Active LED Matrix Discharge Circuitry for Ghosting Control
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Solution Overview
Problem
Time-multiplexing LED display systems suffer from ghosting effects due to residual charges on LEDs caused by parasitic capacitance, leading to unwanted lighting emission and increased complexity and cost in passive discharge methods.
Innovation Solution
Active discharge circuitry is introduced, which includes a control circuit generating timing and digital signals to actuate a switch, creating a discharge path for unwanted charges, comparing discharge voltage with a reference voltage to actively mitigate ghosting by discharging parasitic capacitance before the next scan cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If time-multiplexing is used to reduce driver quantity and cost, then device complexity and cost are reduced, but ghosting effects occur due to residual charges on parasitic capacitance
Solution Approach 1:
The patent extracts the harmful residual charge from the parasitic capacitance by providing a dedicated discharge path that separates the charge discharge function from the normal LED driving function. This allows the time-multiplexing scheme to be used while actively removing the ghosting-causing charges.
Solution Approach 2:
The discharge path is activated during the off-period between PWM cycles, before the next scan cycle begins. This preliminary discharge action prevents residual charges from causing ghosting effects in the subsequent LED activation, solving the problem proactively rather than reactively.
2Object-generated harmful factors
If passive discharge methods are used to eliminate ghosting, then ghosting effects are reduced, but circuit complexity and cost increase
Solution Approach 1:
The patent merges the discharge function with the existing PWM control structure by using the same control circuitry to activate the discharge path during off-periods. This combination eliminates the need for separate complex passive discharge circuits while effectively reducing ghosting effects.
Solution Approach 2:
The system uses its own existing components (control circuit, switch, parasitic capacitance) to perform the discharge function. The control circuit that already exists for PWM modulation is repurposed to also control the discharge path, making the system self-sufficient without requiring additional external discharge circuits.
3Productivity
If residual charge remains on parasitic capacitance, then fewer drivers are needed for time-multiplexing, but unwanted lighting emission occurs
Solution Approach 1:
The discharge path acts as an intermediary element between the parasitic capacitance and ground. It provides a controlled route for residual charges to dissipate safely during off-periods, preventing these charges from causing unwanted LED lighting while maintaining the efficiency of time-multiplexed operation.
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
The active discharge circuitry effectively reduces ghosting effects by rapidly discharging unwanted charges, eliminating the need for dynamic adjustments in input voltage and reducing complexity and cost compared to passive methods.
Implementation Method 1
the circuitry compares a reference voltage signal with a discharge voltage signal attributable to the charge
Implementation Method 2
discharging a charge stored by parasitic capacitance coupled to a channel of an LED display
Data Source
AI summary
Active discharge circuitry for fast discharging of charge on an LED display matrix includes a mechanism to effectuate circuit path switching so as to electrically connect a charged node to a discharge circuit for controlled discharging of unwanted charge until it reaches a desired (e.g., programmable) value. The active discharge circuitry includes a control circuit generating appropriate timing and digital control signals for starting and stopping (e.g., actuating a switch) the discharge activities. The disclosed techniques accommodate variations in channel-to-channel start times for mitigating ghosting effects that would otherwise be presented from the LED display matrix due to residual (i.e., unwanted) charges remaining electrically loaded on display elements via, for example, charged parasitic capacitance or other such transients, after a current driver of a specific channel has stopped driving.