Even automotive-grade LEDs and LED device drivers may need extra protection when used in the harsh electrical environment of exterior car lighting applications. High reliability is paramount, especially as forward and rear-facing lights play an important role in new vehicle safety features that may result in autonomous control of some aspects in a critical situation.
This article will consider how LED driver ICs can be protected against short circuits and other fault conditions with additional circuitry consisting of more highly integrated devices that incorporate diagnostic and protection features. Reference will be made to automotive-grade LED drivers from Linear Technology, and current sense monitors from TI.
Adaptive lighting
Virtually any new model of car on the market today will contain LEDs, for exterior or interior lighting, or both. It’s not only their low power, long life and high efficiency that appeals; nor is it solely their flexible form factor that has brought ‘concept car’ style lighting clusters and bodywork design to the family runaround. The ability to integrate LEDs easily with sensors and electronic controls is one of the most compelling reasons. It is key to advanced driver comfort and safety features, such as adaptive lighting, automatic dipping, object detection and parking aids.
For adaptive lighting multiple LEDs are integrated into a cluster or matrix such that individual LEDs can be turned on and off or dimmed to produce the light output appropriate to conditions. External sensors on the car detect not only changing ambient light levels due to weather conditions or nightfall, but also physical characteristics such as corners and oncoming headlights, so that the beam width and projection can be adjusted automatically (Figure 1).
Figure 1: Audi’s Matrix LED headlight demonstrates its adaptive capability, avoiding blinding oncoming drivers while continuing to light the road. Around 25 individually controllable LEDs are used in each headlamp unit.
Automotive exterior lights are considered safety devices, and are therefore highly regulated. Although more mechanically rugged than the bulbs they replace, LEDs still have to meet tough automotive standards to ensure reliability and safety. Initially, LEDs had to comply with the AEC-Q101 standard (Stress Test Qualification for Automotive Grade Discrete Semiconductors), but now new, more appropriate regulations are evolving for LEDs.
For sure, the essential associated electronic control, power management and circuit protection devices in automotive lighting applications must meet either AEC-Q100 (for integrated circuits), AEC-Q101 (discrete components) or AEC-Q200 (passives). Automotive-qualified electronic devices need to demonstrate reliable operation over extended temperature and humidity ranges, and the ability to withstand vibration and harsh chemicals, as well as survive transients and EMI. In a forward lighting application, for example, such as headlamps or daytime running lights (DLRs), LEDs and drivers will typically be operating at high power levels, up to 75 W in a matrix design, making EMI a particular challenge.
Exterior lighting failures can be catastrophic, whether due to mechanical or electrical problems. Not only are exterior lights essential safety features when driving at night, but they may also be integrated with other safety functions. In addition, short circuits may create electrical arcing, which in the case of a collision, could ignite leaking fuel. Clearly, high-reliability LEDs and drivers are essential, and compliance with the relevant AEC quality standard is a start. However, careful selection of the LED drivers is important and additional protection against electrical short circuit, under and overvoltage, overcurrent and reverse polarity is worth strong consideration.
Topology choices
For reasonably high-power applications, such as automotive forward lighting, a switching regulator is generally used. LEDs typically require constant current to produce consistent light output; an LED driver must be able to vary output voltage to maintain a constant current. Total output voltage will depend on many parameters, such as the LED process and die temperature, and the number of LEDs in series. The designer needs to accurately predict maximum output voltage in order to select the optimum regulator topology, and hence the driver IC and associated components.
A useful Application Note is available from Texas Instruments, Design Challenges of Switching LED Drivers1. It warns that buck regulators, even if capable of a high duty cycle, may run out of steam in certain situations if maximum output voltage exceeds the minimum input voltage. It details further advice on how best to set the tolerance for LED ripple current, and when tighter control and filtering is required. Finally, it outlines how to calculate dynamic resistance for a string of LEDs, showing how it changes as the forward current changes.
In some automotive exterior lighting applications, the LED array or matrix may be located remotely from the driver/controller. In such cases a boost converter may be a more appropriate choice of topology. When driving strings of LEDs, boost-based driver architecture is generally preferred.
However, in both cases, output short-circuit conditions are a real possibility. Very recently introduced LED drivers for automotive lighting include robust short-circuit protection built in. However, a protection circuit can be simply installed using a current shunt monitor, plus, depending on the application, additional components, such as an external MOSFET switch and sense resistors.
Texas Instruments offers a number of AEC-Q100-qualified parts, including the INA20x range, a high-side current shunt monitor capable of sensing drops across shunts at common mode voltages from -16 to 80 V. Operating from a single 2.7 to 18 V supply, drawing a maximum of 1800 µA, the device family is available with three output voltage scales, 20 V/V (INA200-Q1), 50 V/V (INA201-Q1) and 100 V/V (INA202-Q1). The devices incorporate an open drain comparator and internal reference providing a 0.6 V threshold. TI also offers a reference design and datasheet explaining its operation2.
Synchronous buck converters can also be used to regulate the current in LEDs for certain automotive lighting applications. For efficient and safe operation, a current sense monitor, high-side sense resistor, resistive divider and op amp can also be incorporated into these designs. This type of configuration will also control current in a LED precisely, in order to maintain a constant brightness, as well as boost efficiency.
Multi-mode drivers
When selecting an LED driver, it is useful to know that many devices are now available that can be configured in boost, buck and buck-boost mode architectures. Vendors of automotive grade multi-mode devices include, but are not limited to, Allegro Microsystems, Linear Technology, Maxim Integrated, ON Semiconductor, STMicroelectronics and Texas Instruments.
The LT3797 from Linear Technology, for example, is a triple output DC/DC controller designed specifically to drive three strings of LEDs independently. The device can operate in boost, buck, buck-boost modes, SEPIC or flyback topologies. The fixed frequency, current mode design results in stable operation over a wide input voltage range from 2.5 to 40 V. The integrated DC/DC converter produces a regulated 7.5 V supply for the N-channel MOSFET gate drivers of the three channels. Each converter can use the most suitable configuration to drive its LED load, whether step-up, step-down or a combination. Additional features include integrated current sense monitoring, open LED protection, short-circuit protection, fault flags, programmable over and undervoltage lockout, and PWM dimming.
Functional safety
With electronics taking over so many critical functions in vehicles, there is a trend now towards carmakers insisting on hardware and software compliance with the functional safety standard, ISO26262. For new features such as ADAS (advanced drive assistance system), and any function where the vehicle could automatically override the driver, this is likely to become mandatory. Safety features relying on vision systems may also be integrated with exterior lighting, such that even LEDs and their associated electronic devices may need to meet this standard.
While particularly relevant to embedded software, there will be implications for hardware components, too. In essence, functional safety standards require not just extensive testing, but the building of a safety case throughout the design process.
Conclusion
LEDs are becoming the technology of choice for automotive exterior lighting. However, the diodes and their drivers need to meet the relevant automotive standards, and additional short circuit and EMI protection may also need to be added. LED drivers are increasingly becoming more integrated to incorporate this protection.
References:
- Texas Instruments, Application Note: AN1656 Design Challenges of Switching LED Drivers
- Texas Instruments, Reference Design: TIDA-00302: Current Shunt Monitor with Transient Robustness
- Texas Instruments, Reference Design: PMP5518: Universal LED front light for Automotive Applications
