Local Ramp Buffer Assist Current for Faster Ramp Settling
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Solution Overview
Problem
The ramp settling time of the ramp signal generated by the ramp generator in CMOS image sensors limits the maximum frame rate of image sensors, necessitating a reduction in this time to enhance performance.
Innovation Solution
Incorporating a low power ramp settling assist circuit within local ramp buffer circuits, which includes an assist current source that switches on during ramp events to charge an output capacitor, reducing the ramp settling time and improving frame rate by minimizing loading effects and power line current changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the ramp generator generates a ramp signal with conventional settling time, then the circuit operation is stable, but the maximum frame rate of the image sensor is limited
Solution Approach 1:
The assist current source is activated in advance during the ramp phase to pre-charge the output capacitor, preventing the delay that would otherwise occur during the ramp settling period. This preliminary action ensures the output capacitor is ready faster, directly reducing ramp settling time and enabling higher frame rates
2Loss of time
If the output capacitor is charged during ramp events, then the ramp settling time is reduced, but additional current demand is created
Solution Approach 1:
The assist current source operates periodically only during ramp events rather than continuously, activating the additional current demand only when needed to charge the output capacitor. This periodic operation reduces the overall power consumption compared to continuous operation, while still achieving the goal of reduced ramp settling time
3Loss of time
If the assist current source is activated during ramp events, then the ramp settling time is reduced, but power line current changes increase
Solution Approach 1:
The bias current source acts as an intermediary that compensates for the current demand of the assist current source by adjusting its own output. This mediation maintains the total current drawn from the power line relatively constant, reducing power line current changes while still enabling the assist current to reduce ramp settling time
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 solution effectively reduces the ramp settling time, thereby increasing the maximum frame rate and improving the overall performance of image sensors by maintaining constant input current and reducing power consumption during ramp events.
Implementation Method 1
an output capacitor coupled to an output of the local ramp buffer circuit
Implementation Method 2
an assist current source that provides an assist current from the output of the local ramp buffer circuit to ground
Data Source
AI summary
A ramp buffer circuit includes an input device having an input coupled to receive a ramp signal. A bias current source is coupled to an output of the input device. The input device and the bias current source are coupled between a power line and ground. An assist current source is coupled between the output of the input device and ground. The assist current source is configured to conduct an assist current from the output of the input device to ground only during a ramp event generated in the ramp signal.