Development trend of automotive LED and matrix full LED design solution

In recent years, with many advantages such as increased light efficiency, low energy consumption, high reliability, long life, small size and environmental protection, the application of LED in automotive interior and exterior lighting has increased day by day, and has crossed from the initial less important automotive lighting applications, such as cabin lighting, parking lights and instrument panel backlights, to wider applications such as headlights and combination taillights. In particular, due to its small size, LED can be combined with a variety of shapes and lines to help improve the recognition of car lights. It is designated for the headlight system of many mid-to-high-end cars with beautiful exterior design. Figure 1 shows a typical automotive LED lighting application today.

Figure 1: Typical automotive LED lighting applications

Automotive LED lighting - not only beautiful, but also safe

The intuitive benefits that LED lighting brings to cars are not limited to beautiful shapes. According to statistics from the National Highway Traffic Safety Administration (NHTSA) of the United States and the European Commission (EC), although only 25% of driving occurs at night and in low light conditions, 40% of deaths and serious injuries occur during this period. Therefore, improving car lighting, especially lighting at night and in low light conditions, will help improve the active safety of cars. In fact, in order to protect the safety of drivers/occupants/pedestrians on the roadside, the industry has long been committed to developing various automotive lighting solutions, such as adaptive front lighting (AFS) solutions for improving lighting when turning at night, and daytime running lights (DRL) solutions for improving daytime driving safety.

Compared with the traditional incandescent lamps and high-intensity discharge lamps (HID) widely used in automotive lighting, LEDs have incomparable advantages in automotive lighting. For example, LEDs have a short response time, and can increase the braking distance of the following vehicles when used in brake lights, and have a better warning effect when used in turn signals. LEDs are bright, but not as glaring as HIDs, which helps reduce the risk of dazzle for drivers of oncoming vehicles. LED lamps consume much less energy than incandescent lamps or HIDs, which helps reduce fuel consumption and save expenses.

Typical LED driver solutions for automotive lighting applications

Different automotive lighting applications have different requirements for LED current, so it is necessary to select a suitable LED driver solution based on specific application requirements. Typical LED driver solutions include resistors, linear constant current regulators, linear voltage regulators, and switching regulators.

Among them, resistors are the simplest and lowest cost LED current limiting solution, but they are also the least efficient, and there are problems such as LED screening costs and thermal runaway. Constant current regulators (CCRs) have better performance than resistor solutions, but are lower cost than linear or switching regulator solutions, and are suitable for low current LED lighting applications. Linear regulators support multiple lines in parallel configuration to help dissipate heat, provide up to ±2% current stabilization accuracy, no electromagnetic interference (EMI) problems, medium cost, but also low efficiency. Switching regulators are widely used. This solution is more expensive and more technically complex, but supports any type of input voltage and output voltage relationship, and depending on the input/output conditions, the efficiency can be higher than 90%, but there are EMI problems.

Figure 2: Typical automotive lighting applications and LED driver solutions

In addition to these solutions, ON Semiconductor has also launched highly integrated LED lighting management integrated circuits (LMICs). These LMICs integrate a variety of LED drive and control functions, equivalent to complete subsystems, capable of withstanding ambient temperatures up to 125°C, and are used in automotive headlights, combination taillights, and the latest AFS applications.

Full LED headlamp application requirements and highly integrated drive solutions

In 2008, Audi R8 was the first car in the world to adopt full LED headlights. These full LED headlights include low beam, high beam, turn signal and daytime running light modules, each containing a different number of LEDs. According to data from SNE Research, a research and consulting service company, the penetration rate of LED headlights in the global automotive market was less than 5% in 2013, but it is expected to exceed 50% by 2020, which shows that the growth prospects are very impressive.

However, full LED headlights place higher demands on the driving solution, requiring energy-efficient integrated drivers that support different configurations from single LED to multiple LED strings (voltage up to 60 V), and pulse width modulation (PWM) dimming, such as for clearance lights. Full LED headlights also require low EMC radiation from the LED string, and have requirements for heat dissipation, diagnostics, and communication interfaces.

Figure 3: Application circuit diagram In line with the demand for full LED headlight driving, ON Semiconductor has launched the NCV78663 single-chip high-efficiency smart power ballast and dual LED driver system-on-chip (SoC) for advanced LED headlight systems. The NCV78663 uses a buck-boost topology to provide more than 90% total energy efficiency. It is a highly integrated solution that enables designers to control high and low beams, daytime running lights, turn indicators and fog lights with a single SoC. The NCV78663 is very suitable for driving high-current LEDs (current up to 2 A) and supports PWM dimming to maintain LED color temperature and average current control. The NCV78663 provides a complete driving solution for two strings of LEDs with a driving voltage of up to 60 V with two built-in independent buck switch channels with a very small number of external components. Each channel can customize the output current and voltage through the SPI interface and/or OTP settings according to application requirements. The device provides automotive headlamp diagnostics on-chip and also integrates a boost controller, providing designers with a unique input current filter with a limited number of external components. The NCV78663 can be used either standalone or in conjunction with a microcontroller, providing great flexibility. This solution has low EMC from the battery and low EMC radiated to the LED string.

ON Semiconductor's NCV78663 full-LED headlamp driver has been adopted by the Mercedes-Benz E-Class, with each vehicle using up to six NCV78663s and up to three ON Semiconductor NCV70522 stepper motor drivers in its advanced headlamp system.

ON Semiconductor's latest matrix dynamic intelligent full LED headlight solution

In recent years, matrix dynamic intelligent full LED headlights have appeared on the market, such as the intelligent LED headlights equipped in the new generation of Mercedes-Benz S-Class cars. This headlight system contains 56 LEDs, each of which can be lit, turned off or adjusted in brightness. The unique functions of this light are as follows:

● Preventing oncoming vehicles from being dazzled: The LED lights up, turns off, or adjusts the brightness based on the data from the front radar and stereo camera, automatically adjusting the illumination range to ensure your own line of sight while avoiding dazzling oncoming vehicles.

● Avoid the outline of the vehicle in front: When encountering a vehicle traveling in the same direction in front, you can avoid the outline of the vehicle in front and fully illuminate the left and right areas of the vehicle in front. Therefore, you can always turn on the high beam when driving at night, greatly improving driving safety.

● Flash quickly when encountering pedestrians: Smart headlights can identify pedestrians in front and automatically flash the headlights quickly to remind pedestrians to avoid danger and reduce the possibility of accidents at night.

Figure 4: Illustration of the unique functions of the dynamic intelligent full LED headlight

Such matrix dynamic intelligent full LED headlights can adopt a series or parallel drive structure. When using a parallel structure, the differences in the electrical characteristics of each LED have a significant impact on the performance of the lighting system, resulting in increased energy consumption and heat dissipation problems. In the series drive structure, the LED driver provides a constant current source, and the short-circuit switch can turn off a single LED, so that the light beam can be changed as needed. If a companion chip (pixel controller) is added to the series drive circuit, through system partitioning, it is possible to avoid the energy consumption and thermal management problems inherent in the parallel topology.

Figure 5a: Schematic diagram of ON Semiconductor's matrix automotive LED headlight solution

Figure 5b: Schematic diagram of ON Semiconductor's matrix automotive LED headlight solution (continued)

ON Semiconductor's NCV78763 buck-boost LED driver IC acts as a current source in this matrix dynamic intelligent full LED headlamp, which complements the integrated pixel controller/companion chip. This modular approach reduces the number of components and simplifies the application process, thereby accelerating the product's time to market.