At present, non-energy-saving lighting dimming is mainly using TRIAC dimmer (three-terminal triac) for dimming, TRIAC dimmer is also the most widely used dimmer. The working principle of the TRIAC dimmer is shown in Figure 1. The RC circuit connecting R1, R2 and C1 can delay the start of the TRIAC dimmer until the voltage of C1 rises to the trigger point voltage of the alternating diode (Diac); The higher the resistance of the potentiometer (the more the slider moves down), the longer the startup delay time. This shortens the "on time" of the TRIAC dimmer or reduces its "conduction angle" (Θ). Therefore, the average power supply obtained by the load can be reduced. The TRIAC dimmer must constantly provide a holding current (IH) to ensure that the circuit is constantly "on". Wu Zhimin, senior marketing manager for the National Semiconductor Corporation (NS) Asia Pacific, said that in accordance with current industry design standards, in-wall TRIAC dimmers are typically connected to resistive loads such as incandescent or halogen lamps. However, the LED bulb does not belong to the resistive load that the in-wall TRIAC dimmer can access. Therefore, if the traditional in-wall TRIAC dimmer is used to control the brightness of the LED bulb, the control effect will not be optimal. LED drivers currently on the market produce flicker at a frequency of 120 Hz or fail to achieve a 100:1 dimming ratio. However, if a new dimmer is used to fully replace the TRIAC dimmer to support LED dimming, cost will be a problem. Therefore, NS recently introduced an LM3445 LED driver (Figure 2), which can support the traditional in-wall TRIAC dimmer, which can stably control the light and dark of high-brightness LEDs, ensuring no light flicker. The LM3445 can support up to 100:1 dimming ratio, and can output more than 1A constant current to drive multiple strings of LEDs, which makes it suitable for residential, building, commercial and industrial lighting. Wu Zhimin introduced that due to the constant-current floating buck regulator control architecture, the LM3445 ensures that the input LED's ripple current is constant, regardless of the input voltage fluctuations, regardless of the LED voltage (VLED) due to temperature changes. This patented technology ensures that the current flowing into the LED is constant, ensuring uniform and stable brightness of the LED lamp during dimming, which helps to extend the life of the LED. The LM3445 supports an operating voltage range of 80V to 270V (AC). Its conduction angle detector/decoder can achieve a 100:1 dimming range. The device has a built-in 300Ω drain resistor, whenever the line voltage drops to a low level. The bleeder will allow current to continue to flow so that the TRIAC dimmer can continue to operate normally. The LM3445 regulates the output current to 1A or higher and can be controlled by a master/slave system to ensure uniform brightness in a multi-channel illumination system. In addition, the chip's built-in 230W voltage divider circuit and passive power factor correction circuit can intercept current directly from the line for most of the cycle, providing a constant positive voltage to the buck regulator and ensuring the lowest power consumption. .
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