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Use buck-boost LED drivers for automotive headlamp

Posted: 11 Jun 2013     Print Version  Bookmark and Share

Keywords:automobile  high brightness  HB LED  ECU  dimming 

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.

Synchronous buck-boost HB LED driver
The LT3791 is a synchronous buck-boost DC/DC LED driver and voltage controller with 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 enabling to drive a wide range of LEDs in a single string.

A typical 50W headlight application is shown in the figure below. 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 and open or shorted LED condition.

Figure 1: 98% Efficient 50W (25V, 2A) buck-boost LED driver with 50:1 dimming ratio.

Its internal 4 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 design utilises three control loops monitor input current, LED current and output voltage to deliver optimal performance and reliability.

The LT3791 uses four external switching MOSFETs and can deliver from 5W to over 100W of continuous LED power with efficiencies up to 98%. 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% ensures constant lighting in an LED string while + 2% output voltage accuracy offers several LED protection features and also enables the converter to operate as a constant voltage source. The LT3791 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.

Conclusion
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.

About the author
Jeff Gruetter is Senior Product Marketing Engineer, Power Products, at Linear Technology Corporation.

To download the PDF version of this article, click here.


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