Brighter coupling has been discussed in previous session. Darker coupling is easier.

Essentially, darker coupling is the opposite of the brighter coupling. Hence, the root cause is actually the same. Figure 3-23 shows the root cause analysis of the darker coupling. The electric charges from OUT₁ is supposed to fully go to LED₀₁ (and the parasitic capacitor C₀₁) as the current path shown in red dotted line, but shared by LED₀₀ (and the parasitic capacitor C₀₀) as the current path shown in blue dotted line. So, LED₀₁ looks darker than usual.

Though darker coupling has the same root cause with brighter coupling, designs are different to solve those issues. The designs are shown in Figure 3-24 and Figure 3-25.

LED₀₁ is normally lit up as the current path shown in red dotted line. However, LED₀₁ also sets a dummy rising one-shot on OUT₀ which produces a fake one-time switch-ON for darker coupling compensation. The parasitic capacitor of LED₀₀ is charged during the short fake switch-ON time. However, the LED is not lit up during this charge process (called dummy rising one-shot).

Figure 3-23 Root Cause Analysis of Darker Coupling

Figure 3-24 Analysis of Darker Coupling Elimination

Figure 3-25 Darker Coupling Elimination by Dummy Rising One-shot

The line ceiling is used to eliminate brighter coupling and dummy rising one-shot is used to eliminate darker coupling. Why can’t only use either one to solve both issues? Actually, line ceiling can’t fully solve darker coupling due to depending on how strong the clamp circuit is. Dummy rising one-shot can not solve brighter issue and makes the issue worse since unchosen lines are floating.

Figure 3-26 shows the comparison demo with or without darker coupling. You can see that the line ceiling circuit works very well. When enabled, we do not see coupling anymore.

Figure 3-26 Comparison Demo With or Without Darker Coupling