外文翻譯----led照明知識(shí)：pwm調(diào)光-展示頁
2025-01-26 23:26本頁面
【正文】 e silicon design emphasizes low shutdown current over response time. Dedicated switching regulations for driving LEDs will do the opposite, keeping their internal control circuits active while the enable pin is logic low to minimize tD, while suffering a higher operating current while the LEDs are off. Optimizing light control with PWM requires minimum slewup and slewdown delays not only for best contrast ratio, but to minimize the time that the LED spends between zero and the target level (where the dominant wavelength and CCT are not guaranteed). A standard switching regulator will have a softstart and often a softshutdown, but dedicated LED drivers do everything within their control to reduce these slew rates. Reducing tSU and tSN involves both the silicon design and the topology of switching regulator that is used. Buck regulators are superior to all other switching topologies with respect to fast slew rates for two distinct reasons. First, the buck regulator is the only switching converter that delivers power to the output while the control switch is on. This makes the control loops of buck regulators with voltagemode or currentmode PWM (not to be confused with the dimming via PWM) faster than the boost regulator or the various buckboost topologies. Power delivery during the control switch39。附錄AA matter of light：PWM dimming By Sameh Sarhan and Chris Richardson, National Semiconductor Whether you drive LEDs with a buck, boost, buckboost or linear regulator, the mon thread is drive circuitry to control the light output. A few applications are as simple as ON and OFF, but the greater number of applications call for dimming the output between zero and 100 percent, often with fine resolution. The designer has two main choices: adjust the LED current linearly (analog dimming), or use switching circuitry that works at a frequency high enough for the eye to average the light output (digital dimming). Using pulsewidth modulation (PWM) to set the period and duty cycle (Fig. 1) is perhaps the easiest way to acplish digital dimming, and a buck regulator topology will often provide the best peRFormance. Figure 1: LED driver using PWM dimming, with waveforms.PWM dimming preferred Analog dimming is often simpler to implement. We vary the output of the LED driver in proportion to a control voltage. Analog dimming introduces no new frequencies as potential sources of EMC/EMI. However, PWM dimming is used in most designs, owing to a fundamental property of LEDs: the character of the light emitted shifts in proportion to the average drive current. For monochromatic LEDs, the dominant wavELength changes. For white LEDs, the correlated color temperature (CCT) changes. It39。s difficult for the human eye to detect a change of a few nanometers in a red, green, or blue LED, especially when the light intensity is also changing. A change in color temperature of white light, however, is easily detected. Most white LEDs consist of a die that emits photons in the blue spectrum, which strike a phosphor coating that in turn emits photons over a broad range of visible light. At low currents the phosphor dominates and the light tends to be more yellow. At high currents the blue emission of the LED dominates, giving the light a blu
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