The MAX6972–MAX6975 are constant-current LED drivers designed for high-speed color and video display boards. These devices can drive up to 16 LEDs (or 32 multiplexed LEDs) with the MAX6972/MAX6973, and up to 24 LEDs (or 48 multiplexed LEDs) with the MAX6974/MAX6975. Multiplexing allows for more LEDs to be driven using fewer driver channels, which helps reduce system costs.
However, improper circuit design in multiplexed LED systems can lead to unwanted artifacts. These artifacts occur when stray currents flow through LEDs that are supposed to be off, causing a faint glow or flicker on the display. This is especially common in systems where LEDs of different colors are used, as their forward voltage drops may vary significantly, leading to unpredictable current paths.
To address these issues, the MAX6972–MAX6975 family includes built-in features that help prevent such artifacts by carefully controlling the timing and switching of the LED multiplexing signals. This ensures that each LED is properly turned off and that no residual current flows through it during the inactive state.
A typical multiplexing circuit uses transistors controlled by the MUX0 and MUX1 pins to switch between different LED groups. For example, /MUX0 might control the red LEDs while /MUX1 controls the green LEDs. The system alternates between these states at a specific clock frequency, ensuring that only one set of LEDs is active at a time.
During transitions between states, stray capacitances on the LED nodes can cause temporary voltage changes that lead to small currents. These currents can result in visible flickers if not properly managed. To eliminate this, additional resistors (R1 and R2) can be added to provide a discharge path for the stray capacitances, ensuring that the voltage levels return to safe levels before the next cycle begins.
By selecting appropriate resistor values based on the system clock frequency and the expected capacitance, designers can ensure that the LED display remains stable and artifact-free. This approach helps maintain consistent brightness and color accuracy across all LEDs, even in complex multiplexed configurations.
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