外文翻譯----led照明知識(shí)：pwm調(diào)光-文庫吧資料
2025-01-23 23:26本頁面
【正文】 e well suited to PWM dimming. That39。s inductor is connected to the output during the entire switching cycle. This ensures a continuous output current and means that the output capacitor can be eliminated. Without an output capacitor the buck regulator bees a true, high impedance current source, capable of slewing the output voltage very quickly. Cuk and zeta converters can claim continuous output inductors, but fall behind when their slower control loops (and lower efficiency) are factored in. Faster than the enable pin Even a pure hysteretic buck regulator without an output capacitor will not be capable of meeting the requirements of some PWM dimming systems. These applications need high PWM dimming frequency and high contrast ratio, which in turn requires fast slew rates and short delay times. Along with machine vision and industrial inspection, examples of systems that need high performance include backlighting of LCD panels and video projection. In some cases the PWM dimming frequency must be pushed to beyond the audio band, to 25 kHz or more. With the total dimming period reduced to a matter of microseconds, total rise and fall times for the LED current, including propagation delays, must be reduced to the nanosecond range. Consider a fast buck regulator with no output capacitor. The delays in turning the output current on and off e from the IC39。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 th
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