Slew Rate Control Circuit for CMOS Image Sensor Voltage Stability
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Solution Overview
Problem
CMOS image sensors face voltage spikes during transitions in global shutter mode due to uncontrolled current flow, which can lead to undesirable transistor activation and data errors in rolling mode, while sharing regulators to minimize power consumption and silicon area is desirable.
Innovation Solution
A slew rate control circuit is implemented using internal regulators with shared voltage rails, where a third transistor limits the current slew rate between the control terminal and voltage rails to different rates based on the operating mode, preventing spikes in global shutter mode and ensuring accurate readout in rolling mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If regulators are shared to minimize power consumption and silicon area, then power consumption and area are reduced, but voltage spikes occur during mode transitions
Solution Approach 1:
A slew rate control circuit is introduced as an intermediary component between the shared regulators and the pixel array. This circuit actively monitors and controls the rate of voltage change during mode transitions, preventing voltage spikes while allowing the regulators to remain shared. The intermediary circuit manages the harmful effect (voltage spikes) without requiring separate dedicated regulators for each mode.
Solution Approach 2:
The patent dynamically changes the slew rate parameter of the voltage transition based on the operating mode. During global shutter to rolling mode transitions, the slew rate is controlled to prevent spikes. This parameter adjustment allows the system to maintain stable operation during mode changes while benefiting from shared regulators during normal operation.
2Speed
If current flow is uncontrolled during mode transitions, then switching speed is fast, but voltage spikes cause transistor activation errors
Solution Approach 1:
The slew rate control circuit implements feedback control by monitoring the voltage transition and adjusting the current flow accordingly. During mode transitions, the circuit detects voltage changes and actively controls the slew rate to prevent spikes that would cause erroneous transistor activation. This feedback mechanism ensures reliable operation while maintaining acceptable transition speeds.
Solution Approach 2:
The system dynamically adjusts the current flow characteristics based on the operating mode and transition state. The slew rate control circuit modifies the electrical parameters in real-time during mode transitions, creating a dynamic response that prevents voltage spikes while maintaining fast switching when appropriate.
3Stability of the object's composition
If slew rate is limited to prevent voltage spikes, then voltage stability is improved, but transition time increases
Solution Approach 1:
The slew rate control circuit dynamically adjusts the voltage transition characteristics based on the operating mode. During critical transitions (global shutter to rolling mode), the slew rate is limited to prevent spikes. During other transitions or steady-state operation, the circuit allows faster transitions. This dynamic approach optimizes both stability and transition time based on system state.
Solution Approach 2:
The control circuit applies slew rate limiting selectively during specific transition periods rather than continuously. By identifying critical transition windows and applying control only during those periods, the system maintains voltage stability when needed while allowing faster operation during non-critical phases, thus minimizing overall transition time.
Data Source
AI summary
In an embodiment, an image sensor includes: first and second voltage rails; first and second regulators configured to generate first and second regulated voltage at the first and second voltage rails, respectively; and a plurality of pixels coupled to the first and second voltage rails. Each pixel includes: first and second transistor coupled first and second storage capacitor, respectively. A third transistor is coupled between a control terminal of the first transistor and the first or second voltage rails. The third transistor is configured to limit a slew rate of current flowing between the control terminal of the second transistor and the first or second voltage rails to a first slew rate when the image sensor operates in global shutter mode, and to a second slew rate when the image sensor operates in rolling mode, the first slew rate being smaller than the second slew rate.