LCD Power-Off Discharge Control Circuit for Residual Image Reduction
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Solution Overview
Problem
The residual image phenomenon in LCD panels occurs due to uneven discharging of pixel capacitors caused by differences in turn-on and turn-off currents of thin film transistors (TFTs) during the power-off state, which cannot be effectively addressed by simply increasing the turn-off current as it affects other display characteristics.
Innovation Solution
A control circuit integrated into the ASIC of the LCD panel that includes a signal-off detector and a gate delay unit to ensure all TFTs are turned on, allowing for rapid discharging of pixel capacitance by delaying the turn-off of the VGH voltage source, ensuring the gate potential is maintained above the threshold for switching on the TFTs during power-off, thereby reducing residual images.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the turn-off current of TFT is increased to speed up pixel capacitor discharging, then the discharging speed is improved, but other display characteristics such as flickering performance deteriorate
Solution Approach 1:
The patent applies preliminary action by turning on all TFT gates before power-off to pre-establish discharge paths. The gate delay unit delays the turn-off timing of gate driving signals, ensuring TFTs remain conductive during the critical discharge phase. This preliminary preparation of discharge paths enables rapid pixel capacitor discharge without requiring increased turn-off current, thereby avoiding flickering issues.
2Use of energy by moving object
If power is turned off immediately, then power consumption is reduced, but residual image phenomenon occurs due to uneven discharging of pixel capacitors
Solution Approach 1:
The system performs preliminary action by activating all TFT gates before power cutoff to establish discharge paths. The gate delay unit ensures gates remain active during the discharge process, enabling complete pixel capacitor discharge before power is fully removed. This preliminary preparation eliminates residual images while maintaining low power consumption during the off state.
Solution Approach 2:
The patent maintains continuity of useful action by keeping TFT gates turned on during the transition period before power-off. The gate delay unit extends the gate signal duration beyond the normal power-off timing, ensuring continuous discharge capability throughout the power-down sequence. This continuous gate activation ensures uniform discharging across all pixels, eliminating residual images while the system transitions to low-power state.
3Ease of manufacture
If different areas of the glass substrate have different etching rates, then manufacturing process is completed, but off-leakage current varies by location causing uneven discharging
Solution Approach 1:
The patent applies local quality by making the gate delay adjustable or area-specific to compensate for location-dependent etching variations. Different regions of the substrate can have customized gate delay settings that match their specific discharge characteristics. This localized adjustment ensures uniform discharge timing across all areas despite manufacturing variations, eliminating the tide phenomenon while preserving the existing fabrication process.
Data Source
AI summary
A control circuit and a method discharging capacitor/transistor, for a liquid crystal display (LCD), are provided. The control circuit includes a signal-off detector and an all-gate-on delay cell. When an LCD power-off signal is detected, a first control signal is transmitted to a power supply module for turning off power except the gate-on voltage, and turning off VGH after a specific delay time. A second control signal is also transmitted to the gate-on-delay cell, so that all gates of the pixel transistors are turned on after a second specific delay time. The charges on the pixel transistor are discharged via a source thereof before the gate-on voltage decreases below a threshold value, such that a residual image phenomenon caused by heterogeneous filming fabrication is reduced.