Ramp DAC Settling Assist Circuit With Trimmed Assist Current
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Solution Overview
Problem
The ramp settling time of the ramp signal in CMOS image sensors limits the maximum frame rate, which can be improved by reducing the ramp settling time through the use of a low power ramp settling assist circuit with a trimming control circuit.
Innovation Solution
A current integration ramp generator with a low power ramp settling assist circuit, including an assist current source that discharges the output capacitor during the ramp event, and a trimming control circuit to adjust the assist current for variations in pressure, volume, temperature, and process conditions, thereby reducing the ramp settling time and improving frame rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a conventional ramp generator is used without assist circuit, then power consumption is low, but ramp settling time is long which limits frame rate
Solution Approach 1:
The assist current source is activated before the ramp event to pre-charge the output capacitor, and remains active during the ramp event to maintain the ramp signal. This preliminary action prepares the circuit in advance, reducing the settling time required after the ramp event completes, thereby increasing the achievable frame rate.
Solution Approach 2:
The assist current source acts as an intermediary element between the ramp generator and the load capacitor. It provides additional current support during the critical ramp and settling periods, mediating the energy transfer and enabling faster settling without requiring fundamental changes to the ramp generator architecture.
2Loss of time
If assist current is always on to reduce settling time, then ramp settling time decreases, but power consumption increases
Solution Approach 1:
The assist current source operates periodically rather than continuously. It is activated during ramp events and maintains operation only for the duration needed to support the ramp signal and facilitate settling. During non-ramp periods, the assist current is turned off, eliminating unnecessary power consumption while maintaining fast settling performance when needed.
Solution Approach 2:
The assist current source transitions from a static always-on design to a dynamic switched design. The circuit adapts its operation based on the ramp generator's state, being enabled during ramp events and disabled during idle periods. This dynamic behavior optimizes the balance between settling time and power consumption.
3Device complexity
If fixed assist current is used, then circuit is simple, but performance varies with PVT conditions
Solution Approach 1:
The trimming control circuit implements a feedback mechanism that monitors the actual ramp settling performance and adjusts the assist current magnitude accordingly. Based on PVT conditions and measured performance, the control circuit modifies the assist current to maintain optimal settling time, ensuring consistent reliability across different operating conditions without requiring complex circuit redesign.
Data Source
AI summary
A ramp generator includes an operational amplifier having an output to generate a ramp signal. An integration current source is coupled to a first input and a reference voltage is coupled to a second input of the operational amplifier. A feedback capacitor is coupled between the first input and the output of the operational amplifier. A monitor circuit is coupled to the first and second inputs of the operational amplifier to generate an output flag in response to a comparison of the first and second inputs. A trimming control circuit is configured to generate a trimming signal in response to the output flag. An assist current source is configured to conduct an assist current from the output of the operational amplifier to ground in response the trimming signal generated by the trimming control circuit.