AN-1052 GreenPAK Macro Circuit Design LED Brightness Control

This application note demonstrates how to design a simple LED brightness control using a GreenPAK chip. To control the brightness of an LED you can vary the average power which is sent to the LED. PWM from the GreenPAK can provide the varying levels of power by varying the duty cycle. For example, if we turn the LED on for 50% and off for 50% of the time, the LED will appear half as bright. The pulse width is the important factor here. By varying the pulse width we can effectively control the average light power from the LED. The minimum LED oscillating frequency should be at least 50Hz so the switching is not obvious to the human eye. In this application, we will use 50Hz. When using PWM, duty-cycle refers to the percentage of time a pulse is ‘on’ over the duration of the cycle. At the 50Hz frequency the resulting period is 20ms. During that time we have to turn the LED either on or off depending on the required duty-cycle. For example, a 75% duty-cycle requires the pulse to be HIGH for 15ms (LED will be off) and then LOW for 5ms (LED will be on due to open drain pulldown circuit used for this application).

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