LED (LED) and cold cathode fluorescent (CCFL) have gradually become a common source of automotive lighting. These light source key advantages are longer service life and higher efficiency. A typical incandescent lamp life of 10 000 h, and fluorescent lamps up to 50 000 h, LED and up to 100 000 h. Extend the life improves reliability, reduces maintenance work, reduce service costs. LED also has a low operating voltage, low electromagnetic radiation, immunity to mechanical stress, the shape of flexible design, wide operating temperature range and a wide range of brightness adjustment and so on. With the development of LED technology, which is substituted by other power technology trend is more obvious, but the current application, CCFL still occupy certain applications, such as large screen backlight and wherever high-power non-focused light source.
CCFL and LED light sources require an electric power supply, and, different technologies have different specific requirements, the power supply must provide special applications and functions. Here, for various automotive interior and exterior lighting, offers a variety of solutions for the power LED and CCFL light source.
1 automotive interior lighting
Automotive interior lighting applications include instrument panel and instrument panel backlighting, dome and map lamp, mounted on the vehicle body door or door opening, and display backlighting. All internal lighting applications can be used as the LED light source. Map and ceiling lamps typically use a high-brightness LED, cluster lamps and the door usually require more than one LED connected in series. Serial connection helps to avoid the different LED current (corresponding color) mismatch. All applications require an integrated constant current source brightness adjustment function.
1.1 interior automotive lighting design based MAXl6800
FIG. 1 is a typical dimming MAXl6800 application circuit. MAXl6800 having a high input voltage range (up to 40 V), may be directly connected to the car battery without the need for further protection against the surge voltage devices are generally used to bring the battery load change. MAxl6800 is possible to provide a constant current an LED, a current value through the sense resistor RSENSE is provided in series with the LED. In order to improve the current accuracy and to increase immunity to external noise, MAXl6800 differential current sense input.
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LED chromaticity changes with its current varies, and therefore the best way to adjust the LED brightness signal is a fixed current chopping is achieved by pulse width modulation (the PWM), instead of changing the actual magnitude of the current. MAXl6800 adjusting LED brightness by a PWM signal at the enable input of the device. Turned on and off according to the PWM signal frequency current through the LED.
Electromagnetic interference (EMI) suppression is very important in automotive applications, the design requirements and does not generate EMI are equally important. Anyway EMI radiation of the LED current on and off control. Accordingly, in order to reduce the EMI when the PWM dimming, MAXl6800 integrated waveform shaping circuit for smoothing the switching edges.
1.2 based on internal MAX16805 / MAXl6806 automotive lighting design
Since many of the lighting system is not available on the microcontroller generates a PWM dimming signal, and therefore can be MAXl6805 / MAXl6806 LED driver. These devices generate a PWM signal by the internal, the PWM signal provided by the external analog voltage applied to the DIM input. When no analog dimming, MAXl6806 a switch input (SW), only the input switch state detection switch, has a debounce function as a switch with a wetting current. FIG 2 is a MAXl6805 / MAXl6806 typical application circuit.
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Automotive lighting applications, LED temperature must be closely monitored, for which a compact space, in particular the poor heat dissipation system is particularly important. LED overheating will reduce the life of the LED, thus weakening the key advantages of this light source. However, by momentarily reducing the LED brightness, in most applications avoid overheating. Thus MAXl6806 provide inputs for an external temperature sensor. When overheating is detected, the device may reduce the brightness, until the temperature returns to an acceptable range. Temperature and brightness adjustment threshold can be programmed through the serial port and stored to the EEPROM. This feature eliminates the need for expensive, large heat sink. Internal reference MAXl6805 / MAXl6806 for monitoring LED current feedback loop, through the serial port and adjusting. Therefore, the actual application may be used subject to the brightest LED fixed resistor, to simplify equipment and reduce costs. Technology area
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