Illuminance Detection Circuit Parasitic Capacitance Compensation
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Solution Overview
Problem
Existing illuminance detection circuits in liquid crystal display devices face challenges in achieving accurate illuminance detection, especially under low light conditions, due to the influence of parasitic capacitance and instability in the hysteresis characteristic of Schmidt inverters.
Innovation Solution
The proposed solution involves an illuminance detection circuit with a photosensor, capacitor, comparator, switching circuit, and selection circuit, where a second capacitor is connected to reduce the impact of parasitic capacitance, and a dark current correcting transistor is used to enhance accuracy, with key components formed on the same substrate as the pixels and semiconductor chip, allowing for improved frequency output independence from parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a Schmidt inverter is used as an illuminance-frequency converting circuit, then the circuit can convert illuminance to frequency, but the hysteresis characteristic cannot acquire sufficient accuracy
Solution Approach 1:
The patent introduces a comparator as an intermediary component between the photosensor and the frequency output stage. The comparator compares the voltage across the integration capacitor with a reference voltage, providing a stable switching point that eliminates the hysteresis instability inherent in Schmidt inverters. This intermediary component ensures accurate illuminance detection by creating a well-defined threshold for frequency conversion.
2Ease of manufacture
If the photosensor is formed using a TFT with low-temperature polysilicon, then the photosensor can be integrated on the same substrate, but the photosensor becomes large-sized thus increasing parasitic capacitance
Solution Approach 1:
The patent extracts the parasitic capacitance effect from the measurement equation by using a comparator-based circuit that compares voltage levels rather than directly measuring current. By converting the photosensor output to a voltage signal and comparing it with a reference voltage, the circuit eliminates the influence of parasitic capacitance on the frequency output, thereby maintaining detection accuracy despite the large photosensor size required for TFT integration.
3Ease of manufacture
If the photosensor size is increased to improve manufacturing integration, then parasitic capacitance increases, but this causes the output frequency to become dependent on parasitic capacitance
Solution Approach 1:
The patent implements a feedback mechanism through the comparator that continuously monitors the voltage across the integration capacitor and compares it with a stable reference voltage. This feedback loop ensures that the output frequency is determined by the rate of voltage change (which is proportional to illuminance) rather than by the absolute voltage level affected by parasitic capacitance. The feedback stabilizes the frequency output against variations caused by parasitic capacitance changes.
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 enhances detection accuracy without dependence on parasitic capacitance, even at low illuminance levels, and allows for precise control of backlight brightness, improving display visibility and power efficiency.
Implementation Method 1
a photosensor which changes an optical current in response to illuminance of an external light
Data Source
AI summary
The present invention provides a display device having an illuminance detection circuit. The illuminance detection circuit includes: a photosensor which changes an optical current in response to illuminance of an external light; a capacitor which discharges a charge when the optical current flows in the photosensor; a comparator which compares a voltage at one end of the capacitor and a comparison reference voltage; a switching circuit which is connected to one end of the capacitor and charges the capacitor in response to a level of an output signal of the comparator; and a selection circuit which applies either a first voltage or a second voltage to the other end of the capacitor in response to the level of the output signal of the comparator.