1、2000单词，3100汉字附录 A A matter of light： PWM dimming By Sameh Sarhan and Chris Richardson, National Semiconductor Whether you drive LEDs with a buck, boost, buck-boost or linear regulator, the common thread is drive circuitry to control the light output. A few applications are as simp
2、le 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
3、 eye to average the light output (digital dimming). Using pulse-width modulation (PWM) to set the period and duty cycle (Fig. 1) is perhaps the easiest way to accomplish digital dimming, and a buck regulator topology will often provide the best peRFormance. Figure 1: LED driver using PWM dimm
4、ing, 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, o
5、wing 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. It's difficult for the human eye to detect a chan
6、ge 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 coatin
7、g 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
8、, 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
9、 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 con
10、trol 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 challenge to the output current accuracy. Almost every LED driver uses a resistor of some type in series with
11、the output to sense current. The current-sense voltage, VSNS, is selected as a compromise to maintain low Power dissipation while keeping a high signal-to-noise ratio (SNR). Tolerances, offsets, and delays in the driver introduce an error that remains relatively fixed. To reduce output current in a
12、closed-loop system, VSNS, must be reduced. That in turn reduces the output current 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
13、dimming. Dimming frequency vs. contrast ratio The LED driver'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
14、 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 targ
15、et 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, TDIM.The lower limit for fDIM is approxima
16、tely 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 on-time, i.e., CR = 1 / tON-MIN : 1 &nb
17、sp;where tON-MIN = tD + tSU. Applications in machine vision and industrial inspection often require much higher PWM dimming frequencies because the high-speed cameras and sensors used respond much more quickly than the human eye. In such applications the goal of rapid turn-on and turn-off of the LED
18、 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 regulator-based LED drivers require special consideration in order to be shut off and turned on at hun
19、dreds or thousands of times per second. Regulators designed for standard power supplies often have an enable pin or shutdown pin to which a logic-level PWM signal can be applied, but the associated delay, tD, is often quite long. This is because the silicon design emphasizes low shutdown current ove
20、r 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 r
21、equires minimum slew-up and slew-down 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 soft-start and often a soft-shutdown, b
22、ut 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 ra
23、tes 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 voltage-mode or current-mode PWM (not to be confused with the dimming via PWM) faster than th
24、e boost regulator or the various buck-boost topologies. Power delivery during the control switch's on-time also adapts easily to hysteretic control, which is even faster than the best voltage-mode or current-mode control loops. Second, the buck regulator's inductor is connected to the output
25、 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 becomes a true, high impedance current source, capable of slewing the output voltage very quickly. Cuk and zeta converters can claim continuous output inducto
