LED (Light Emitting Diode) is a solid-state semiconductor device that directly converts electricity into light. In recent years, its applications have expanded continuously, moving from traditional backlighting for portable devices to backlighting for medium and large-sized LCD displays/LCD TVs, automotive, and general lighting. This article will specifically introduce various LED driver solutions suitable for automotive lighting applications and discuss some typical applications.
1. LEDs offer numerous advantages for automotive lighting applications.
LEDs have a wide range of applications in automobiles, but they can be simply categorized into interior lighting and exterior lighting (see Figure 1). Exterior lighting includes headlights and taillights, while interior lighting includes interior convenience and comfort lighting and dashboard backlighting.
Many factors are driving the adoption of LED lighting in automobiles. Taking headlights as an example, using LEDs as the light source offers numerous advantages:
1.1 Facilitates flexible or novel styling designs
LEDs are physically very small, enabling the development of extremely compact and thin modules. Compared to halogen and xenon lamps, multi-string LED modules require smaller and simpler lens and light diffuser hardware. Furthermore, the light from multiple LED sources is easier to guide, significantly reducing the impact of engineering design on the overall shape.
1.2 Light intensity and energy efficiency are continuously improving.
LED luminous intensity is on an extremely rapid growth curve, showing a trend of doubling its luminous flux every 18 to 24 months. LED light output has already surpassed that of halogen lamps, and in the future, the actual luminous efficacy of LEDs will rival that of xenon lamps.
1.3 High reliability and long lifespan
As long as LED headlight modules are effectively thermally managed to maintain a low junction temperature and are protected from current spikes and battery pulses that may occur during switching on/off, dimming, and other processes, it is not unrealistic to expect them to be used continuously throughout the entire lifespan of the vehicle.
1.4 Significant energy savings
Compared to other solutions, LED headlights use significantly less fuel/energy. They are even more energy-efficient by using high-efficiency smart power technologies/chips, rather than general ICs requiring multiple external components.
Therefore, LEDs are increasingly used in automotive lighting and are being used as a design selling point by many car manufacturers.
2. Comparison of LED driver solutions for automotive lighting
LEDs have a wide variety of applications in automotive lighting, and different applications have different requirements for LED current. Therefore, it is necessary to select a suitable LED driver solution based on the specific application requirements.
A key function of LED driver solutions is to maintain current stability under a variety of operating conditions, regardless of input conditions or changes in forward voltage. Driver solutions must meet application requirements in terms of energy efficiency, form factor, cost, and safety. Simultaneously, the chosen solution must be easy to use and robust enough to withstand the demanding environments of specific applications.
Typical LED driver solutions in automotive applications include resistors, linear LED drivers, switching LED drivers, and innovative lighting management LED drivers. Generally, depending on the LED current required for the application, discrete components (i.e., resistors) or linear drive solutions can be used in low-current applications (20 to 200 mA); linear or switching drive solutions can be used in medium-current applications (200 to 500 mA); and switching drive solutions are generally chosen for high-current applications (greater than 500 mA).
In reality, resistors are the simplest and lowest-cost current-limiting solution for LEDs. However, they do not "stabilize the current"; they simply limit the maximum current of the LED when changes in the LED's forward voltage and input power supply voltage cause current changes, resulting in variations in LED brightness. Although this solution is low-cost, it also has the lowest energy efficiency and suffers from issues such as LED selection costs and thermal runaway.
CCRs offer higher performance than resistive solutions but at a lower cost than linear or switching drivers, making them suitable for low-current LED lighting applications with current less than 200 mA. CCRs are inexpensive and robust, providing constant brightness over a wide voltage range, protecting LEDs from overdrive at higher input voltages, and maintaining high brightness even at low battery voltages. This approach reduces or eliminates inventory from LED screening, resulting in a lower overall cost. CCRs operate up to 50 V and can withstand battery load drops. Available in SOD123, SOT123, and DPAK packages, CCRs can operate in harsh thermal environments (125°C), and their negative temperature coefficient protects both the device and the LED at high ambient temperatures. Furthermore, CCRs do not generate electromagnetic interference and are easy to design.
LEDs are a new type of highly efficient and environmentally friendly semiconductor light source with advantages unmatched by other light sources. They have a bright future in automotive lighting applications. LEDs can be combined into LED arrays in various ways, such as series and parallel connections, to meet the intensity requirements of automotive lighting. This paper focuses on the design and characteristics of LED drivers, considering their luminous characteristics, and briefly describes the current problems and solutions related to LEDs.
1. Functions and requirements of automotive lights
Currently, automobiles are becoming increasingly commonplace and have become a major means of transportation, making road safety a widespread concern. According to incomplete statistics, 25% of a car's total mileage is driven at night or in insufficient natural light, and 40% of traffic accidents occur during this period, with more than half of fatal accidents happening at night. Therefore, exterior lights and signal lights are crucial components for safe driving and must meet the following conditions:
(1) The car lights turn on without delay and have a faster response time, giving the driver more reaction time.
(2) The lighting is bright, which improves the driver's vision at night or when there is insufficient natural light, and at the same time makes the indicator function of the external signal lights stronger.
(3) High shock resistance and high operational reliability, avoiding accidents caused by lighting failure.
(4) Energy saving, which can effectively reduce the amount of exhaust gas emissions and protect the environment.
(5) Given the increasingly fierce competition in automobile sales, the design of car lights should be both practical and aesthetically pleasing.
2. LEDs are becoming a rising star in automotive lighting materials.
2.1 Working principle of LED
An LED is a special type of diode, a semiconductor device formed by doping and other methods to create a PN junction. When the diode conduction condition is met, current flows through the LED, releasing energy in the form of light and heat. An LED is a current-controlled device; its luminous intensity depends primarily on the magnitude of the current passing through it. During forward conduction, its voltage drop is very high and exhibits a certain fluctuation range.
Since LEDs do not emit infrared or ultraviolet radiation, almost all the energy they consume, besides being converted into light energy, is heat energy, which can only be dissipated through heat conduction. Therefore, the junction temperature of an LED gradually increases during operation. LEDs are devices with a negative temperature coefficient; the current flowing through an LED increases with temperature, creating positive feedback and causing a further increase in junction temperature. If left uncontrolled, this will burn out the LED. The thermal parameters of an LED are closely related to the junction temperature of the PN junction. The relationship between the dominant wavelength and temperature is as follows:
mp(Tl) = m0(T0)+ 3Tg #0.1nm/°C
As shown in the above formula, whenever the junction temperature of an LED increases by 10°C, the dominant wavelength (observable by the human eye) will shift by 1nm to a longer wavelength (1nm = 10⁻⁹m), resulting in a decrease in LED brightness and light decay. Therefore, overheating of individual LEDs will cause a deterioration in the uniformity of light emission from the LED array.
2.2 Significant Lighting Advantages of LEDs
LEDs are called a new light source because they possess the characteristics of both point light sources and solid-state light sources, thus having advantages that other lighting sources cannot match.
(1) Theoretically, the lifespan of LEDs can reach 100,000 hours, and the actual lifespan can reach more than 20,000 hours, which is more advantageous than the 1,000 hours of ordinary incandescent bulbs and the 10,000 hours of fluorescent lamps. They generally do not need to be replaced during the lifespan of a car.
(2) No delay in lighting up and faster response time. The startup time of LED is only tens of nanoseconds, which is much shorter than that of incandescent bulbs.
(3) It greatly reduces the incidence of car accidents when the light intensity is high and the natural light visibility is low; it is basically radiation-free and belongs to the "green light source".
(4) LEDs occupy a small volume, have a simple structure, and are highly shock resistant. Designers can freely change the lighting mode to diversify the car's styling and meet the needs of different consumers.
(5) LED light sources are far less affected by voltage changes than incandescent bulbs, demonstrating excellent safety and reliability. At the same time, they consume 80% less energy than incandescent bulbs with the same luminous efficacy, making them very energy-efficient.
Based on the above advantages, LEDs can be widely used in automotive lighting. However, a single LED cannot meet the requirements of automotive lighting intensity. Multiple LEDs must be connected in series, parallel, or series-parallel to form an LED array, as shown in Figure 1.
Figure 1 Lighting source composed of LEDs
2.3 Design and Features of LED Driver
LED driving methods can be categorized into three types: resistor current limiting, linear regulators, and switching converters. Resistor current limiting is suitable for low-efficiency applications, so it's not used in automotive lighting where extremely high efficiency and a wide input voltage range are required. Linear regulators are suitable for low-current applications or where the LED forward voltage drop is slightly lower than the power supply voltage, but they also suffer from efficiency and a limited input voltage range. Switching converters offer flexible circuit topologies, high efficiency, and a wide input voltage range. Therefore, considering factors such as operating efficiency, mounting size, quiescent current, operating voltage, noise, and output regulation, switching converters are commonly used in driving circuits. Switching converter topologies include Buck, Boost, and Buck-Boost types. Currently, in automotive lighting applications, the driving power source for LEDs is invariably a lead-acid battery.
Because the input voltage range of a battery can deviate significantly from the normal range, the driver circuit typically uses a Buck-Boost topology to meet the voltage requirements of the LED array. The DC gain (the ratio of output voltage to input voltage) of this topology is related to the duty cycle D (the ratio of on-time to the cycle period within one switching cycle). When the battery voltage is lower than the voltage required by the LED, D is adjusted > 0.5 to put the circuit in boost mode; when the battery voltage is higher than the voltage required by the LED, D is adjusted < 0.5 to put the circuit in buck mode. LEDs are current-controlled current-type devices, and their brightness is directly proportional to the current flowing through them. If the LED is not driven by a constant current, fluctuations in the current will cause changes in the LED's brightness, even if the voltage is constant. To ensure stable and reliable brightness, the LED requires a constant current to drive it, and the ripple current must be controlled to an acceptable level under all circumstances. Therefore, the output of the LED driver circuit must be a constant current output, not a constant voltage output.
The Buck-Boost circuit converts battery power to supply power to the LED array. A sampling circuit samples the current flowing through the LEDs and transmits the signal to the control circuit. The control circuit analyzes the sampled information and adjusts the duty cycle of the switching transistors in the Buck-Boost circuit to ensure a constant current through the LEDs. When a circuit malfunctions, the control protection circuit cuts off the power supply to prevent damage to the LEDs. Generally, LED driver circuits must meet the following requirements:
(1) Step-up and step-down function. When the input voltage or the voltage drop of the LED itself fluctuates, the output voltage is adjusted to meet the requirement of constant output current, ensuring stable and reliable LED light emission.
(2) High power conversion efficiency. This reduces drive losses, saves costs, and also reduces the number of battery charging cycles, extending battery life.
(3) Brightness adjustment function. When the surrounding environment is dark, the indicator light often does not require maximum current drive. At this time, the brightness of the LED can be changed by controlling the drive current, thereby reducing the power consumption of the LED. The common method for adjusting the drive current is to use PWM signal control.
(4) It has a complete protection circuit. Various protection measures should be set up to ensure the reliable operation of itself and the LED. For example, low voltage latch, overvoltage protection, overheat protection, output open circuit or short circuit protection, etc.
(5) Good heat dissipation function. As can be seen from the thermal characteristics of LEDs, temperature is one of the important factors affecting LED operation. When driving at night, LEDs are lit for a long time. Therefore, good heat dissipation function is necessary to ensure the lifespan and reliable operation of LEDs.
3. Defects of LEDs and their solutions
3.1 Problems caused by inconsistency
In theory, all LEDs are light-emitting diodes. However, due to differences in material purity, manufacturing processes, and packaging (which are difficult to avoid even within the same manufacturer), the performance of individual LEDs in an actual LED array varies. This results in inconsistencies in the luminous intensity and driving current of the LEDs, naturally leading to differences in overcurrent tolerance and heat generation. Because of these differences, one LED will inevitably fail first, causing an increase in current and damaging other LEDs. This is the result of inconsistency and one of the factors restricting its development. Therefore, manufacturers should improve LED quality to avoid significant variations and incorporate appropriate protection circuits into the driver circuit design to prevent these phenomena.
3.2 Problems with Complex Drive Circuits
First, regarding voltage matching, LEDs, unlike ordinary incandescent bulbs, cannot be directly connected to 220V AC mains. LEDs require a low voltage drive of 3.6 to 4.5V, necessitating the design of complex conversion circuits. Second, in terms of drive current, constant current and constant voltage circuits are required to ensure the normal operation of LEDs, along with additional protection circuits. This increases the complexity of the power supply circuit and the failure rate, significantly limiting market competitiveness and the customer base.
Therefore, dedicated driver chips should be used as much as possible during the design process to simplify the driver circuit structure and enhance the stability of the system. For example, chips such as the FAN5608 series, CAT4201, and LT3754 perform well in LED driving.
3.3 Cost Issues of LEDs
If all the lights in a car were replaced with LEDs, approximately 200 to 300 LEDs would be needed both inside and outside, making the cost considerably higher compared to other lighting sources. Although the long lifespan of LEDs can compensate for the high cost, overall, the cost is still significantly higher than other light sources. This is the main reason why LEDs, despite their superior performance, have not captured a large market share.
However, governments around the world attach great importance to LED technology and invest a lot of money in research every year. It is believed that this problem will be solved satisfactorily in the near future.
LEDs can be used in all parts of automotive lighting, from headlights to fog lights, instrument panel backlights to rearview mirror lighting, taillight lighting, interior lighting, etc. ON Semiconductor offers a range of standard lighting analog products, such as the NCV3065, and discrete devices such as the NSI45 series, as well as a range of products acquired from AMIS, such as ASSP, providing professional automotive-grade lighting solutions for different lighting subsystems.
With the development of the automotive industry, increasingly advanced lighting technologies are being used in automobiles. Among them, LED lighting has advantages that halogen lamps and xenon lamps cannot match, such as long lifespan, fast response time, energy saving and environmental protection, which has accelerated its widespread adoption in automotive lighting.
In response, ON Semiconductor offers a broad product portfolio for automotive headlight lighting applications, ranging from general bulb driver solutions to stepper drivers, LED drivers, and xenon drivers. The company continues to focus on providing a comprehensive range of solutions, including standard and custom products, for automotive lighting applications such as headlights, dashboard backlighting, interior lighting, door lighting, and taillights, particularly in the markets for dedicated ICs for xenon lamp drivers for headlight adjustment and deflection, and stepper drivers, which have become the de facto standard.
Features of various product types and overall solutions
ON Semiconductor linear regulators are designed for low-power circuit applications, such as 20mA LEDs. They have low total power consumption, can operate in environments up to 85 degrees Celsius, and have low system integration. For example, the NSI45 series CCR or NUD series can meet these requirements.
For switching regulators, the characteristics are relatively high LED power, reaching around 700mA, which is considered medium to high total power consumption. They can operate in environments up to 125 degrees Celsius. Their system integration is also relatively low, and they can be controlled using the NCV3056/66 series.
Lighting management integrated chips have relatively high power and can withstand ambient temperatures up to 125 degrees Celsius. They are complete and independent subsystems, so their integration is relatively high. NCV78663 and NCV7680 are designed for this type of application.
