Automotive interior and exterior lighting LED driver solutions

LED (Light Emitting Diode) is a solid-state semiconductor device that can directly convert electricity into light. In recent years, its application areas have been continuously broadened, from traditional portable device backlight to medium and large-sized LCD display/LCD TV backlight, automotive and general lighting. This article will specifically introduce various LED driver solutions suitable for automotive lighting applications and explore some typical applications.

1 LEDs provide many advantages for automotive lighting applications

LEDs are widely used in automobiles, but can be simply categorized into interior lighting and exterior lighting (see Figure 1). Exterior lighting includes headlights, taillights, etc., while interior lighting includes interior convenience and comfort lighting and instrument panel backlighting.

Figure 1: Various automotive LED lighting applications

There are many factors driving the application of automotive LED lighting. Taking headlights as an example, using LED as a light source has many advantages:

1.1 Conducive to flexible or novel design

LEDs are physically small and can be used to develop extremely compact and thin modules. Compared with halogen and xenon lamps, multi-string LED modules require smaller and simpler lens and light diffuser hardware. And the light from multiple LED light sources is easier to guide, greatly reducing the impact of engineering design on styling.

1.2 Light intensity and energy efficiency continue to improve

LED light intensity is on a very fast improvement curve, showing a trend of doubling luminous flux every 18 to 24 months. LED light output has exceeded halogen lamps, and in the future, LED actual light performance will be comparable to xenon lamps.

1.3 High reliability and long life

As long as LED headlamp modules are thermally managed effectively to keep junction temperatures low and are protected from current spikes that may occur during switching on/off, dimming, and battery pulses, it is not unrealistic to expect them to last throughout the life of the vehicle.

1.4 Significant energy saving

LED headlights use much less fuel/energy than other solutions. Using highly efficient smart power technology/chips instead of general ICs that require multiple external components can save more energy.

Therefore, the application of LED in automotive lighting is increasing day by day, and is used as a design selling point by many automobile manufacturers.

2 Comparison of LED driver solutions for automotive lighting

LEDs are used in a variety of automotive lighting applications, and different applications have different requirements for LED currents. Therefore, it is necessary to select a suitable LED driver solution based on specific application requirements.

A key function of an LED driver solution is to maintain a constant current under a variety of operating conditions, regardless of input conditions and changes in forward voltage. The driver solution must meet application requirements in terms of energy efficiency, form factor, cost, and safety. At the same time, the selected solution must be easy to use and robust enough to adapt to the rigorous environment of a specific application.

Typical LED driver solutions in automotive applications include resistors, linear LED drivers, switch LED drivers, and innovative lighting management LED drivers. Generally speaking, depending on the size of the LED current in the application, discrete components (i.e. resistors) or linear driver solutions can be selected in low current applications of 20 to 200 mA; linear or switch driver solutions can be selected in medium current applications of 200 to 500 mA; and switch driver solutions are generally selected in high current applications greater than 500 mA.

In fact, resistors are the simplest and lowest cost LED current limiting solution, but they do not "stabilize the current". They simply limit the maximum LED current when the LED forward voltage changes and the input power supply voltage changes, causing the current to change and thus causing the LED brightness to vary. Although this solution is low cost, it is also the least energy efficient and has problems such as LED screening costs and thermal runaway.

CCR is a solution that has higher performance than resistor solutions but lower cost than linear drivers or switch drivers. It is suitable for low-current LED lighting applications with current less than 200 mA. CCR is cheap and rugged, provides constant brightness over a wide voltage range, protects LEDs from overdriving when the input voltage is high, and still provides high brightness under low battery voltage conditions. This solution can reduce or eliminate the inventory generated by LED screening, making the total cost of the solution lower. The maximum operating voltage of CCR is 50 V, and it can withstand battery load dump. CCR is available in different packages such as SOD123, SOT123 and DPAK, and can operate in strict thermal environments (125°C). The negative temperature coefficient characteristic protects the device itself and the LED under high ambient temperature conditions. In addition, CCR does not generate electromagnetic interference and is easy to design.

Figure 2: Two-terminal and three-terminal CCR circuit diagram

Linear regulators support parallel configuration of multiple lines to spread heat dissipation, provide up to ±2% current regulation accuracy, no electromagnetic interference (EMI) issues, medium cost, but low energy efficiency. Switching regulators are widely used due to their high energy efficiency and flexibility. This solution is more expensive and technically more complex, but it also provides significant advantages, such as supporting any type of input voltage and output voltage relationship, and energy efficiency can be higher than 90% depending on input/output conditions. Unlike linear drivers, they are sensitive to EMI, which brings design constraints that designers need to pay attention to.

It is worth mentioning that in addition to these common LED driver solutions, ON Semiconductor has launched highly integrated LED lighting management integrated circuits (ICs). These lighting management integrates a variety of LED driving and control functions, equivalent to a complete subsystem, capable of withstanding ambient temperatures up to 125°C, and are used in automotive headlights, combination taillights, and the latest advanced front lighting system (AFS) and other applications, which will be discussed in detail later.

3 Typical automotive LED driver products and their applications

3.1 CCR for Low Current LED Applications

ON Semiconductor has developed the NSI45 series of CCRs using patented self-biased transistor (SBT) technology (see Figure 2). This series of CCRs includes two types: two-terminal fixed output and three-terminal adjustable output, with current levels ranging from 10 to 180 mA. The three-terminal adjustable output CCR includes different options such as 30 to 80 mA (SOT-223) and 60 to 150 mA (DPAK), which can be used to meet specific current setting requirements.

CCR is easy to design and is suitable for high-side and low-side applications. A single CCR can drive one or more strings of LEDs, or multiple CCRs can be connected in parallel to drive one or more strings of LEDs (see Reference 1 for details).

Figure 3: CCR automotive center high-mounted stop lamp (CHMSL) application circuit diagram

3.2 Linear LED Drivers for Backlight, Interior Lighting Control and Tail Light Applications

In the linear driver solution, typical products include ON Semiconductor's NUD4001 and NUD4011 linear LED drivers. The rated input voltage ranges of the two devices are 6 to 30 V and 48 to 200 V, respectively, and the rated output currents are up to 500 mA and up to 70 mA, respectively. The application areas in automobiles include taillights, turn signals, brake lights, and roof lights. Both devices are designed to replace discrete solutions in AC-DC applications, and designers can use external resistors to set the drive current for different LED arrays. The difference is that the NUD4001 is suitable for low-voltage applications of 5 V, 12 V, or 24 V, while the NUD4011 supports a maximum voltage of 200 V. Figure 4 shows the application of NUD4001 in automotive parking lights.

Figure 4: Example of NUD4001 application in car parking lights

In high-brightness LED automotive lighting applications that require driving high current, ON Semiconductor's CAT4101 high-brightness linear LED driver can be used. The CAT4101 can drive a string of high-brightness LEDs with a current of up to 1 A, and the voltage drop at full load is only an extremely low 0.5 V. This simple solution does not require an inductor, eliminates switching noise, minimizes the number of components, and simplifies the design.

3.3 LED switch drivers with different configurations including buck, boost and buck-boost

The energy efficiency of the switching regulator driving LED is generally between 50% and 90%, involving different topologies such as buck, boost and buck-boost, and paying attention to EMI control methods. It is mainly used in applications such as automotive interior lighting, parking lights, fog lights, daytime running lights and side marker lights. ON Semiconductor's switching regulator products used in automotive LED lighting are shown in Table 1.

Among them, a typical boost controller is NCV8871. This is a boost controller with a multi-function enable/synchronization pin that can be synchronized with an external clock to drive an external N-channel MOSFET. This device can operate under a wide battery voltage range of 3.5 V to 40 V, is specifically designed for -40°C to 150°C junction temperature, provides ±2.0% output voltage accuracy under operating temperature conditions, and provides low shutdown current (typically less than 10 μA) to help reduce power consumption.

In terms of buck switch drivers, products such as NCV8842, NCV8843 and NCV51411 are suitable for applications such as body/telematics systems. NCV8842/3 is a 1.5 A, 170/340 kHz buck driver with synchronization function, supporting 4.5 V to 40 V input voltage, and the output voltage can be programmed from 1.27 V. The synchronization pin function is used to improve EMI performance. NCV5141x (such as NCV51411 and NCV51413) is a version of CS5141x for automotive applications, providing high energy efficiency. Under 9 to 15 V input voltage conditions, the energy efficiency of driving 2 LEDs with a forward voltage drop of 3.5 V at 700 mA is basically higher than 85%. Another automotive-grade synchronous buck controller that supports 4.5 to 45 V input voltage even provides more than 93% energy efficiency under wide load current conditions, and the maximum quiescent current in sleep mode is only 1 μA.

Table 1: ON Semiconductor switching regulator products for automotive LED lighting

In addition, ON Semiconductor also provides a variety of multi-topology switch drivers that can be used for automotive LED lighting, including NCV33063AV, NCV3065 and NCV3163. Among them, NCV33063AV and NCV3065 both support a maximum current of 1.5 A and can be configured as buck, boost, SEPIC or buck-boost topologies, which are suitable for automotive interior lighting applications. NCV3065 can also be used for interior lighting, targeting applications that are very cost-sensitive, and can be configured as a controller if higher current is required. NCV3163 is a monolithic switching regulator with a maximum current of 3.4 A, allowing system designers to apply buck, boost or buck-boost (voltage reversal) topologies with a minimum number of external components.

3.4 Highly integrated lighting management ICs for headlights and combination taillights

Automobile headlights include different sub-areas, such as daytime running lights (DRL), low and high beams, signal lights and fog lights. The earliest headlights to use LEDs were the daytime running lights. As the cost of high-brightness LEDs continues to decrease and the performance continues to improve, more and more new car designs are using LEDs more widely in other headlight areas, such as low and high beams, signal lights and fog lights, and even using LEDs to provide emerging functions such as beam shaping and road spot.

Generally speaking, automotive headlights require support from a single LED to multiple strings of LEDs, with voltages up to 60 V; in addition, they also require support for pulse width modulation (PWM) dimming (such as for clearance lights), high integration, high energy efficiency, low electromagnetic radiation from LED strings, integrated diagnostics and communication interfaces, etc. ON Semiconductor has developed the NCV78663 power ballast and dual LED driver for LED automotive headlight systems. The NCV78663 is an advanced, highly reliable, and energy-efficient system-on-chip (SoC) LED driver that supports currents up to 2 A, provides a total energy efficiency of more than 90%, drives multiple strings of LEDs up to 60 V, can maintain color temperature and control average current with PWM dimming, and enables designers to control high and low beams, daytime running lights, turn indicators, and fog lights with a highly integrated SoC, with minimal external components required. The device communicates with an external microcontroller via an SPI interface, dynamically controls system parameters, detects LED operating status, and feeds back diagnostic information after power-on. Figure 5 shows a circuit diagram of the NCV78663 in an advanced automotive headlight application.

Figure 5: a) Automotive LED headlight application example; b) Automotive LED headlight application circuit diagram based on NCV78663

For automotive taillight applications, low-cost, low-current LED drive solutions such as CCR can be used, depending on the application requirements, or highly integrated solutions that provide higher performance and higher energy efficiency levels can be used, such as ON Semiconductor's highly integrated linear current regulator and controller NCV7680. This device contains 8-way linear programmable constant current sources, which are designed for current regulation and control of automotive combination taillights, and can output up to 75 mA of LED drive current per channel. Due to the high degree of functional integration, NCV7680 enables engineers to easily achieve two brightness levels, one for parking and the other for tail lighting. If necessary, optional PWM control can also be applied. Designers only need one external resistor to set the output current of all channels. The operating temperature range is 40°C to +150°C.

Figure 6: a) LED combination taillight example; b) NCV7680 application circuit diagram in LED combination taillight

4 Conclusion

As LED performance continues to improve and costs continue to decrease, its application areas in automotive interior and exterior lighting are becoming wider and wider. Designers need to choose suitable LED driver solutions for specific applications, such as linear constant current stabilizers, linear voltage regulators or switching regulators. As the leading supplier of high-performance silicon solutions for energy-efficient electronic products, ON Semiconductor provides a complete LED driver product line for automotive lighting that is AEC Q100 certified and covers low current to high current, including a recently launched series of highly integrated, energy-efficient lighting management integrated circuits, which facilitates designers to choose suitable automotive LED driver solutions according to specific application requirements.