Shift Register Circuit Feedthrough Voltage Reduction

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Solution Overview

Problem

In liquid crystal displays (LCDs) using amorphous Si transistors, the feedthrough voltage caused by coupling capacitors prevents accurate gray level representation due to voltage drops when the gate voltage changes, leading to incorrect gray level display.

Innovation Solution

A shift register circuit with three driving voltage levels is implemented, utilizing a series of cascading shift registers and pull-down modules to manage voltage levels, reducing or eliminating feedthrough voltage by using transistors and multiplexers to control output signals across different voltage levels.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If a coupling capacitor Cgs is used between gate and source in amorphous Si transistor, then the transistor can function as a switch for pixel control, but feedthrough voltage is generated when gate voltage changes, causing incorrect gray level display

Engineering Contradiction:

Improvetransistor switching functionVSAvoidgray level accuracy

Core Design Contradiction:

Ease of operationVSManufacturing precision

Solution Approach 1:

The invention divides the conventional two-voltage-level shift register into a three-voltage-level system, segmenting the voltage control into distinct levels (first voltage level for high state, second voltage level for intermediate state, third voltage level for low state). This segmentation allows the TFT to maintain stable voltage levels and prevents feedthrough voltage from causing incorrect gray levels, while preserving the switching function through controlled voltage transitions.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention changes the voltage level parameters from conventional two levels to three levels. By introducing an intermediate second voltage level between the first (high) and third (low) voltage levels, the system can control the TFT to stay at predetermined voltage levels. The pull-down module dynamically adjusts the output voltage among these three levels based on clock signals and neighboring pixel states, eliminating gray level inaccuracies caused by feedthrough voltage while maintaining proper transistor switching.

Inventive Principle:

Principle #35Parameter changes

2Device complexity

If conventional two-voltage-level shift register is used, then the circuit structure is simple, but the TFT cannot stay at predetermined voltage level due to coupling capacitor, resulting in incorrect gray level

Engineering Contradiction:

Improveshift register structureVSAvoidvoltage level stability

Core Design Contradiction:

Device complexityVSManufacturing precision

Solution Approach 1:

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention introduces a pull-down module as an intermediary component between the shift register output and the pixel. This module includes additional transistors and control circuits that actively manage the output voltage level by connecting to the second and third voltage levels. The intermediary module compensates for the coupling capacitor effect by dynamically adjusting the voltage, ensuring the TFT maintains predetermined voltage levels without requiring complete redesign of the basic shift register structure.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Duration of action of moving object

If gate voltage changes from high to low level, then the transistor switches off to maintain pixel voltage, but coupling capacitor generates feedthrough voltage that disrupts the voltage level

Engineering Contradiction:

Improvepixel voltage retentionVSAvoidvoltage level accuracy

Core Design Contradiction:

Duration of action of moving objectVSManufacturing precision

Solution Approach 1:

The invention applies preliminary action by proactively managing the voltage transition before the feedthrough effect can cause problems. The pull-down module detects when the gate voltage is changing and preemptively adjusts the output voltage level through the additional control transistors. By anticipating the feedthrough voltage generation and counteracting it in advance, the system maintains accurate voltage levels throughout the switching process, ensuring both proper transistor cutoff and accurate gray level display.

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentUS7317780B2Shift register circuit

Publication Date: 2008.01.08 OPTRONIC SCIENCES LLC

AI summary

A shift registers circuit having a series of cascading shift registers comprises a first transistor coupling to an output signal of a pre-stage shift register, a second transistor coupling to the first transistor, an output and a first clock signal, and a pull-down module coupling to the output, output signals of pre-stage and post-stage shift register, a second and a third voltage level. When the second transistor turns on and the first clock signal is at high voltage level, the output is at a first voltage level. When the signal of post-stage shift register is at first voltage level, the output is at the third voltage level.