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Jeff Gruetter, Senior Product Marketing Engineer of Power Products at Linear Technology Corporation explores why automotive headlamps are demanding more buck-boost LED drivers
Although LEDs have been used in daytime running lights, brake lights, turn signals and interior lighting for several years, headlamp specific applications are relatively new. However, every major automotive manufacturer has either released a model with LED headlamps or will shortly. Initially, most LED headlamps are offered in premium models, but the universal improvements offered by LED lighting will be demanded by customers for all models in the very near future. Some estimate that the current LED headlamp market was around $1B in 2013 and is expected to surpass $3B in 2014.
One of the biggest challenges for automotive lighting systems designers is how to optimise all the benefits of the latest generation of HB LEDs. As HB LEDs generally require an accurate and efficient DC current source and a means for dimming, the LED driver IC must be designed to address these requirements under a wide variety of conditions. As a result, power solutions must be highly efficient, robust in features and reliable while being very compact and cost effective.
Arguably, one of the most demanding applications for driving HB LEDs is found in automotive headlamp applications as they are subjected to the rigors of the automotive electrical environment, must deliver high power, typically between 50W to 75W, and must fit into very space constrained enclosures, all while maintaining an attractive cost structure.
Automotive LED Headlamps
Benefits, such as small size, extremely long life, low power consumption and enhanced dimming capability are the catalyst for the wide spread adoption of HB LED headlights. Several manufacturers, such as Audi, Mercedes and most recently, Lexus have used LEDs to design very distinctive driving lights or “eyebrows” around the headlights to make clear that it is in fact their brand, long before the car can be seen.
Although these applications are very distinctive from a design perspective, they do not have the same level of design challenges as do both the low beam and high beam of the headlights.
The benefits of using LEDs in automotive headlights have several positive implications. First, they never need to be replaced, since their solid state longetivity of up to 100K+ hours (11.5 service years) surpasses the life of the vehicle. This allows automobile manufactures to permanently embed them into headlight designs without requiring accessibility for replacement.
This also enables styling to be dramatically changed as LED lighting systems do not require the depth or area as HID or Halogen do. HB LEDs are also more efficient than Halogen bulbs (and are soon to surpass HIDs) at delivering light output (in lumens) from the input electrical power.
This has two positive effects. First, it drains less electrical power from the automotive bus, which is especially important in EVs and hybrids, and equally important, it reduces the amount of heat that needs to be dissipated in the display eliminating any requirement for bulky and expensive heat sinking.
Finally, by using arrays of HB LEDs and electronically steering or dimming them, they can easily be designed to optimise lighting for many different driving conditions.
In order to ensure optimal performance and long operating life, LEDs require an effective drive circuit. These driver ICs must deliver an accurate and efficient DC current source and accurate LED voltage regulation regardless of wide variations in the input voltage source. Secondly, they must offer a means of dimming and offer a wide array of protection features just in case an LED open or short circuit is encountered. In addition to operating reliably from the electrically caustic automotive power bus, they must also be both cost and space effective.
Stop/Start Cold Crank and Load Dump Conditions
In order to maximise fuel mileage while minimising carbon emissions, alternative drive technologies are continuing to evolve. Whether these new technologies incorporate electric hybrids, clean diesel or a more conventional combustion engine designs, the chances are that they will also incorporate a stop-start motor design.
Already prevalent in virtually all hybrid designs throughout the world, many European and Asian car manufacturers have been incorporating this design into conventional gas and diesel vehicles as well. In the USA, Ford recently announced that it will incorporate stop-start systems into many of its 2014 domestic models.
The extreme loading of the battery on vehicles equipped with stop/start technology introduces yet another design challenge for LED headlamps. As a large draw of current is required to restart the engine, it can temporarily pull the battery voltage as low as 5V. The challenge for the LED driver is to continually deliver a well regulated output voltage and LED current when the battery bus voltage briefly drops to 5V, then returns to a nominal 13.8V when the charger returns to steady state conditions.
This is very similar to an automotive “cold crank” condition, which occurs when a car’s engine is subjected to cold or freezing temperatures for a period of time. The engine oil becomes extremely viscous and requires the starter motor to deliver more torque, which in turn, draws more current from the battery.
Alternatively, a “load-dump” condition occurs when the battery cables are accidentally disconnected while the alternator is still charging the battery. This can occur when a battery cable is loose while the car is operating, or when a battery cable breaks while the car is running.
Such an abrupt disconnection of the battery cable can produce transient voltage spikes up to 60V as the alternator is attempting to fully charge an absent battery. Transorbs on the alternator usually clamp the bus voltage somewhere between 30V and 34V and absorb the majority of the surge; however DC/DC converters and LED drivers downstream of the alternator are subjected to transient voltage spikes as high as 36V.
These LED drivers are not only expected to survive, but must also continually regulate output voltage and LED current through this transient event.
Fortunately, there is a new solution to these dilemmas, Linear Technology’s LT3791 LED driver. This device is a synchronous buck-boost DC/DC LED driver and voltage controller that can deliver over 100W of LED power. Its 4.7V to 60V input voltage range makes it ideal for a wide variety of applications including automotive, truck and even avionics HB LED headlights. Similarly, its output voltage can be set from 0V to 60V driving a wide range of LEDs in a single string.
A typical 50W headlight application is shown in Figure 2. This application uses a single inductor to accurately regulate a 25V string of LEDs at 2A to deliver 50W of LED power. This circuit offers a 50:1 PWM dimming ratio which is ideal for anti-glare auto-dimming requirements. Both input and output (LED) current is monitored while fault protection is provided to survive and report an open or shorted LED condition.
Its internal four switch buck-boost controller operates from input voltages above, below or equal to the output voltage making it 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 in spite of wide variations of supply voltage. The LT3791’s unique design utilises three control loops monitor input current, LED current and output voltage to deliver optimal performance and reliability.
This device uses four external switching MOSFETs and can deliver from 5W to over 100W of continuous LED power with efficiencies up to 98 percent as can be seen in Figure 3. In conventionally powered vehicles high efficiency is important as it minimises the need for heat sinking enabling a very compact low profile footprint. However, in EVs this power savings adds valuable miles of vehicle range between charges.
LED current accuracy of +/-6 percent ensures constant lighting in an LED string while +/-2 percent output voltage accuracy offers several LED protection features and also enables the converter to operate as a constant voltage source. This device can utilise either analogue or PWM dimming as required by the application. Furthermore, its switching frequency can be programmed between 200kHz and 700kHz or synchronised to an external clock. Additional features include output disconnect, input and output current monitors, and integrated fault protection.
HB LED applications
The continual acceleration of HB LED applications, especially those found in automotive head lights, is being driven by an insatiable demand for higher performance and cost effectiveness. These demands must be enabled by new HB LED driver ICs. Therefore, these LED drivers must provide constant current in order to maintain uniform brightness, regardless of input voltage or LED forward voltage variations, operate with high efficiency, offer very wide dimming ratios and have a variety of protection features to enhance system reliability.
Of course, these LED driver circuits must also offer a very compact, low-profile and thermally efficient solution footprint.
Linear Technology Corporation
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