Brightness Control for Multiplexed LEDs
To add PWM intensity control to connected LEDs, a switched-LED power supply approach can also be used. A circuit using a PWM-based supply to provide additional intensity control to some LEDs is shown on figure below. The PWM waveform and I²C commands to the LED driver is sent from the same microcontroller. A software routine can control the PWM waveform. We can apply this technique to the MAX6969 with constant-current LED ports but no built-in intensity control, and to the MAX6956 with adjustable constant-current LED ports.
In this approach, the duty cycle of the PWM waveform controls the intensity of the LEDs through a transistor. We can control LED intensity using microcontroller indirectly through the LED driver and directly through the transistor. For the MAX6956 example, additive effects with no interference can be produced by the combination of constant-current and PWM duty-cycle adjustments. [Circuit’s schematic diagram source: maxim-ic.com]