Image Sensor Reset Circuit Floating Diffusion Voltage Control
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Solution Overview
Problem
The existing image sensing devices, such as those described in Japanese Patent No. 4048415, face issues with voltage leakage during reset operations, leading to an increase in the floating diffusion voltage, which prevents the reset transistor from raising the gate-source voltage to the threshold voltage or higher, thereby preventing the floating diffusion from being reset to the intended voltage.
Innovation Solution
An image sensing device with a reset circuit that supplies a first reset voltage based on the power-source voltage before light exposure and a second reset voltage, generated from a reset correction voltage lower than the power-source voltage, during light exposure, to the floating diffusion, allowing for precise voltage control and elimination of voltage shifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single reset voltage based on power-source voltage is used in both reset operations, then the reset transistor can be simplified, but leak current causes voltage increase in the floating diffusion preventing proper reset in the second operation
Solution Approach 1:
The reset voltage is segmented into two distinct voltages: a first reset voltage (VDD) applied during the first reset operation, and a second reset voltage (VRR) applied during the second reset operation. This segmentation allows each voltage to be optimized for its specific operational context, preventing the leak current issue while maintaining circuit functionality.
Solution Approach 2:
The reset circuit dynamically switches between different reset voltages based on the operational phase. The reset voltage control circuit changes the reset voltage from VDD to VRR between the first and second reset operations, adapting to the changing electrical conditions caused by leak current accumulation.
2Manufacturing precision
If the gate-source voltage of the reset transistor is raised to threshold voltage or higher, then the floating diffusion can be properly reset, but leak current prevents achieving the required voltage level
Solution Approach 1:
The second reset voltage VRR is specifically designed to counteract the voltage increase caused by leak current. By applying this corrected voltage during the second reset operation, the system preemptively compensates for the harmful effects of leak current, ensuring the floating diffusion reaches the intended reset voltage level.
Solution Approach 2:
The reset voltage parameter is changed from the power-source voltage VDD to a corrected reset voltage VRR during the second reset operation. This parameter change accounts for the electrical conditions prevailing at that stage, specifically the voltage drift caused by leak current, thereby restoring precise voltage control.
3Measurement precision
If a reset correction voltage circuit is added to generate the second reset voltage, then voltage control precision is improved, but the device complexity increases
Solution Approach 1:
A reset voltage control circuit is introduced as an intermediary component between the power source and the floating diffusion. This mediator generates the appropriate reset voltage (VRR) based on the operational phase, providing precise voltage control without requiring complex modifications to the core photodetector structure.
Solution Approach 2:
The reset voltage control circuit serves multiple functions: it generates both the first reset voltage VDD and the second reset voltage VRR, manages the timing of voltage application, and compensates for leak current effects. This multi-functionality achieves precise voltage control while minimizing the addition of separate dedicated circuits for each function.
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 approach effectively reduces the variation in the reset voltage of the floating diffusion, preventing voltage leakage and ensuring the floating diffusion can be reset accurately, even when leak currents are present, thereby improving the reliability and efficiency of the image sensing device.
Implementation Method 1
a photoelectric conversion element that generates electric-charge corresponding to an amount of received light
Data Source
AI summary
A conventional image sensing device has a problem of a large variation of a reset voltage of a floating diffusion. According to an embodiment, an image sensing device includes a reset circuit switching a voltage to be supplied to a floating diffusion when the floating diffusion is reset. In a first reset operation (PD reset) performed prior to a light-exposure period for exposing a photoelectric conversion element with light, the reset circuit supplies a first reset voltage generated based on a power-source voltage to the floating diffusion. In a second reset operation (FD reset) performed during the light-exposure period for exposing the photoelectric conversion element with the light, the reset circuit supplies a second reset voltage generated based on a reset correction voltage lower than the power-source voltage to the floating diffusion, and then supplies the first reset voltage.