Local Ramp Buffer Assist Circuit for Faster Ramp Settling
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Solution Overview
Problem
The ramp settling time of the ramp signal in image sensors limits the maximum frame rate, which in turn restricts the performance of image sensors, as it affects the conversion of image charge to digital representations.
Innovation Solution
A low power ramp settling assist circuit is introduced in local ramp buffer circuits, which includes an assist current source that discharges the output capacitor during the ramp event, reducing the ramp settling time and improving the maximum frame rate by providing an assist current from the output of the local ramp buffer circuit to ground.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the ramp generator generates a ramp signal for ADC conversion, then the image charge can be converted to digital representation, but the ramp settling time limits the maximum frame rate
Solution Approach 1:
The output capacitor is pre-charged to the starting voltage level before each ramp event begins. This preliminary action eliminates the settling time that would otherwise be required to charge the capacitor from zero, allowing the ramp signal to immediately reach its target voltage and significantly reducing the time loss, thereby increasing the maximum frame rate
Solution Approach 2:
The circuit employs periodic charging and discharging of the output capacitor synchronized with each ramp event. A control circuit periodically resets the capacitor to the starting voltage level at the beginning of each conversion cycle, enabling repeated rapid ramp operations without accumulating settling delays, thus improving productivity
2Loss of time
If the output capacitor is charged to the starting voltage level before each ramp event, then the ramp settling time is reduced, but additional circuit complexity is introduced
Solution Approach 1:
The circuit merges the capacitor charging function with the existing ramp buffer circuitry by integrating a charging current source and control logic into the same circuit block. This consolidation achieves the beneficial settling time reduction while minimizing additional complexity through shared components and unified control
Solution Approach 2:
The control circuit automatically detects the start of each ramp event and autonomously triggers the capacitor charging sequence without external intervention. The circuit serves itself by monitoring its own operational state and initiating the pre-charging action, reducing the need for complex external control mechanisms
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
The implementation of the low power ramp settling assist circuit significantly reduces the ramp settling time, thereby enhancing the maximum frame rate and overall performance of the image sensor.
Implementation Method 1
an assist current source coupled between the output of the local ramp buffer circuit and ground and configured to provide an assist current from the output of the local ramp buffer circuit to ground only during a ramp event generated in the ramp signal
Data Source
AI summary
A ramp buffer circuit includes a ramp buffer input device having an input coupled to receive a ramp signal. A current monitor is circuit coupled to a power line and the ramp buffer input device to generate a current monitor signal in response to an input current conducted through the ramp buffer input device. A corner bias circuit is coupled to the current monitor circuit to generate an assist bias voltage in response to the current monitor signal. A bias current source is coupled to an output of the ramp buffer input device. An assist current source is coupled to the corner bias circuit and coupled between the output of the ramp buffer input device and ground to conduct an assist current from the output of the ramp buffer input device to ground in response to the assist bias voltage.