AMOLED Pixel Driving Circuit Threshold Voltage Compensation
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Solution Overview
Problem
Variations in threshold voltage and power supply voltage in active matrix organic light emitting diode (AMOLED) displays lead to non-uniform brightness due to the low temperature polysilicon process and voltage drops, necessitating a pixel driving circuit with compensation for improved display uniformity.
Innovation Solution
A pixel driving circuit is designed with a storage capacitor and multiple transistors, including NMOS and PMOS types, that compensates for threshold voltage and power supply voltage variations through specific switching periods and signal control, ensuring consistent brightness by decoupling the electroluminescent element from threshold voltage and power supply voltage fluctuations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional 2T1C pixel driving circuit is used, then the device complexity is low, but the brightness uniformity deteriorates due to threshold voltage variations and power supply voltage drops
Solution Approach 1:
The pixel driving circuit is segmented into multiple functional blocks: a compensation circuit block (including first to sixth transistors and storage capacitor) that handles threshold voltage and power supply voltage compensation, and a driving circuit block (including seventh to tenth transistors) that drives the electroluminescent element. This segmentation allows independent optimization of compensation functions without affecting the overall circuit structure.
Solution Approach 2:
A storage capacitor is introduced as an intermediary element between the compensation circuit and the driving circuit. The capacitor stores the compensated voltage signal during the first period and releases it during the second period, effectively decoupling the electroluminescent element from threshold voltage and power supply voltage fluctuations. This intermediary mechanism enables brightness uniformity improvement without requiring continuous active compensation.
2Manufacturing precision
If threshold voltage compensation is implemented, then the brightness uniformity improves, but the device complexity increases due to additional transistors and capacitors
Solution Approach 1:
The compensation circuit block performs multiple functions simultaneously: it compensates for threshold voltage variations of the driving transistor, compensates for power supply voltage drops, and generates the compensated voltage signal for storage. By integrating these functions into a single circuit block, the patent avoids the need for separate compensation circuits for each parameter, thereby limiting the increase in device complexity.
Solution Approach 2:
The patent changes the operating parameters of the compensation transistors dynamically across different periods. During the first period, compensation transistors are configured to capture voltage signals; during the second period, they are configured to release stored voltages. This temporal parameter changing strategy allows the same hardware components to serve multiple purposes, reducing the overall component count while maintaining compensation effectiveness.
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 solution achieves uniform brightness in AMOLED displays by isolating the electroluminescent element from threshold voltage and power supply voltage variations, thereby enhancing display quality and reducing non-uniformity issues.
Implementation Method 1
a storage capacitor coupled between a first node and a second node
Implementation Method 2
an electroluminescent element coupled between the drain of the first transistor and a second voltage and emitting light in the fourth period
Data Source
AI summary
An image display system comprises a pixel driving circuit. A storage capacitor is coupled between the first and second nodes. The first switch is turned on in the first and second periods. The second switch, coupled to the first node, is turned on in the first and second periods. The third switch, coupled between the second node and the first switch, is turned on in the first, third and fourth periods. The fourth switch, coupled between the second switch and the first voltage, is turned on in the first, third and fourth periods. The fifth switch, coupled between the second node and the first voltage, is turned on in the first, second and third periods. The sixth switch, coupled between the first node and the reference voltage, is turned on in the fourth period. The first transistor is coupled between the first and second switches and is turned on in the fourth period. During the second period, the voltage between source and gate of the first transistor is a threshold voltage. The electroluminescent element emits light in the fourth period.