OLED Driving Circuit Threshold Compensation via Capacitive Voltage Holding
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Solution Overview
Problem
In OLED displays driven by active matrix methods, the threshold voltage characteristic of driving transistors can cause uneven brightness across pixels, leading to deteriorated display quality, especially when the driving transistor is of the p-channel type and current flow is small, making it difficult to apply the required voltage to the gate during the writing period.
Innovation Solution
The solution involves a driving method and circuit configuration that includes a driving transistor, switching elements, and a capacitive element, where the switching elements control the flow of current to the OLED element, allowing the application of a voltage corresponding to the current flowing into the OLED element, thereby compensating for the threshold voltage deviation of the driving transistor without directly applying it to the gate, reducing the number of control lines needed and shortening the time required for voltage application.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a voltage corresponding to gray scale is applied to the gate of the driving transistor during the writing period, then the OLED element can be driven with the desired brightness, but the threshold voltage deviation of the driving transistor causes uneven current flow and deteriorated display quality
Solution Approach 1:
The patent introduces a capacitor as an intermediary element between the gate of the driving transistor and the data line. The capacitor stores the gate voltage during the writing period and releases it during the light emission period, decoupling the voltage application timing and enabling stable current flow through the OLED element despite threshold voltage variations in the driving transistor
Solution Approach 2:
The patent applies the gate voltage to the driving transistor in advance during the writing period before the light emission period begins. The capacitor charges during this preliminary phase, ensuring that the gate voltage is already established and will maintain stable current flow through the OLED element during the subsequent light emission period, preventing display quality deterioration
2Manufacturing precision
If the driving transistor is diode-connected to compensate for threshold voltage deviation, then the gate voltage can be programmed to flow the required current, but when the current is small the gate voltage becomes high making it difficult for current to flow between source and drain
Solution Approach 1:
The patent divides the operation into distinct periodic phases: a writing period for voltage application and a light emission period for current flow. During the writing period, the transistor operates in voltage-mode to program the gate voltage. During the light emission period, the capacitor maintains this voltage while the transistor operates in current-mode. This periodic separation allows the transistor to function optimally in different modes without conflict
Solution Approach 2:
The patent dynamically changes the circuit configuration by controlling the switching elements. During the writing period, the switching elements connect the data line to the gate through the capacitor. During the light emission period, the switching elements are opened, isolating the gate and allowing the capacitor to maintain the voltage. This dynamic reconfiguration enables the circuit to adapt to different operational requirements
3Manufacturing precision
If the voltage corresponding to the current is directly applied to the gate of the driving transistor, then the current can be controlled accurately, but the time required for voltage application increases
Solution Approach 1:
The capacitor acts as an intermediary that can be charged rapidly during the writing period and then maintain the voltage during the light emission period. This allows the voltage application to be completed quickly during the writing period, reducing the writing time, while still achieving accurate voltage control for the subsequent current flow through the OLED element
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 configuration ensures consistent current flow to the OLED element, independent of the driving transistor's threshold characteristic, maintaining uniform brightness across pixels and reducing the time needed for voltage application, thus improving display quality and efficiency.
Implementation Method 1
a capacitive element one end of which is connected to the gate of the driving transistor
Data Source
AI summary
To reduce a time for applying a target voltage to a gate of a driving transistor. During an initializing period, both ends of a capacitive element become a short-circuited state by turning on transistors, so that node A and B becomes a voltage made by subtracting the threshold voltage Vthp of the driving transistor from a power source voltage VEL. During a writing period, the transistor is turned on and a data signal X-j is supplied to change the voltage at the node B as much as a voltage corresponding the current which is to flow into an OLED element. The node A is changed from the threshold voltage as much as the value obtained by dividing the voltage change by capacity ratio. During a light-emitting period, the transistor is turned on, so that the current corresponding to the voltage at the node A flows through the OLED element.