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LM3445MM Datasheet(PDF) 9 Page - Texas Instruments |
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LM3445MM Datasheet(HTML) 9 Page - Texas Instruments |
9 / 42 page DELAY DELAY (a) (b) (c) ? ? LM3445 www.ti.com SNVS570M – JANUARY 2009 – REVISED NOVEMBER 2015 Feature Description (continued) An RC network consisting of R1, R2, and C1 delay the turn on of the TRIAC until the voltage on C1 reaches the trigger voltage of the diac. Increasing the resistance of the potentiometer (wiper moving downward) increases the turn-on delay which decreases the on-time or conduction angle of the TRIAC ( θ). This reduces the average power delivered to the load. Voltage waveforms for a simple TRIAC dimmer are shown in Figure 10. Figure 10a shows the full sinusoid of the input voltage. Even when set to full brightness, few dimmers will provide 100% on- time, i.e., the full sinusoid. Figure 10. Line Voltage and Dimming Waveforms Figure 10b shows a theoretical waveform from a dimmer. The on-time is often referred to as the conduction angle and may be stated in degrees or radians. The off-time represents the delay caused by the RC circuit feeding the TRIAC. The off-time be referred to as the firing angle and is simply 180° - θ. Figure 10c shows a waveform from a so-called reverse phase dimmer, sometimes referred to as an electronic dimmer. These typically are more expensive, microcontroller based dimmers that use switching elements other than TRIACs. Note that the conduction starts from the zero-crossing, and terminates some time later. This method of control reduces the noise spike at the transition. Since the LM3445 has been designed to assess the relative on-time and control the LED current accordingly, most phase-control dimmers, both forward and reverse phase, may be used with success. Copyright © 2009–2015, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM3445 |
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