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 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 portfolio comprises 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).
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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.
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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.
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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 marketing manager LED Drivers for Rohm Semiconductor Europe.
This article was first published on EE Times Europe.
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