LED Drivers for Automotive Applications
While the basic operating requirement for an LED driver is to supply a constant current to LEDs in order to produce consistent lighting, automotive application - unlike other market segments - have more stringent guidelines in 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 carmakers 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% 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 case, 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.
Automotive Design Considerations
Unlike other market segments, automotive applications have several more demanding requirements that are reflected in industry standards and purchasing specifications including temperature and humidity range, supply voltage range, ability to withstand harsh chemicals, electromagnetic interference and electromagnetic compatibility (EMC) as well as reliability requirements dictated by qualification testing.
The automotive voltage range extends from normal operation of 9 to 16V (nominally 14V) to charge the 12.6V battery under ambient temperatures and includes extreme conditions such as reverse battery (-12V), jump start conditions of continuous double battery voltage (+24V) to faults conditions such as load dump which occurs when the battery is disconnected from the alternator, and other voltage transients. An unclamped load dump can be as long as several hundred milliseconds and can easily exceed 80V but today many manufacturers have centralized load dump clamping circuits and subsequently require that components must withstand the transient for levels of approximately 40 to 60V. In addition to high voltage requirements, cranking conditions may cause lower voltages that require countermeasures to be prepared for worst case situations.
High reliability in automotive applications is indicated by the need of protection circuitry such as overvoltage, undervoltage, reverse polarity, overcurrent, short circuits and overtemperature protection in many ICs. Also the component's life must be verified by extensive testing. The vehicle manufacturer’s target life warranty requirements could be 10 years and 150.000 km.
Discrete versus integrated LED Outputs
Depending on the power demand of LEDs, it can be useful to deploy LED drivers with integrated or external transistors. However, there are advantages and disadvantages when integrating the MOSFETs into the LED driver IC. Integrated LED switches reduce the number of components saving board space and simplifying inventory as it is also guaranteed that transistor and driver are ideally concerted. If the power level increases – such as when the LED driver controls HB LEDs or when the option for driving LED arrays is required, it is significantly better to consider an external layout, as heat dissipation is much easier permuted.
ROHM’s LED drivers are divided into three main areas:
a) One- and multi-channel drivers for backlighting
b) Drivers for switches and single parallel LEDs
c) Complex drivers for exterior lighting
Backlighting is especially required for display (from mid-sizeto large) in radio-navigation systems and the central information display. ROHM’s portfoliocomprises of LED drivers with one, two, three or four channels in order to address the different requirements and sizes of the display. A buck-boost system is used as topology which additionally enables brightness adjustment and display readability over a wide range of ambient light levels and sources – from full sunlight to darkness. The three-channel BD81A33 requires only a few external components and is available in a high-temperature SSOP28 package (see image 1).
Image 1. For full resolution, click here.
The switching frequency of the backlight LED drivers can be up to 2.2MHz in order to reduce the size of the external components (e.g. the coil) and to exclude EMI influence to the AM band. The supply voltage ranges from 4.5 to approx. 40V and the maximum output current from 120 to 400 mA per channel (depending on the driver). The maximum amount of serial LEDs in each array depends on the forward bias of the diodes and the predefined value of the protective mechanisms. Usually, about 6 to 7 LEDs can be driven in series. As a result, with a three channel matrix, 20 LEDs are available to lit a display.
In order to prevent EMI problems – which is especially important in regards to car radio applications – there is an external synchronization option.
The built-in LED abnormal state detection has two output channels: Fail 1 for the undervoltage-lockout, the thermal shut-down as well as the overvoltage and overcurrent protection. Output Fail 2 indicates whether hot-wired or open LEDs can be detected in one or more LED arrays. If required, the respective channel can be switched off preventing further deficiencies in parts of the system.
Fully integrated drivers with serial input and parallel output are shift registers which are used controlling single, parallel assembled LEDs. In order to achieve a consistent brightness of nearby LEDs each channel can be dimmed singularly and together with the entire system. The ROHM portfolio incorporates drivers with 8 and 12 channels such as the BD8105, BD8115 and BD8377. Based on the BD8377, a device including diagnosis functionality has been designed addressing the needs of the European market. All drivers are ideal for dashboard displays, telltale indicators for instrument clusters, particularly clusters and center stack controls such as HVAC. The ICs only require a few external components and enable a space-saving design. The BD8377 and BD8105 feature 12 parallel outputs (image 2), the BD8115 eight channels. The maximum output current is 50mA (DC) respectively pulsed 150mA for each channel.
The driver ICs can be cascaded so that two or more devices can be driven in series (image 3) when more than eight or 12 LEDs are controlled, without increasing the number of I/O pins of the microprocessor. Packaged in a SSOP-B20W, the driver with its built-in transistors provides standard protection circuits for the prevention of overheating, overcurrents and overvoltage integrated . Additional diagnosis functions enable the detection of open or hot-wired LEDs in order to switch off single channels until the malfunction is eliminated and the respective channel is reactivated by the microcontroller.
Image 3. For full resolution click here.
ROHM’s latest LED driver series are drivers for exterior lighting such as taillights and low-beam headlamps as well as daytime running lights. The BD8372 in a HTSSOP28 package is designed for driving multiple HB LEDs in headlamps. The BD8372 in a HTSOP8 package is designed for rear lamps. Both devices tolerate a maximum input voltage of 50V in order to be directly driven by the battery. They feature the same protection circuits and diagnosis functions which have been already described above and which are particularly important for the automotive market.
The application diagram (image 4) shows the usage of external MOSFETs by the BD8381. The transistor phase is kept external in order to enable designers to select the switch rating needed for different application requiring various brightness levels. Whether multi- or single chip LEDs – both kinds of diodes are supported. The integrated PWM generator would even allow the operation without a microcontroller.
Image 4. For full resolution, click here.
The trend in exterior lighting is going towards increasing functionality integrated in the LED driver and multiple channels for all kinds of functions –
turn signals, low- and high beam, daylight running light (front) as well as flash lights, rear lamps, fog lamps, tail and brake lights (rear). With the LED drivers for white LEDs deployed in automotive applications, ROHM Semiconductor has further extended its portfolio of highly integrated LED driver ICs and offers a package of different devices for almost any kind of LED lighting in vehicles. And the matching ROHM LED is available as well.
About the Author: Finn Lange is Product Markeing Manager LED Drivers at Rohm Semiconductor Europe.
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