The need to provide higher performance and higher cost performance for LEDs has never been met, prompting the rapid adoption of LED headlamps for a variety of new models. The new LED driver IC meets these needs. It has constant current, high efficiency, wide dimming ratio range, and various protection functions to achieve system reliability. In the solution proposed in this paper, the LED driver circuit footprint is very compact, flat and highly efficient.

By 2015, the market for high-brightness (HB) LEDs is expected to reach $20.2 billion (source: Strategies Unlimited). One of the key application areas driving this growth is the LEDs used in automotive design, including headlights, daytime running lights, brake lights, instrument panel backlighting, and various interior dressing lighting. This amazing growth rate is not only due to the high reliability, low power consumption and compact form factor of LEDs, but also because LEDs can be used to achieve innovative designs such as steerable headlights and anti-glare dimming. In the automotive environment, all of these improvements must be optimized while still withstanding the rigors of automotive electrical and physical environments. Needless to say, these solutions must also be very flat, with a very compact footprint and an overall price/performance ratio.

Although LEDs have been used for daytime running lights, brake lights, turn indicators and interior lighting for many years, a few headlight-specific applications have only recently appeared, such as those used in the Audi A8 and R8 and the Lexus LS600h. However, in the past year, a number of car manufacturers have introduced LED headlights in their most popular models. In 2014, Toyota introduced LED headlights for its best-selling Corolla models. In addition, several other Toyota models also used LED headlights. Honda, Nissan, Porsche and Audi also introduced LED headlights in most of the upcoming models. Many people estimate that the LED headlamp market will exceed $3 billion in 2014.

One of the biggest challenges facing automotive lighting system designers is how to take advantage of all the benefits of the latest generation of HB LEDs. HB LEDs typically require an accurate and efficient DC current source and a dimming method. LED driver IC designs must be able to meet these requirements under a variety of conditions. Therefore, power conversion solutions must be very efficient and reliable, and must be very compact and cost effective. It can be said that one of the most demanding applications for driving HB LEDs is automotive headlamp applications, because such applications must withstand the rigors of automotive electrical environments and must provide significant power (generally 50W~75W) must be able to fit into a housing with limited heat and space, while maintaining an attractive cost structure.

Car LED headlights

The advantages of small size, long life, low power consumption, and powerful dimming capability are the catalysts widely used in HB LED headlamps. A few years ago, automakers such as Audi, Daimler, and Lexus began to use LEDs to design a very distinctive "eyebrow" of running lights or headlights to highlight the brand's distinctiveness. The LED headlamp design reflects a similar trend in Acura's gemstone design, Porsche's 4-quadrant design and countless emerging designs.

There are several positive effects of using LEDs for automotive headlights. First, the headlights do not 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. As a result, automakers can permanently embed LEDs into headlamp designs without having to consider replacement issues. This allows the headlamp style to vary greatly, as the LED lighting system does not require the depth or area of ​​a xenon lamp (HID) or halogen lamp. HB LEDs are also more efficient than halogen lamps (and will soon be higher than HID) when providing light output (in lumens) by input electrical power. This has two positive effects. First, LED lighting systems consume less power from the car's bus, which is especially important in electric and hybrid vehicles, and equally important, LED lighting systems reduce the amount of heat that needs to be dissipated from the casing, eliminating the need for any bulkiness. Expensive heat sink. Finally, by using HB LED arrays and electronically changing their direction or dimming them, such a solution can be designed to optimize illumination under many different driving conditions.

Design Parameters

To ensure optimum performance and long operating life, LEDs require an efficient drive circuit. This type of driver IC must provide an accurate DC current source and provide tightly controlled LED voltage regulation regardless of the range of input power supply variations. The extreme voltage range seen in automotive battery buses is 4.7V in cold-start/start-stop situations to 60V in load dump conditions. More commonly, the battery bus voltage typically operates at 9V~16V. Many of these new LED headlamp applications use 4 to 8 series HB LEDs with a voltage drop of 12V to 25V, and the battery bus voltage range may be 4.7V to 60V, so a buck-boost topology is required to power the LEDs. The input voltage may be higher, lower or equal to the desired 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 reliable operation in automotive electrical bus lines where electrical environments are critical, LED drivers must also be cost-effective and take up less space.

A new synchronous buck-boost HB LED driver

Linear Technology's LT3791 LED driver can handle these dilemmas. The LT3791 is a synchronous buck-boost DC/DC LED driver and voltage controller that delivers over 100W of LED power. Its 4.7V~60V input voltage range makes the device ideal for a wide range of applications, including HB LED headlights for automobiles, trucks, and even avionics. Similarly, its output voltage can be set from 0V to 60V, enabling it to drive multiple LEDs in a single string.

Figure 1 shows a typical 50W headlamp application. The application uses a single inductor to accurately adjust the 25V/2ALED string to provide 50W of LED power. This circuit provides a 50:1 PWM dimming ratio that is well suited to anti-glare auto dimming requirements. The input and output LED currents are monitored while providing fault protection to withstand and report open or shorted LED conditions.

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

Its internal 4-switch buck-boost controller can operate at input voltages above, below or equal to the output voltage, making it ideal for automotive applications. The transition between buck, pass-through, and boost modes of operation is seamless, providing a good, stable output despite a wide range of supply voltage variations. The LT3791's unique design uses three control loops to monitor input current, LED current, and output voltage for optimum performance and reliability.

The LT3791 uses four external switching MOSFETs to continuously deliver LED power from 5W up to over 100W with up to 98% efficiency. See Figure 2. In a conventionally powered vehicle, high efficiency is important to minimize heat dissipation. The need for measures to achieve a very compact flat solution with a small footprint. In electric vehicles, this power savings also extends the mileage that the vehicle can travel between charges.

Figure 2: LED efficiency in Figure 1

±6% LED current accuracy ensures constant brightness for each LED in a string, while ±2% output voltage accuracy can be used to provide several LED protection functions, while also allowing the converter to be constant The voltage source works. The LT3791 can be used with 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 monitoring, and integrated fault protection.

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