OLED Pixel Sensing Circuit for Burn-In Compensation
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Solution Overview
Problem
Organic light-emitting diode (OLED) elements suffer from irreversible variations in light intensity leading to issues like image burn-in and residual images, necessitating a method to estimate and control light emission based on deterioration to reduce intensity differences among pixels.
Innovation Solution
A display device with pixel circuits, sense lines, and measurement circuits that include sensing transistors and a correlated double sampling (CDS) circuit to measure and adjust light intensity by supplying sense current from a power line, using a source follower circuit and CDS to calibrate voltage measurements, reducing threshold voltage variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sensing circuit with a sensing transistor is used to measure OLED element voltage, then measurement capability is provided, but threshold voltage variations of the sensing transistor affect measurement precision
Solution Approach 1:
The patent implements feedback by using the measured voltage information to adjust the driving current supplied to the OLED element. The measurement circuit measures the voltage across the OLED element, and this measurement is fed back to control the driving transistor, which adjusts the current to compensate for OLED deterioration and maintain consistent light emission.
Solution Approach 2:
The patent uses a sensing transistor that replicates the electrical characteristics of the driving transistor to measure the OLED voltage. By copying the transistor structure and operating conditions, the sensing circuit can accurately measure voltage without directly interfering with the driving circuit's operation, thereby maintaining measurement precision despite threshold variations.
2Speed
If sense current is supplied from a power line through a switching transistor, then measurement speed is improved, but voltage drop across the switching transistor affects measurement accuracy
Solution Approach 1:
The patent applies preliminary action by pre-charging a capacitor with the OLED voltage before the measurement phase. During the measurement period, the stored voltage on the capacitor is used to drive the sensing transistor, eliminating the need for real-time current switching during measurement. This approach maintains fast measurement speed while avoiding voltage drop errors from switching transistors during the actual measurement.
3Device complexity
If circuit components are minimized in pixel circuits, then device complexity is reduced, but measurement capability is insufficient for accurate OLED deterioration detection
Solution Approach 1:
The patent achieves multi-functionality by designing the sensing transistor to serve dual purposes: it acts as a switching transistor during normal display operation and as a sensing transistor during measurement periods. This universal component performs both switching and measurement functions, reducing the need for additional dedicated sensing components and maintaining low device complexity while enabling accurate OLED deterioration detection.
Data Source
AI summary
Each of pixel circuits includes a light-emitting element, a first power line to supply the light-emitting element with a power-supply voltage, a driving transistor to control current to the light-emitting element, a sensing transistor whose gate electrode is connected to an end of the light-emitting element, a second power line connected to a drain electrode of the sensing transistor, and a first switching transistor connected between a source electrode of the sensing transistor and a sense line. The driving transistor controls sense current from the first power line to the light-emitting element in order to measure a voltage at an end of the light-emitting element. The sensing transistor supplies a source voltage to the sense line when the light-emitting element is being supplied with the sense current. A measurement circuit determines a voltage of the light-emitting element based on an output of the sense line.