4T2C Pixel Circuit for Top-Emitting AMOLED Panels
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Solution Overview
Problem
The high impedance of the transparent cathode in large-size top-emitting AMOLED panels leads to a significant IR voltage drop in the low voltage power source VSS, causing the driving TFT to operate in the linear zone instead of the saturation zone, resulting in circuit failure.
Innovation Solution
A 4T2C pixel circuit is introduced, featuring four thin film transistors and two capacitors, with a scan signal timing sequence that includes a compensation phase and a light-emitting phase, maintaining an unchanged voltage difference between the high and low voltage power sources, and using LTPS or oxide semiconductor TFTs to ensure the driving TFT operates in the saturation zone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a transparent cathode is used in large-size top-emitting AMOLED panels, then the aperture ratio is improved, but the impedance increases causing significant IR voltage drop
Solution Approach 1:
The patent changes the voltage parameters of the power sources. Specifically, it increases the voltage of the high voltage power source VDD and adjusts the voltage difference between VDD and VSS to compensate for the IR voltage drop caused by the high impedance transparent cathode, ensuring the driving TFT operates in the saturation zone.
2Reliability
If the voltage difference between VDD and VSS is increased to compensate for IR drop, then the driving TFT can operate in saturation zone, but power consumption increases
Solution Approach 1:
The patent applies different voltage adjustments to different parts of the circuit. The high voltage power source VDD is adjusted to provide sufficient voltage headroom for the driving TFT to operate in saturation zone, while the low voltage power source VSS is kept relatively stable, thus compensating for IR drop locally at the cathode without unnecessarily increasing overall power consumption.
Data Source
AI summary
The invention discloses a pixel circuit for top-emitting AMOLED panel and driving method thereof. The pixel circuit comprises: first TFT (T1), connected to first node(G), second node(S) and third node(D); second TFT(T2), connected to scan signal(Scan), first node(G) and data signal(Data1); third TFT(T3), connected to scan signal(Scan), second node(S) and reference voltage(Ref); fourth TFT(T4), connected to scan signal(Scan), third node(D) and high voltage power source(Data2); first capacitor(Cst), connected to first node(G) and second node(S); second capacitor(C), connected respectively to third node(D) and reference voltage(Ref); OLED, connected to second node(S) and low voltage power source(VSS); the voltage difference between high voltage power source(Data2) and low voltage power source(VSS) maintaining unchanged. The invention also provides a corresponding driving method. The invention can effectively relieve the IR voltage drop caused by increased impendence of the transparent cathode.