Shift Register Circuit for OLED Scan Driving
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Solution Overview
Problem
In OLED display technology, the low level signal provided by the low level signal terminal VGL is not low enough to switch on the transistor M9 due to threshold loss in PMOS transistors, leading to errors in the shift register output.
Innovation Solution
A scan driving circuit with a shift register design that includes a first node control module, a second node control module, and an output control module, where the second node control module ensures a lower low level is maintained at the second node by controlling the levels at the third and fourth nodes, allowing complete transmission of the low level signal and preventing errors in output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional shift register design is used, then the circuit structure is simple, but the low level signal is not low enough to switch on the transistor due to threshold loss
Solution Approach 1:
The shift register is divided into multiple stages, with each stage independently controlling the voltage level at its output node. This segmentation allows each stage to maintain proper voltage levels despite threshold losses in PMOS transistors, ensuring reliable transistor switching throughout the entire circuit.
Solution Approach 2:
Different nodes within the shift register are assigned different voltage control characteristics. Specifically, the second node is designed to maintain a lower voltage level compared to other nodes, which compensates for the threshold voltage loss in PMOS transistors and ensures proper switching operation.
2Reliability
If the low level signal from VGL is used directly, then the signal transmission is simple, but the level is not low enough due to threshold loss in PMOS transistors
Solution Approach 1:
The circuit performs preliminary voltage adjustment by controlling the second node to maintain a lower voltage level before the signal is transmitted through the PMOS transistor. This preliminary action compensates for the expected threshold loss, ensuring the signal remains at an adequate low level for reliable transistor switching.
Solution Approach 2:
The voltage level parameter at the second node is specifically adjusted to be lower than conventional designs. By changing this voltage parameter, the circuit compensates for threshold loss in PMOS transistors and ensures the low level signal remains sufficient for reliable transistor switching.
3Reliability
If the second node voltage is not sufficiently low, then the circuit operation is simple, but transistor M9 cannot be switched on, leading to output errors
Solution Approach 1:
The circuit incorporates feedback mechanisms where the voltage level at the second node is controlled based on the state of the third node and clock signals. This feedback ensures that the second node maintains the appropriate low voltage level needed to switch on transistor M9, preventing output errors.
Solution Approach 2:
The voltage level at the second node is dynamically adjusted based on the operating phase and clock signals. During specific phases, the second node is pulled down to a lower voltage level to ensure transistor M9 can be reliably switched on, while maintaining flexibility for different operating conditions.
Data Source
AI summary
The present disclosure provides a shift register, a driving method thereof, a scan driving circuit and a display device. The shift register includes: a first node control module configured to control level at the first node based on an input signal and a second clock signal; a second node control module configured to control level at a second node based on the input signal, the first clock signal, the second clock signal, a low level signal and a high level signal; and an output control module configured to control the output terminal to output high level or low level based on level at the first node, level at the second node and the second clock signal. The second node can be provided with a relatively low level, which is conductive to maintaining a normal output of the shift register.