The usage of light-emitting diodes (LEDs) in automotive applications is increasing for the same reasons that LED lighting is penetrating non-automotive sectors. LEDs are more efficient and smaller in size, have a substantially longer life, allow considerably greater design freedom for improved aesthetics, and more.
While the basic operating requirement for an LED driver is to supply a constant current to LEDs in order to produce consistent lighting, automotive applications -unlike other market segments- have more stringent guidelines with regards to temperature and humidity range, voltage, ability to withstand harsh chemicals, electromagnetic interference and electromagnetic compatibility (EMI) as well as protection circuitry.
This article describes the different options designers have to integrate a LED driver solution. Rohm has expanded its range of highly integrated LED driver ICs to provide a variety of design options with integrated or externally switched outputs, parallel/series control and extensive protection and fault detection functions in small surface mount packages.
Automotive applications for LED include interior lighting (such as dome, dash and footwell lighting), indicator and telltale lights and infotainment backlighting as well as exterior (signalling) functions such as tail lights, turn signals, brake lights including CHMSL (center high-mount stop lamps), parking lights, side marker lights, fog lamps and daytime running lights (DRLs).
A few car manufacturers have introduced LED headlamps on production models based on high-brightness (HB) LEDs. In some cases, the capabilities of an LED driver can enable more than one application to be addressed with the same LEDs. With leading automotive headlamp manufacturers providing prototypes with HB-LEDs, almost all car makers have displayed concept vehicles with LED headlights, and it is predicted that several standard vehicles will have LED headlights in 2012. As LEDs continue to improve in efficiency and reduce in cost (the light output levels from packaged LED devices roughly doubles every 18 months), an increasing amount of LEDs and LED drivers will be used in vehicles. With the low power consumption of LEDs compared to conventional lighting, an estimated 0.2 liters of fuel per 100 km and about 4 grams lower CO2 emissions/km are being cited as the ultimate advantage of replacing incandescent lighting with LEDs in the DRL application alone. In electric and hybrid vehicles, an 85-percent reduction in energy consumption from LED usage instead of incandescent bulbs translates into increased range. As a result, there are several compelling reasons to implement LEDs in automotive applications. One essential part is the power management provided by the IC drivers. LED driver capabilities
LEDs require a constant current to produce consistent lighting. Consequently, this forms the basic operating requirements for an LED driver. The accuracy of the current source determines its customer appeal. Current fluctuations occurring with voltage supply variations in vehicles must be avoided. Linear regulators provide a simple control and do not require electromagnetic interference (EMI) filters. However, their power dissipation can become excessive for high power applications. Buck DC-DC converters are commonly used as the next step. When the driver controls several LEDs in series, a boost converter topology is used. In some cases, a buck-boost topology provides the capability to address a variety of application requirements including the ability to handle varying supply voltage.
LED drivers can be designed to offer a combination of series and parallel LED control. Devices with this capability are providing circuit designers the flexibility to control LEDs in different applications with a single driver rather than requiring different devices that increase layout work and qualification testing. Dimming the light level is a common requirement for interior lighting. However, exterior lighting has applications with the requirement to provide different brightness from the same LED. For example, brake lights/taillights, low beam/daytime running lights and high beam/low beam headlights are so-called bi-level lightings. In some cases, lighting design may be able to address both situations with the same LED by using the appropriate LED driver. For harsh automotive environments, several protection circuits are required to prevent device failure under fault conditions.