Capacitance-Independent Current Measurement for OLED Pixel Circuits
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Solution Overview
Problem
In organic EL display devices, capacitance variation in current measurement circuits affects accurate measurement of current flowing through pixel circuits, leading to suboptimal video signal correction and image quality.
Innovation Solution
An active-matrix display device with a current measurement circuit that includes a capacitor charged with the current from the pixel circuit and a discharge circuit to discharge the charge at a constant speed, allowing for accurate current measurement independent of capacitor capacitance, using operational amplifiers, switches, and voltage comparators to control the measurement process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a current measurement circuit including a capacitor is used to measure current flowing through the pixel circuit, then current measurement capability is provided, but capacitance variation occurs due to semiconductor process variation and chip differences, degrading measurement precision
Solution Approach 1:
The patent changes the measurement parameter from voltage (which depends on capacitance) to time (which is independent of capacitance). By measuring the time required for the capacitor to charge to a reference voltage level, the system achieves current measurement that is not affected by capacitance variations. This is accomplished by using a counter to measure the number of clock cycles during the charging period, transforming the measurement from an voltage-based to a time-based parameter.
2Illumination intensity
If video signal correction is performed based on current measurement, then image quality should improve, but inaccurate current measurement due to capacitance variation prevents high image quality display
Solution Approach 1:
The patent transforms the measurement approach by changing from voltage measurement to time measurement. The measurement unit measures the time period during which the capacitor charges to a reference voltage, and this time value is used for video signal correction. Since time measurement is independent of capacitance variations, accurate current measurement is achieved even when capacitance varies, enabling high image quality display through accurate video signal correction.
3Adaptability or versatility
If multiple current measurement circuits are provided on multiple semiconductor chips, then current measurement is enabled across the display device, but capacitance variation between chips increases, degrading measurement consistency
Solution Approach 1:
The patent measures current by timing how long it takes for a capacitor to charge to a reference voltage level, rather than measuring the voltage directly. Since the charging time depends on the current magnitude and not on the capacitor's capacitance value, this time-based measurement method produces consistent results across multiple chips even when capacitance values vary due to manufacturing tolerances. The counter unit records the number of clock cycles during charging, providing a capacitance-independent measurement that ensures consistency across all measurement circuits in the display device.
Data Source
AI summary
In a current measurement period of a pixel circuit (20), a measurement voltage is applied to a non-inverting input terminal of an operational amplifier (31), switches (35, 36) turn on, and a switch (37) turns off. Next, the switch (36) turns off, and a capacitor (32) is charged with charge of a current flowing through the pixel circuit (20). Next, the switch (35) turns off, the switch (37) turns on, a constant current flows through a current source (33), and a voltage of a second terminal (terminal connected to an output terminal of the operational amplifier (31)) of the capacitor (32) decreases. A voltage comparator (42) and a counter (43) measure the current flowing through the pixel circuit (20) by measuring time until a voltage of the second terminal of the capacitor reaches a voltage Vcomp. A video signal VS1 is corrected based on a current measurement result. With this, it is possible to compensate for capacitance variation of a capacitance in a current measurement circuit and perform high image quality display.