2.5 Operating Principles of the LED Driver

The LED Driver implementation is fully firmware-based and uses various peripherals of the SAM R21 MCU to drive the LEDs.

Figure 2-4 shows the block diagram of the scheme used in the implementation of buck LED driver.

Figure 2-4. Circuit Block Diagram

The SAMR21 controls the LED peak current using boundary-conduction-mode. The peripherals used for the implementation are:

  1. One Analog Comparator to monitor inductor or MOSFET peak current
  2. One Timer (TC3) for reference voltage generation for the inductor peak current for Amplitude Dimming
  3. One Timer (TC4) for PWM Dimming
  4. One PWM Timer (TCC0) for MOSFET control
  5. One external interrupt input channel for inductor Zero-Current-Detection (ZCD)
  6. Event system

The event system connects the output of the analog comparator and external interrupt (ZCD). It sends a signal to the timer TCC0 to start or stop the PWM signal for the buck MOSFET depending on the mode-of-transition (high-to-low or low-to-high) of analog comparator output and ZCD signals.

The reference for the peak current is generated using timer TC3. Timer TC3 generates a PWM signal at 10kHz with a desired duty-cycle and the PWM signal is passsed through a low-pass filter to give a DC reference for the analog comparator. The duty-cycle of TC3 is varied to adjust the reference voltage to achieve amplitude dimming.

The LED Driver implementation is available in \Light\src\LightPwm.c and corresponding header file.