LEDs are being used with increasing frequency in automotive lamps because they offer advantages over traditional automotive lighting. The biggest advantage of LED lighting is higher reliability and long service life compared with incandescent lamps. This feature is important in LED backlighting automotive instrument clusters and other LEDs embedded into a car’s interior because it means they will probably never require replacement during the life of the car.
Another advantage of LEDs is a significant safety performance benefit when employed in stop lights. Power applied to LEDs causes light output to rise to full intensity approximately 250 milliseconds faster than incandescent bulbs. Fast rise time improves the recognition of a stop lamp for drivers following the vehicle, which provides increased time in which to react to the lighting of the stop lamps.
Also, LEDs permit considerably shallower packaging compared with most bulb-type assemblies. LEDs are also much smaller in size, which enables much more flexibility in package design and overall appearance. A typical cluster assembly can be made much thinner by using LED backlighting.
And, LEDs offer extreme vibration resistance compared with incandescent lamps that employ a filament. Present incandescent lamp filaments must be heavy enough to survive high vibration, which establishes their operating voltage and current.
Other advantages of using LEDs include lower voltage operation, low electromagnetic radiation, thinner form factors, wide operating temperature range, and wide dimming range.
However, use of LEDs in automotive applications becomes a much more complex issue. The automotive industry has strict guidelines with regard to temperature and humidity range, ability to withstand adverse environments, electromagnetic interference (EMI), and voltage protection circuitry. In addition, automotive components must pass reliability requirements dictated by qualification testing. In particular, LED driver ICs must pass AEC-Q100, a critical stress test qualification. Despite the higher complexity and stress testing, there has been a large increase in the use of LED lighting in cars over the last few years, and their potential applications are expected to expand even further.
A decade ago, it would not have been feasible to even consider using LEDs for automotive use because of their limited temperature range, low light output levels, and illumination only in certain colors, such as red or green. But the technology of LEDs has continued to improve, where now it seems that eventually this technology will replace all other traditional lighting technologies.
It has been estimated that the light output levels from packaged LED devices roughly doubles every 18 months. With the increased efficiency and lower costs, it becomes apparent that more LEDs and driver ICs will be used in vehicles. The technology of power management functions in LED driver ICs has advanced rapidly also, and this allows much more efficient control of LED lighting than ever before.
Today, LED lamps are a feasible alternative to traditional light sources for flashing beacon lights on vehicles such as maintenance trucks. Previously, traditional light sources needed the engine to continue running to save the battery if the lights were to be used for more than a few hours. The energy-efficient capability of LED light sources allows the engine to remain turned off while the light continues to flash.
Typical automotive applications for LEDs include cluster or instrument backlighting, dome or map reading lights, courtesy lights, and other forms of interior lighting. Exterior applications include tail lights, turn signals, brake lights, parking lights, side marker lights, fog lamps, daytime running lights, and most recently headlights. Heat built up within LEDs are a big issue, as is the fact that the LEDs need to deal with the heat from the engine compartment.
A workable solution for most automotive lighting applications is relatively simple compared with the additional development needed for the headlamp LED lighting system. The main reason for this is the thermal design considerations of the LEDs and their associated driver circuits. Traditional techniques have been far simpler to implement, as the entire circuit consisted of a battery and switch connected to the headlamp with three wires through an assortment of automotive connectors.
The newly developed high-brightness (HB) LEDs are now being used more extensively as headlamps in the automotive industry. Most of leading headlamp manufacturers have now taken steps to produce (HB) LEDs, and it is predicted that many production models will have LED headlights within a year or two.
Some car manufacturers, such as Audi, have introduced LED headlamps on a few production models. The Audi R8 (Fig. 1) features a distinctive curved bar of LED daytime running lamps (DRLs) mounted inside xenon HID headlamp casings. Among the other automobiles that employ LED lighting are the Lexus LS 600 that features LED low beam, position and side marker lamps in North America. Also, a model of the Cadillac Escalade uses LEDs for the low and high beams, as well as for the position and side marker lamps. These higher priced vehicles can absorb the additional cost of LED lighting. And, as the cost of LED lighting goes down, even lower cost vehicles can take advantage of LED lighting.
LED headlamps would be a major advantage for future electric and hybrid vehicles. LEDs could offer an 85% reduction in energy consumption compared with incandescent bulbs, which would translate into increased driving range.
Limiting factors on the use of LED headlamps include high system expense and temperature constraints. Headlamp LEDs have much higher power requirements compared with interior LEDs. LED performance depends on its temperature because it will produce more light at a low temperature than at a high temperature. Therefore, maintaining a constant light output requires the temperature of an LED headlamp to be kept relatively stable. LEDs actually produce a significant amount of heat per unit of light output and are damaged by high temperatures, so prolonged operation above the maximum junction temperature will permanently degrade the LEDs and ultimately shorten their life. The need to keep LED junction temperatures low at high power levels requires thermal management, such as heat sinks or cooling fans, which add to system cost.
Sometimes, it is not convenient to install an adequate heat sink in the headlamp housing, so the temperature must be controlled electronically, where a sensor would detect an over-temperature condition, the controller would reduce the LED brightness momentarily until the temperature returns back to normal. It is possible to use pulse-width modulation (PWM) techniques to lower the temperature of the LEDs with minimal effect on the brightness level, and it would hardly be noticeable.
Also, there are thermal issues with LED headlamps in cold ambient temperatures. Excessively low temperatures decease LED light output beyond the regulated maximum. And, in some cases, heat must be added to thaw snow and ice from the front lenses.
Although its lifetime is not reduced at low temperatures, an LED just fails to provide light output. This can also be monitored electronically, and a special current overdrive mode would be activated that quickly heats up the lamp whenever it is sensed that the temperatures is too low. Again, this would take place so fast, that it would almost appear that the LED was turned on instantly.
Immunity to electromagnetic interference (EMI) is an important consideration for all automotive electrical components. Switching the current on and off through the LED at high pulse rates can be an EMI radiation source. PWM switching pulses are normally applied for dimming. To control radiated EMI during PWM dimming, the rise times and fall times of the pulses are reduced with special wave-shaping circuitry.
LED Driver Capabilities
High power LED drivers in an automotive environment should be able to supply a constant current over the wide voltage supply variations that commonly occur in a vehicle. Along with the large input voltage variation, the LED driver should also have a large output voltage range, as there are different types of LED configurations and brightness levels need to be controlled as the pulse width is changed for temperature control.
The basic linear regulator would provide a simple control while avoiding the need for electromagnetic interference (EMI) filters. However, there could be problem with power dissipation when it is used to supply current to LED headlamps. A buck DC-DC converter is sometimes used instead to avoid the dissipation problem, and when higher voltage outputs are needed, a buck-boost topology can be used. This would happen when the driver controls several LEDs in series. This type of converter would handle both the varying supply voltage and the output voltage variation.
The requirements for headlamp LED drivers are much more complex than those of the driver ICs used for interior lighting. Although drivers for interior lighting in vehicles tend to supply power through the MOSFETs integrated into an LED driver IC, it would be better to consider using external MOSFETs instead for headlamp lighting to handle the heat dissipation. Besides supplying extra power, the headlamp LED driver IC should be able to handle dimming requirements for headlamps. Normally, the exterior lighting of a car has to be dimmed by varying the brightness of each individual LED, such as for brake lights/taillights and high beam/low beam. These are called bi-level settings, and the power requirements put extra strain on the voltage converter/driver. Sometimes two separate drivers are needed for the two settings, and at other times, one LED driver can address both situations.
In either case, extensive protection and fault detection functions are required to prevent device failure. To insure high reliability in automotive applications, ICs and other electronic components need to be protected from over-voltage, under-voltage, reverse polarity, over-current, short circuits and over-temperature.
LED driver ICs provide power to either a series or parallel combination of LEDs, and sometimes a combination of both. A combination IC allows designers to use the same chip for a wider variety of applications. This avoids extra layout work and qualification testing. But normally, a parallel system is used, where one output on the LED driver will activate only one LED. A series arrangement, where one output will activate several LEDs is typically used in LCD backlighting of instrument clusters, where the light is not focused, but spread out over an area. In this case the same current will flow through each LED to ensure a continuous brightness level and color over the area.
Some of the newer LED drivers can generate the PWM controlling signal under various conditions without the need for a microcontroller. All functions needed to efficiently control LEDs are on the driver chip, including a ramp generator, pulse detector, regulator, band-gap reference, and others.
Examples of constant current controllers for LEDs are the LM3421 and LM3423 series from Texas Instruments (Fig. 2). They include a 1A peak gate driver for external N-channel MOSFET that drive the LEDs. These ICs can be configured as buck, boost, buck-boost and SEPIC topologies. Their input voltage range is 4.5V to 75V, making them ideal for illuminating LEDs in a large family of applications. These ICs include an LED output status flag, a fault flag, a programmable fault timer, and a logic input to select the polarity of the dimming output driver.
Linear Technology’s LT3791 is a synchronous buck-boost DC/DC LED driver that can deliver over 100W of LED power (Fig. 3). Its 4.7V to 60V input voltage range makes it ideal for automotive applications. Its output voltage can be set from 0V to 60V, enabling it to drive a wide range of LEDs in a single string. Its internal 4-switch buck-boost controller operates from input voltages above, below or equal to the output voltage, ideal for applications such as automotive, where the input voltage can vary dramatically during stop/start, cold crank and load dump scenarios. Transitions between buck, pass-through and boost operating modes are seamless, offering a well-regulated output even with wide variations of supply voltage.
The LT3791 utilizes either analog or PWM dimming as required by the application. Furthermore, its switching frequency can be programmed between 200kHz and 700kHz or synchronized to an external clock. Additional features include output disconnect, input and output current monitors, open and shorted LED detection and integrated fault protection.
The ASL1010 series from NXP Semiconductor features buck-boost topology, temperature feedback, fault detection, PWM generation, short circuit protection and flexible current settings. This series is specially designed to support the growing replacement of traditional automotive lighting solutions with high-brightness (HB) LEDs, meeting the automotive industry’s performance requirements. The devices also deliver low electromagnetic emission (EME) and high electromagnetic immunity (EMI). Supporting a supply voltage range of 6 to 60 V, the IC is ISO 7637 compliant for transient and reverse battery voltages. The output DC voltage capability of up to 60 V makes it possible to drive up to 20 LEDs in series. Maximum output current is 1 A over the complete operating temperature range, with high LED current accuracy and very low (less than 2%) LED current ripple.
Compatible with current bulb lighting architectures, the ASL1010 series offers a high degree of flexibility. It includes all the required features for automotive lighting, including temperature feedback, fault detection, PWM generation, short circuit protection and flexible current setting by selecting an external binning resistor. It provides full functionality and flexibility without a microcontroller, while the high integration minimizes the number of external components.
The MAX16800 from Maxim is a high-current regulator capable of providing up to a total of 350mA of current to one or more strings of high-brightness LEDs (Fig. 4). A +6.5V to +40V input voltage range makes the MAX16800 ideal for automotive applications. A +5V regulated output provides up to 4mA of current to power external circuitry. In addition, the MAX16800 features thermal and output short-circuit protection. The wide operating input voltage range helps protect the MAX16800 against large transients such as those found in load-dump situations up to 40V.
The MAX16800 uses a feedback loop to control the output current. The differential voltage across a sense resistor is compared with a fixed reference voltage, and the error is amplified to serve as the drive to the internal power series pass device.
The combined technology of both headlamp LEDs and their associated drivers have reached the point where a headlamp system can be designed using these components. The thermal effects can now be controlled effectively while delivering the maximum light output from the lowest possible level of power. It would not been possible to utilize one technology without the corresponding development in the other.