The main cause of downside ghosting is the buildup charge in the parasitic capacitance on OUTn when transitioning between scan lines, the charge on the parasitic capacitance discharges through the LED causing the LED to light up when the LED is not supposed to be.

Figure 3-3 lists the LED legend that is used in the following analysis.

Figure 3-3 LED Legend

Figure 3-4, Figure 3-5 and Figure 3-6 show the root cause analysis of downside ghosting with the simplified time-multiplexing LED driver structure for easier understanding.

So, in P1 operation, when only wanting to light up LED₀₂, the driver turns ON SW₀ and the current path is shown in red dotted line. At this moment, the yellow parasitic capacitor on channel OUT₂ is charged.

During P1 operation, SW₀ is ON, LED₀₂ lights up.

Figure 3-4 Downside ghosting - P1 Operation

Next, in P2 operation, when only wanting to light up LED₁₁, the driver turns on SW₁ and the current path is shown in red dotted line. And at this moment, the yellow parasitic capacitor on channel OUT₁ is charged. However, LED₁₂ is also slightly lit up even channel OUT₂ is closed. Because the parasitic capacitor has been charged previously, and LED₁₂ is lit up through the current path in blue dotted line. So, LED₁₂ is called downside ghosting.

During P2 operation, SW1 is ON, LED11 lights up, and LED12 slightly lights up.

Figure 3-5 Downside ghosting - P2 Operation

Similarly, in P3 operation, when only wanting to light up LED₂₀,LED₂₁ is also slightly lit up and LED₂₂ is slightly lit up, through the current paths in blue and orange dotted line, respectively. The reason is the same as previously due to the channel parasitic capacitors.

The reason that LED₂₁ and LED₂₂ have different brightness is that the corresponding channel parasitic capacitors have different charges to light up the LED. Different quantities of charge flow through the LED to discharge the corresponding channel parasitic capacitor.

During P3 operation, SW2 is ON, LED20 lights up, LED21 slightly lights up, LED22 ever so slightly lights up.

Figure 3-6 Downside ghosting - P3 Operation

So, how to solve this issue? We know that the downside ghosting is because of charges on channel parasitic capacitors. So, if we can remove these charges, then we can possibly to solve this issue.

As shown in Figure 3-7, TLC698x uses a pre-discharge circuit to pull all channels OUT_n down to the set level (Vpd) during the line switch period to maintain LEDs in the next scan line are not lit up, thus eliminating downside ghosting.

Figure 3-7 Downside Ghost Effect Elimination by Pre-discharge Circuit

Figure 3-8 shows the comparison demo with or without downside ghosting. You can see that the pre-discharge circuit works well. When enabled, we do not see ghosting anymore.

Figure 3-8 Downside Ghosting Comparison