CMOS Sensor Row Drivers with Dual Drive Capability
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Solution Overview
Problem
CMOS image sensors face issues with partition noise due to charge injection during transfer control signal transitions and coupling noise from parasitic capacitances between adjacent rows, which degrade image sensor performance and power efficiency.
Innovation Solution
A buffer with dual drive capabilities is implemented, where each row control signal driver has a low drive capability when active to reduce partition noise and a higher drive capability when inactive to minimize coupling noise, using an auxiliary pull-down transistor to maintain inactive rows at a low DC voltage and control the slew rate of the control signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the row control signal driver has high drive capability to quickly switch rows, then row switching speed is improved, but partition noise increases due to charge injection during transitions
Solution Approach 1:
The driver circuit dynamically adjusts its drive capability based on the state of the row control signal line. When transitioning between active and inactive states, the circuit uses a first drive capability that limits slew rate to reduce charge injection. When the row is clearly active or inactive, a second drive capability is used for faster switching. This dynamic adjustment resolves the contradiction between switching speed and partition noise reduction.
Solution Approach 2:
The invention changes the drive capability parameter of the row control signal driver based on the operational state. By detecting whether the row control signal line is in an active or inactive state, the circuit adjusts its output drive strength accordingly. This parameter change allows the system to optimize between speed and noise reduction for different operational phases.
2Speed
If the slew rate of control signals is increased to improve switching speed, then row switching is faster, but partition noise increases due to faster transitions
Solution Approach 1:
The invention implements dynamic slew rate control where the rate of change of the control signal is adjusted based on the operational state. During transitions to or from the inactive state, a higher slew rate is used to quickly establish the new state. During transitions between active states or when already in a stable state, a lower slew rate is used to minimize charge injection and partition noise. This resolves the contradiction between switching speed and partition noise.
Data Source
AI summary
A CMOS image sensor includes a pixel array including a plurality of unit pixels with individual rows of unit pixels being coupled to respective row control signal lines, and a buffer including plural row control signal drivers. Each driver is coupled to a respective one of the row control signal lines and is configured to provide a row control signal pulse to a respective row control signal line in response to an input pulse when the row control signal line is in an active state and to bias the row control signal line at a ground voltage when the respective row control signal line is in an inactive state. Each driver has a first drive capability when the row control signal line is in the active state and a second drive capability greater than the first drive capability when the row control signal line is in an inactive state.