In many lighting applications, the brightness of the device needs to be controlled. When designing an LED driver, this can be accomplished by either varying the current through the LED, or by varying the duty cycle. It is preferable to vary the duty cycle via PWM rather than vary the current, as the color of the LED may change slightly with current. This ensures that the LED retains it’s color characteristics, as the LED is either fully on or fully off. Ideally when using PWM the average brightness of the LED would be equal to the duty cycle * maximum brightness. However, this calculation assumes that the time to switch between on and off is zero. As the pulse width is shortened, the difference between the calculated and actual pulse width increases. If the lighting application requires a smooth dim from 0% to 100% brightness, this becomes an issue.
The width of the pulse will be distorted by both the rise and fall time. The rise time will retard the leading edge of the pulse, shortening it. The fall time will extend the trailing edge of the pulse, lengthening it. Thus, the total pulse distortion will be the difference of the rise and fall times. In order to accurately reproduce the correct lightness, the difference between the rise and fall times must be calculated and corrected.
Pulse Width Distortion Simulation
The transient response of an LED driver will vary heavily with the design. The two primary methods of implementing PWM are to either switch the entire converter off and on, or to add another MOSFET dedicated to interrupting current flow. The former is cheaper and simpler whereas the latter is more precise. In an application that demands a 0-100% smooth fade a dedicated MOSFET will almost always be used. This dedicated MOSFET is usually driven directly from the microcontroller’s PWM output. This causes an issue as the output pin of a microcontroller can often source more current than it can sink. This will cause uneven rise and fall times for the MOSFET, which will distort the length of the pulse. This can be calculated by using a few parameters of the parts to be utilized. The MOSFET switch time can be roughly modelled by dividing the gate charge by the charge current. Thus:
As the MOSFET will not turn on until the gate is charged, and will not turn off until the gate is discharged, the actual pulse width will be:
Using typical values for an IC of 10mA source and 25mA sink, we get a -1.2uS distortion to the pulse width. That is, every pulse width will be 1.2uS shorter than it should be. Even at a PWM frequency of 200 Hz, this causes a significant error as you approach 0.1% duty cycle. Due to the non-linear response to light of the human eye, the driver must be able to accurately reproduce extremely low light levels. To do this, the pulse distortion must be corrected to allow for smooth dimming throughout this range. To correct the error, all pulses are simply extended by the same amount lost by the switching. This software fix can easily be implemented on the microcontroller. At 200 Hz and 12 bit PWM each part represents 1.22uS . As this is close to the determined value of 1.2uS, we can simply add 1 to every 12-bit PWM value to correct.