外文翻譯--- -led照明知識(shí)：pwm調(diào)光-文庫(kù)吧
2025-01-02 23:26 本頁(yè)面
【正文】 rent accuracy and ultimately the output current cannot be specified, controlled, or guaranteed. In general, dimming with PWM allows more accurate, linear control over the light output down to much lower levels than analog dimming. Dimming frequency vs. contrast ratio The LED driver39。s finite response time to a PWM dimming signal creates design issues. There are three main types of delay (Fig. 2). The longer these delays, the lower the achievable contrast ratio (a measure of control over lighting intensity). Figure 2: Dimming delays. As shown, tn represents the propagation delay from the time logic signal VDIM goes high to the time that the LED driver begins to increase the output current. In addition, tsu is the time needed for the output current to slew from zero to the target level, and tsn is the time needed for the output current to slew from the target level back down to zero. In general, the lower the dimming frequency, fDIM, the higher contrast ratio, as these fixed delays consume a smaller portion of the dimming period, lower limit for fDIM is approximately 120 Hz, below which the eye no longer blends the pulses into a perceived continuous light. The upper limit is determined by the minimum contrast ratio that is required. Contrast ratio is typically expressed as the inverse of the minimum ontime, ., CR = 1 / tONMIN : 1 where tONMIN = tD + tSU. Applications in machine vision and industrial inspection often require much higher PWM dimming frequencies because the highspeed cameras and sensors used respond much more quickly than the human eye. In such applications the goal of rapid turnon and turnoff 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 ma
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