MOS Capacitor Boosting Pixel Circuit Luminance Control
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Solution Overview
Problem
In organic EL display devices, the existing pixel circuits with P-channel type transistors face challenges in accurately controlling the gate voltage during the writing period, leading to luminance issues as the image is shifted to the lower luminance side due to inadequate charge holding and voltage control, especially when the scanning line changes from low to high level.
Innovation Solution
The introduction of a MOS capacitance connected to the scanning line and the control terminal of the driving transistor, which changes capacitance value based on the scanning signal level, allows for greater charge holding during the writing period and reduced pulling when the period ends, enabling accurate voltage control and luminance-dependent image display.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a low transconductance transistor is used for the driving transistor to facilitate control of drive current, then the control capability is improved, but the writing speed deteriorates causing charge shortage
Solution Approach 1:
A boost capacitor is introduced as an intermediary component between the data line and the gate of the driving transistor. This capacitor enables rapid voltage application to the gate during the writing period, allowing the low transconductance transistor to still achieve proper charging despite its limited current control capability. The boost capacitor acts as a mediator that compensates for the slow response of the low transconductance transistor.
Solution Approach 2:
The boost capacitor is pre-charged to the data voltage before the writing period begins. When the writing period starts, this pre-charged capacitor rapidly discharges to charge the gate of the driving transistor, achieving fast writing action before the low transconductance transistor can limit the charging speed. This preliminary charging action ensures that the gate reaches the required voltage level within the writing period.
2Productivity
If the scanning line potential changes from low to high level, then the writing period ends and the circuit transitions, but voltage pulling occurs causing luminance shift to lower side
Solution Approach 1:
The boost capacitor is designed to counteract the voltage pulling effect that occurs when the scanning line potential changes. By maintaining a charge on the boost capacitor during the writing period and controlling its discharge, it creates an opposing voltage action that compensates for the pulling effect. This preliminary anti-action prevents the gate voltage from dropping when the scanning line transitions, thereby maintaining accurate luminance control.
3Manufacturing precision
If the gate voltage is not charged to data voltage within the writing period, then the writing transistor cannot complete charging, but extending the writing period increases the risk of voltage pulling
Solution Approach 1:
The circuit utilizes periodic action by charging the boost capacitor during the initialization period and then utilizing this pre-charged state during the writing period. This periodic charging and discharging cycle allows the gate to be rapidly charged during the brief writing period without extending the writing time, thereby avoiding voltage pulling while achieving accurate voltage control.
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
This solution ensures that the pixel circuit can charge to the data voltage for a short time during the writing period, maintaining image luminance based on the data voltage, and suppresses voltage pulling when the writing period ends, resulting in improved luminance control and display accuracy.
Implementation Method 1
a MOS capacitance configured to control a voltage applied to a control terminal of the driving transistor by use of a capacitance changing depending on a level of a scanning signal supplied to the scanning line
Data Source
AI summary
The present application discloses to provide a display device capable of displaying an image with a luminance depending on a data signal by controlling pulling of a gate voltage of a driving transistor occurring when a writing period starts and ends and a driving method of the display device.A pixel circuit including a compensation circuit compensating variation of a threshold value of a driving transistor is provided with a boost capacitor including a MOS capacitor between a node connected to a gate terminal of the driving transistor and a scanning line. A current value of a drive current is controlled by the driving transistor by using the pulling of the potential of the node being different between a case that a low level voltage is applied the scanning line connected to the boost capacitor and a case that a high level voltage is applied.