外文翻譯--- -led照明知識(shí)：pwm調(diào)光-全文預(yù)覽
【正文】 rnoff of the LED light source is not to reduce the average light output, but to synchronize the light output with the sensor or camera capture times. Dimming with a switching regulator Switching regulatorbased LED drivers require special consideration in order to be shut off and turned on at hundreds or thousands of times per second. Regulators designed for standard power supplies often have an enable pin or shutdown pin to which a logiclevel PWM signal can be applied, but the associated delay, tD, is often quite long. This is because the 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。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 blue cast, leading to a higher CCT. In applications with more than one white LED, a difference in CCT between two adjacent LEDs can be both obvious and unpleasant. That concept extends to light sources that blend light from multiple monochromatic LEDs. When we have more than one light source, any difference between them jars the senses. LED manufacturers specify a certain drive current in the electrical characteristics tables of their products, and they guarantee the dominant wavelength or CCT only at those specified currents. Dimming with PWM ensures that the LEDs emit the color that the lighting designer needs, regardless of the intensity. Such precise control is particularly important in RGB applications where we blend light of different colors to produce white. From the driver IC perspective, analog dimming presents a serious challen
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