Dual-Switch Scan Line Control for LED Display Ghosting
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Scanning LED displays experience ghost images and caterpillar phenomena due to parasitic capacitance and short circuits, which cause unintended lighting effects when switching scan lines.
Innovation Solution
Implementing a dual-switch structure with USW(N) for charging and DSW(N) for discharging each scan line, where USW(N) is turned on for a first time interval and DSW(N) is turned on for a second time interval, with the option for overlapping or non-overlapping intervals, and both switches are controlled by external or internal signals to manage the charging and discharging process, ensuring the voltage difference across LEDs is below their forward voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a single switch SW(N) is used to control scan line N, then the device complexity is low, but ghost images and caterpillar phenomena occur due to parasitic capacitance discharge issues
Solution Approach 1:
The single switch SW(N) is segmented into two independent switches: USW(N) for charging the scan line and DSW(N) for discharging the parasitic capacitance. This segmentation allows independent control of charging and discharging operations, enabling the DSW(N) to actively discharge parasitic capacitance before the next scan line is activated, thereby eliminating ghost images and caterpillar phenomena without significantly increasing overall system complexity.
Solution Approach 2:
The DSW(N) is activated in advance before the next scan line switch SW(N+1) is turned on to discharge the parasitic capacitance CpN. This preliminary discharging action ensures that when the next scan line is activated, there is no residual voltage from the previous scan line that could cause unintended LED activation, thus preventing ghost images and caterpillar effects.
2Reliability
If DSW(N) is activated to discharge parasitic capacitance, then ghost images are eliminated, but the device complexity increases due to additional switches and control signals
Solution Approach 1:
The control signals for USW(N) and DSW(N) are merged into a unified control scheme where the enable signal S(N) simultaneously controls both switches. When S(N) transitions from high to low, it triggers both the turn-off of USW(N) and the turn-on of DSW(N). This merged control approach eliminates the need for separate control circuits while achieving the desired discharge function, thus minimizing the increase in control circuit complexity.
Solution Approach 2:
The enable signal S(N) is given multi-functionality: it not only controls the charging switch USW(N) but also controls the discharging switch DSW(N). This universal control signal performs multiple functions (charging control and discharging control) without requiring additional dedicated control circuits, thereby reducing the overall control circuit complexity despite the addition of the DSW(N).
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively eliminates ghost images and caterpillar phenomena by preventing parasitic capacitance-induced glitches and short circuit-related visual effects, ensuring accurate image representation in LED displays.
Implementation Method 1
Each of the scan line will be scanned by activating the corresponding enable signal of the scan line in a pre-determined order, for example, S1→S2→S3→S4. In each scan line(N), there is a corresponding parasitic capacitance Cp1, Cp2, Cp3, Cp4, coupled from the anodes of the LED(s) of the scan lines to the ground voltage, respectively
Implementation Method 2
when the S(N) is enabled, the SW(N) is turned on, or closed; otherwise, SW(N) is turned off or opened. As shown on FIG. 1, when the SW(1) is turned on and the voltage of the anodes of the LED(S) in scan line (1) will be charged to Vs1
Data Source
AI summary
A method to eliminate caterpillar phenomenon in a scanning LED display is disclosed, wherein each scan line comprises a USW(N) for charging the scan line(N) and a DSW(N) for discharging the scan line(N), the method comprising: turning on the USW(N) to charge the scan line(N) for a first time interval; turning on the DSW(N) to discharge the scan line(N) for a second pre-determined time interval; and turning off the DSW(N) after the second pre-determined time interval is elapsed.