CMOS Image Sensor Bias Circuit for Low Voltage Charge Transfer
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Solution Overview
Problem
Existing image sensor technologies face difficulties in maintaining proper charge transfer and linearity when the supply voltage is decreased, making it challenging to adjust voltages for efficient charge transfer and ensuring transistor M2 operates in a linear state across the desired voltage range.
Innovation Solution
The solution involves a bias circuit that temporarily increases the voltage at the sense node during charge transfer using capacitive coupling, and adjusting circuit parameters to maintain transistor M2 in a linear state during read cycles, while also using a switch to connect the read column directly to the charge voltage source without intermediate transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If the supply voltage is decreased, then power consumption is reduced, but charge transfer efficiency and linearity are compromised
Solution Approach 1:
The patent applies dynamics by making the voltage at the sense node time-dependent through capacitive coupling. During the charge transfer phase, the voltage at the sense node is dynamically increased above the supply voltage to ensure efficient charge transfer, while during other phases it is maintained at the supply voltage. This dynamic voltage adjustment allows the system to operate reliably at reduced supply voltages.
Solution Approach 2:
The patent changes the voltage parameter at the sense node during charge transfer by using capacitive coupling to temporarily raise it above the supply voltage. This parameter change enables the transfer transistor to operate efficiently even when the supply voltage is reduced, as the effective voltage swing during charge transfer is enhanced by the capacitive coupling effect.
2Use of energy by stationary object
If the supply voltage is decreased, then power consumption is reduced, but transistor linearity is compromised
Solution Approach 1:
The patent maintains transistor linearity by dynamically adjusting the voltage at the sense node during the read phase. The capacitive coupling mechanism allows the voltage to be raised during charge transfer, while during reading it is maintained at appropriate levels to ensure transistor M2 operates in its linear region. This dynamic control preserves linearity even at reduced supply voltages.
3Reliability
If voltage is increased during charge transfer, then charge transfer efficiency is improved, but voltage swing requirements increase
Solution Approach 1:
The patent uses capacitive coupling as an intermediary mechanism to transfer charge. The capacitor stores charge during the voltage swing phase and releases it during the charge transfer phase, effectively mediating the charge transfer process. This allows efficient charge transfer without requiring large voltage swings, as the capacitor acts as an energy buffer.
Solution Approach 2:
The patent performs preliminary action by charging the capacitor during a voltage swing phase before the actual charge transfer. The capacitor is charged when the voltage at the sense node is raised, and then this stored charge is used during the charge transfer phase to enhance the transfer efficiency without requiring continuous large voltage swings.
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 ensures efficient charge transfer from the photodiode to the sense node while maintaining linearity properties, even with reduced supply voltage, by optimizing voltage levels and transistor operation.
Implementation Method 1
a bias circuit (102) capable of temporarily increasing the voltage at said sense node during charge transfer
Data Source
AI summary
A device for controlling an image sensor including at least one photosensitive cell including a photodiode capable of discharging into a sense node via a first MOS transistor, the sense node being connected to the gate of a second MOS transistor having its source connected to a processing system. The device includes a bias circuit capable of increasing the voltage of the source during the discharge of the photodiode into the sense node.