How to Eliminate False Image Current in Color LED Display Systems

Multiplexing LED drivers can help improve efficiency and reduce costs; however, designing multiplexed LED circuits can be tricky. Poorly designed circuits can create unwanted LED currents and artifacts in the application. This application note details the issues associated with multiplexing LEDs and explains how to overcome them using the MAX6972–MAX6975 family of pulse-width-modulation LED driver boards.

The MAX6972–MAX6975 are constant-current LED drivers for high-speed color and video display boards. The MAX6972/MAX6973 can directly drive 16 LEDs, or 32 multiplexed LEDs, while the MAX6974/MAX6975 can directly drive 24 LEDs, or 48 multiplexed LEDs. The benefit of multiplexing is that it doubles the number of LEDs that can be driven by each driver, thereby substantially reducing costs.

However, poorly designed LED multiplexing circuits can create artifacts. Artifacts can occur when stray current flows through the LED when it is off (i.e., no current is flowing); this results in very faint displays or artifacts. These artifact currents typically come from stray capacitance associated with long traces that share a common anode with the LEDs, as well as stray capacitance caused by the LEDs themselves being slightly forward biased. The MAX6972–MAX6975 family of constant-current LED drivers can prevent these artifacts in display systems through careful multiplexing circuit design.

Typical Multiplexing Circuit

Figure 1 shows a typical multiplexing circuit for the MAX6972–MAX6975 (also known as the MAX6972 and MAX6974 evaluation kit).



The multiplexing transistors (Q1 and Q3) are alternately turned on by the MAX6972–MAX6975, while the constant-current sink driver pins (OUT0–OUTn) alternately control the setting between two states. In state 1, /MUX1 is low, Q1 is turned on, and node A is pulled up to VLED, thereby connecting all green LED anodes to the LED supply. Similarly, in state 0, /MUX0 is low, Q3 is turned on, and all red LEDs are connected to the VLED supply. The /MUX0 and /MUX1 outputs are driven by an open-drain circuit that sinks the base current flowing through the 562Ω resistor, turning on the pnp transistor. When /MUX0 and /MUX1 are off, the open-drain outputs are effectively open circuits, allowing the base-emitter resistors (182Ω each) to turn off the pnp transistor. Between each /MUX0 and /MUX1 state, Q1 and Q3 are off for 16 internal clock cycles (CLKI), as shown by tEMUX in Figure 2.



Ghost Image Current in a Typical Circuit

When the multiplexing state changes from /MUX0 to /MUX1 and vice versa, stray currents can cause ghost images to appear. This effect is most pronounced when the LEDs in the multiplexing circuit are different colors (emission wavelengths), so the voltage drops can be very different at certain currents.

For simplicity, the Figure 1 multiplexing circuit is simplified in the following discussion to show only one red and one green LED. In the example below, /MUX0 drives the red LED through Q3 and /MUX1 drives the green LED through Q1.

The voltage drop across the LED is: VRED = 2.0V; VGREEN = 3.1V

The power supply is: V+ = 3.3V; VLED = 5.0V

The stray current caused by multiplexing LEDs with different forward voltage drops is best described in State 0, where /MUX0 is set low and the red LED is on (Figure 3).



When Q3 turns on, the anode of the red LED (node ​​B) is pulled up to 4.9V. Current flows through the red LED and the constant current driver (OUT0) at the active port (i.e., the channel driving the LED for any PWM cycle). The stray capacitance at node B (shown as lumped parameter CB) is charged to 4.9V. The LED cathode is forced to the following voltage, which is approximately equal to:

4.9V - VRED = 2.9V (Equation 1)

At the end of State 0, the OUT0 driver stops working and /MUX0 goes high (inactive), disconnecting the anode voltage from the LED power supply. Since there is no discharge path, the voltage on the red LED PN junction remains close to the 2.0V forward voltage drop. Similarly, since there is no discharge path, the voltage VCB on the stray node capacitance remains at 4.9V. This voltage state remains unchanged during the intermediate state stage of 16 CLKI cycles.

When State 1 begins, /MUX0 is set to a low level, Q1 is turned on, the anode of the green LED is connected to 5V, and the OUT0 current driver of the selected LED begins to work. The final stable state is shown in Figure 4.




The cathode voltage is lower than the green LED voltage drop, which is approximately equal to:

4.9V - VGREEN = 1.8V (Equation 2)

The 1.8V voltage on the cathode of the red LED indicates that the anode cannot be higher than 1.8V + VRED = 3.8V. At the beginning of State 1, the common cathode voltage (OUT0 voltage in the figure) must change from 2.9V to 1.8V. This voltage change requires CB to discharge from 4.9V to 3.8V or even lower. The CB discharge current flowing through the red LED causes the display to flicker slightly, as shown in Figure 5.



In the previous state, there is always a CB discharge current, whether the red LED is on or off. In state 0, the voltage at node B is always charged to 4.9V. Since VRED is less than VGREEN when sharing a common cathode connection, node B will discharge through the red LED. Depending on the slightly different forward voltage drops across the various LEDs, the CB discharge can result in a faint flickering of one or more red LEDs, as shown in Figure 1.

Eliminating ghost image currents

Giving stray node capacitance a discharge path and enough time to discharge can eliminate ghost image currents. This can be achieved by adding resistors R1 and R2, as shown in Figure 6. Select appropriate resistor values ​​to achieve sufficient discharge during the idle period of the multiplexed state.



Adjust resistors R1 and R2 to discharge nodes A and B during the intermediate state interval to prevent the LED from being forward biased when starting the next operating cycle. In the example shown, node B must be discharged from 4.9V to less than 3.8V before starting state 1. The

intermediate state time is controlled by the system clock frequency, and the maximum clock frequency is 33MHz. With this maximum frequency, the value of R2 can be determined.

The intermediate state time (tEMUX in Figure 2) comes from the system clock frequency:

tCLKI = 1/33MHz = 30.3ns (Equation 3)

and

tEMUX = 16×tCLKI = 485ns (Equation 4)

Each LED is 150pF (from the combined capacitance of the trace, package leads and the small amount of bias on the LED PN junction). Multiplying by the 8 LEDs at each node, the approximate stray anode capacitance can be estimated:

CB = CA = 150pF × 8 = 1.2nF (Equation 5)

Substituting the above values ​​into the equation, the required discharge current for CB can be estimated:

IDIS_B = CB × ΔVCB / Δt (Equation 6)

Substituting the above values ​​into the equation, the required discharge current for CB can be estimated:

IDIS_B = 1.2nF × (4.9V - 3.8V) / 485ns

IDIS_B = 2.7mA

The resistor value that produces a rated 2.7mA discharge current at the lowest voltage in the required range is:

R2 = 3.8V/2.7mA (Equation 7)

R2 = 1.4kΩ

The same calculations can be made for IDIS_A and CA. However, due to the different effects of the LED forward voltage drop, the ghost-image current has different effects on the transition from state 1 to state 0. In the circuit of Figure 6, it can be seen that no ghost-image current occurs during the transition from state 1 to state 0. However, with the same values ​​for R1 and R2, the red and green LEDs can be interleaved between the /MUX0 and /MUX1 states.

Resistors R1 and R2 add a small current load to transistors Q1 and Q3 during each state:

IRn = 4.9V/1.4kΩ = 3.5mA (Equation 8)

The current does not flow through the constant-current driver output OUT0 or through the LEDs, so it does not affect the calibrated LED current.

Conclusion



The MAX6972–MAX6975 multiplexing circuit eliminates ghost-image current in multiplexed display systems by ensuring intermediate state dwell time for stray node capacitance to discharge. Each MAX6972–MAX6975 device adds two resistors at very little cost, ensuring a clear image without artifacts.