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LED street lamp power circuit design considerations

2018-04-25 12:48:22

The following points should be done in the design: 1. When selecting a white LED driver, you need to take into account the frequency of use of the display. If the display will be backlit for long periods of time, having a high-efficiency converter is crucial for battery life. Larger displays require more leds, while applications with longer display life benefit from a more energy-efficient booster topology. Conversely, if the display is only used for short periods of backlighting, then efficiency may not be a key design parameter. 2. LED technology continues to improve rapidly, and manufacturers are using new materials, manufacturing techniques, and LED designs to produce a larger light output for the same amount of current, so that a display that required four leds to backlight a few years ago may now use two leds to achieve the same backlight brightness. Not only that, larger 4 - to 7-inch displays, which used to be backlit by cold cathode fluorescent lamps (CCFL), are now switching to LED backlighting. In addition, the forward voltage of leds is trending lower. Therefore, it is not only necessary to consider the driver efficiency on the driver manufacturer's datasheet curve, but also based on the evaluation of the driver with the selected LED. Table 1 lists some important specifications for several leds, showing the differences between these leds in terms of forward voltage and luminance. It should be noted that the forward voltage range varies greatly, which means that the efficiency of the driver should be evaluated using the limit value of the LED specification. 3. Note the wiring Even if the individual leds are driven with very low currents of 10 to 20mA, the peak current flowing through the converter is significantly higher. This is especially true for inductive topologies, as the peak switching current can be 10 to 20 times the average LED current. Therefore, it is necessary to use appropriate low-loss wiring technology. For charge pump topologies, capacitors should be positioned near the drivers to minimize the loop area to avoid radiated switching noise. For inductive boost converters, input and output capacitors and induction are located near the driver. In addition, the current setting resistor (Rfb) should be connected directly to the ground of the chip, because errors between the internal reference and detection voltage can directly affect the LED current accuracy. 4. Test your test product in a real environment Consider the performance of the display under high external light conditions, and ensure that the software dimming control has sufficient dynamic range to adequately dimmer the display in the expected light environment.