Global Shutter Pixel Circuit for Fast ToF Gating in Mobile Vision
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Solution Overview
Problem
Conventional CMOS pixel sensors for time-of-flight (ToF) depth measurement systems face challenges due to large size and high power consumption, necessitating an improved pixel circuit and method for mobile computer vision applications.
Innovation Solution
A pixel circuit using a global shutter CMOS image sensor process flow with fast time-of-flight gating, implemented with a low capacitance and low resistance device structure, employing a shuttering mechanism based on static gate integration and dynamic LDM, and a control method that involves a sequence of temporal windows for light pulses and convolution processing to determine distance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional CMOS pixel sensors are used for ToF depth measurement, then depth measurement capability is achieved, but sensor size and power consumption increase
Solution Approach 1:
The patent combines multiple functions (photodetection, charge storage, transfer gating, and readout) into a single integrated pixel circuit architecture. The photodiode, storage node, and transfer gates are merged into one compact structure, eliminating the need for separate components and reducing overall sensor area while maintaining ToF measurement capability
Solution Approach 2:
The pixel circuit is designed to perform multiple functions: photodetection during exposure, charge storage during transfer, and signal readout. This multi-functional design allows a single circuit structure to replace what would traditionally require multiple separate components, thereby reducing sensor size while preserving depth measurement functionality
2Measurement precision
If conventional CMOS pixel sensors are used for ToF depth measurement, then depth measurement capability is achieved, but power consumption increases
Solution Approach 1:
The patent employs periodic gating signals to control charge transfer between the photodiode and storage node only during specific time windows corresponding to light pulse emissions. This periodic operation allows the sensor to remain in a low-power state during non-exposure periods while maintaining measurement accuracy during active periods
Solution Approach 2:
The transfer gates are dynamically controlled with time-varying signals that enable charge transfer only when needed (synchronized with light pulses). This dynamic control allows the circuit to switch between active measurement mode and low-power standby mode, reducing overall power consumption while preserving depth measurement capability
3Speed
If fast time-of-flight gating is implemented, then measurement speed improves, but device complexity increases
Solution Approach 1:
The patent pre-configures the pixel circuit with storage nodes and transfer gates positioned and biased in advance, ready for immediate charge transfer when light pulses arrive. This preliminary preparation eliminates setup delays and enables fast time-of-flight gating response without requiring complex real-time control mechanisms
Solution Approach 2:
The circuit uses self-aligned gate structures and automatically timed transfer sequences that reduce the need for external control complexity. The transfer gates are controlled by locally generated signals that automatically synchronize with the measurement timeline, simplifying the overall control architecture while maintaining high-speed operation
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
Enables accurate and efficient ToF depth measurement with reduced sensor size and power consumption, suitable for mobile computer vision applications, by using a single control line per pixel and minimizing interference between feedback and active regions.
Implementation Method 1
activating the photodiode in a first plurality of time windows to sense light reflected from a target as a result of a corresponding plurality of emitted light pulses
Implementation Method 2
a shutter gate disposed between the photodiode and the drain region. The shutter gate is controlled by a global shutter signal to apply a bias voltage
Implementation Method 3
The storage diode is coupled to the photo diode through a first transfer gate controlled by a first transfer signal. The floating diffusion region is coupled to the storage diode through a second transfer gate controlled by a second transfer signal
Data Source
AI summary
An imaging system includes an illumination unit and a sensor unit disposed on a printed circuit board. The illumination unit includes a diode laser source inside an illumination housing. The sensor unit includes an image sensor having a pixel array and a lens barrel mounted on the image sensor with an adhesive, and an optical fiber coupled between the illumination housing and image sensor. The optical fiber is configured to collect a portion of light from the interior of the illumination housing that is emitted by the diode laser source and direct the portion of light to a corner of the pixel array of the image sensor that is located outside the lens barrel.