Shift Register Bootstrap Capacitor Discharge Circuit
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Solution Overview
Problem
Active-matrix flat screens with integrated shift registers experience a reverse scan effect due to charges on bootstrap capacitors, leading to a mirror effect in image display, where the image is written from bottom to top instead of top to bottom, causing double writing and an inverted visual impression.
Innovation Solution
Incorporating a discharge circuit within each stage of the shift register, activated by a discharge control signal, specifically at startup and potentially with each new video frame, to discharge the bootstrap capacitors and prevent the reverse scan effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If bootstrap capacitors are used in the shift register stages, then the transistor stress is reduced and the same transistor technology can be used for both register and active matrix, but charges accumulate on the capacitors causing a reverse scan effect and mirror effect in image display
Solution Approach 1:
The patent applies preliminary action by discharging the bootstrap capacitors at the beginning of each frame period before the shift register operation starts. This preliminary discharge prevents charge accumulation that would otherwise cause the reverse scan effect, while maintaining the benefits of bootstrap capacitors during normal operation.
Solution Approach 2:
The patent implements periodic action by repeatedly discharging the bootstrap capacitors at regular intervals (each frame period). This periodic discharge prevents continuous charge accumulation and eliminates the mirror effect, allowing the bootstrap capacitors to function reliably throughout operation.
2Productivity
If the shift register operates continuously without discharge, then the image writing proceeds without interruption, but charges on bootstrap capacitors cause double writing and inverted visual impression
Solution Approach 1:
The discharge operation is performed preliminarily at the start of each frame, ensuring that no residual charges from the previous frame affect the current frame's image writing. This preliminary discharge maintains image integrity while allowing continuous operation.
Solution Approach 2:
The patent briefly interrupts the shift register operation during the discharge phase, then quickly resumes it. This short interruption is sufficient to clear charges without significantly impacting overall productivity, and prevents information loss from charge accumulation.
3Reliability
If a discharge circuit is added to each stage of the shift register, then the mirror effect is eliminated, but the device complexity increases
Solution Approach 1:
The patent merges the discharge circuit functionality with the existing shift register structure by using the same transistor components (T1, T2, T3, T4) and capacitors (C1, C2) that are already present in each stage. The discharge function is achieved by controlling existing transistors T2 and T4 through the output signals of following stages, eliminating the need for separate discharge components.
Solution Approach 2:
The transistors T2 and T4 serve dual functions: they control the output node during normal shift register operation and simultaneously act as discharge paths for the bootstrap capacitors when activated by the R-last signal. This multi-functionality adds discharge capability without increasing device complexity.
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 discharge circuit effectively eliminates the mirror effect by ensuring proper initialization and operation of the shift register, maintaining the intended top-to-bottom image writing and display.
Implementation Method 1
Incorporating a discharge circuit within each stage of the shift register, activated by a discharge control signal, specifically at startup and potentially with each new video frame, to discharge the bootstrap capacitors and prevent the reverse scan effect
Data Source
AI summary
A shift register driven by two complementary clock signals is integrated on an active-matrix substrate of a flat screen for controlling selection of rows of image points of the flat screen is disclosed. The shift register includes a control transistor driven by one of the clock signals, and connected between this clock signal and an output node connected to a corresponding row. The shift register also includes a capacitor connected between an internal node connected to the gate of the control transistor. The output node is discharged each time the flat screen starts.