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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