Some applications simply implement the "on" and "off" functions, but more application requirements are to adjust the brightness of the light from 0 to 100%, and often have high precision.
Designers have two main choices: linearly adjusting the LED current (analog dimming), or using a switching circuit to operate at a frequency high enough relative to the human eye to change the average of the light output (digital dimming). Using pulse width modulation (PWM) to set the period and duty cycle (Figure 1) is probably the easiest way to implement digital dimming, and the Buck regulator topology often provides the best performance.
Recommended PWM dimming
Analog dimming can usually be done very simply. We can proportionally change the output of the LED driver with a control voltage. Analog dimming does not introduce potential electromagnetic compatibility/electromagnetic interference (EMC/EMI) frequencies. However, PWM dimming is used in most designs due to one of the fundamental properties of LEDs: the characteristics of the emitted light are shifted with the average drive current.
For a monochrome LED, its dominant wavelength will change. For white LEDs, the associated color temperature (CCT) will change. For the human eye, it is difficult to detect changes in the wavelength of a few nanometers in red, green or blue LEDs, especially when the light intensity is also changing. However, the color temperature change of white light is very easy to detect.
Most LEDs contain an area that emits blue spectral photons that provide a wide range of visible light through a phosphor face. At low currents, phosphorescence dominates and the light approaches yellow. At high currents, the LED blue light dominates and the light appears blue, achieving a high CCT. When more than one white LED is used, the difference in CCT of adjacent LEDs is clearly undesirable. The same problem arises when mixing multiple single-color LEDs into a light source application. When we use more than one light source, any difference in the LED will be noticed.
LED manufacturers have specifically developed a drive current in their product's electrical characteristics table to ensure that the wavelength of light or CCT produced by these specific drive currents is guaranteed. PWM dimming ensures that the LED emits the color that the designer needs, and the intensity of the light is another matter. This fine control is especially important in RGB applications to mix different colors of light to produce white light.
From the perspective of driver ICs, analog dimming faces a serious challenge, which is the accuracy of the output current. Almost every LED driver uses a series resistor to identify the current. The current discrimination voltage (VSNS) is selected by a compromise between low energy loss and high signal to noise ratio. The tolerances, offsets, and delays in the drive result in a relatively fixed error. To reduce the output current in a closed loop system, the VSNS must be reduced. This in turn reduces the accuracy of the output current and, ultimately, the output current cannot be specified, controlled, or guaranteed. In general, PWM dimming improves accuracy and linearly controls light output to a lower level than analog dimming.
Dimming frequency vs contrast
The LED drive's non-negligible response time to the PWM dimming signal creates a design problem. There are three main delays here (Figure 2). The longer these delays, the lower the contrast that can be achieved (the control scale of the light intensity).
As shown, tn represents the transition delay from when the time logic signal VDIM is raised enough to cause the LED drive to begin to increase the output current. In addition, the time required for the tsu output current to rise from zero to the target level, on the contrary, tsn is the time required for the output current to fall from the target level to zero. In general, the lower the dimming frequency (fDIM), the higher the contrast because these fixed delays consume a small portion of the dimming period (TDIM). The lower limit of fDIM is about 120Hz. Below this lower limit, the naked eye will no longer mix the pulses into a continuous light. In addition, the upper limit is determined by reaching the minimum contrast.
Contrast is usually represented by the reciprocal of the minimum pulse width value:
CR = 1 / tON-MIN : 1
Here tON-MIN = tD + tSU. Higher PWM dimming frequencies are often required in machine vision and industrial inspection applications because high speed cameras and sensors require much faster response times than the human eye. In this application, the purpose of the fast turn-on and turn-off of the LED light source is not to reduce the average intensity of the output light, but to synchronize the output light with the sensor and camera time.
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