lighting systems reduce the heat that needs to be dissipated from the housing, eliminating the need for any bulky, expensive heat sinks. Finally, using an array of HB LEDs and electronically redirecting and dimming them allows the design of LED lighting systems to optimize lighting under many different driving conditions.

Design Parameters

To ensure optimal performance and long operating life, LEDs require an effective drive circuit. Such driver ICs must provide an accurate DC current source and tightly controlled LED voltage regulation regardless of the wide range of input power variations. The extreme voltages seen in the automotive battery bus range from 4.7V in cold crank/stop-start conditions to 60V in load dump conditions. More commonly, however, the battery bus voltage typically operates in the 9V to 16V range. Since many of these new LED headlight applications use 4 to 8 HB LEDs in series, the total voltage drop is 12V to 25V, and the battery bus voltage can range from 4.7V to 60V, so a buck-boost topology is required to power the LEDs because the input voltage may be higher, lower, or equal to the required LED string voltage. Second, the LED driver must provide a dimming method and provide a variety of protection features to prevent LED open or short circuit problems. In addition to operating reliably in the harsh electrical environment of the automotive power bus, the LED driver must also be cost-effective and small in size.

A New Synchronous Buck-Boost HB LED Driver

Fortunately, there is a new solution to the above dilemma, that is Linear Technology's LT3791 LED driver. The LT3791 is a synchronous buck-boost DC/DC LED driver and voltage controller that can provide more than 100W of LED power. Its 4.7V to 60V input voltage range makes the device ideal for a variety of applications, including HB LED headlights for cars, trucks and even avionics. Similarly, its output voltage can be set from 0V to 60V, allowing it to drive a variety of single-string LEDs.

Figure 2 shows a typical 50W headlight application. This application uses a single inductor to accurately regulate a 25V/2A LED string to provide 50W of LED power. This circuit provides a 50:1 PWM dimming ratio, which is a good match for anti-glare automatic dimming requirements. Input and output (LED) currents are monitored, and fault protection is provided to withstand and report open or shorted LED conditions.

Figure 2: 50W (25V, 2A) buck-boost LED driver with 50:1 dimming ratio and 98% efficiency

Its internal 4-switch buck-boost controller operates with input voltages above, below or equal to the output voltage, making it ideal for automotive applications. Transitions between buck, pass-through and boost modes of operation are seamless, providing well-regulated outputs despite wide variations in supply voltage. The LT3791's unique design employs three control loops that monitor input current, LED current and output voltage to provide optimal performance and reliability.

Using four external switching MOSFETs, the LT3791 can continuously deliver from 5W to over 100W of LED power at efficiencies up to 98%, see Figure 3. In conventionally powered vehicles, high efficiency is important because it minimizes the need for heat sinking, allowing for a very compact, low-profile solution footprint. In electric vehicles, this power savings also extends the range that the vehicle can travel between charges.

±6% LED current accuracy ensures constant brightness for each LED in an LED string, while ±2% output voltage accuracy can be used to provide several LED protection functions and also enables the converter to operate as a constant voltage source. The LT3791 can use analog or PWM dimming as required by the application. In addition, its switching frequency can be set from 200kHz to 700kHz or synchronized to an external clock. Other features include output disconnect, input and output current monitors, and integrated fault protection.

Figure 3: LED efficiency in Figure 2

in conclusion

The insatiable demand for the high performance and cost-effectiveness offered by LEDs is driving the rapid adoption of LED headlights in a variety of new vehicle models. New HB LED driver ICs are meeting this need. These new LED drivers must provide constant current, maintain consistent brightness regardless of changes in input voltage or LED forward voltage, while operating at high efficiency, providing a very wide range of dimming ratios, and providing a variety of protection features to improve system reliability. Of course, these LED driver circuits must also provide solutions with very compact footprints, low profiles, and high thermal efficiency. Linear Technology continues to redefine its LED driver portfolio to meet these challenges with HB LED driver ICs such as the LT3791. In addition, Linear Technology has developed a comprehensive family of high current LED driver ICs specifically targeted for automotive applications, including forward headlighting, LCD backlighting, and more. As automotive lighting systems continue to demand higher performance LED drivers, designers have innovative IC solutions to meet this need.