CMOS Imager Output Driver With Constant Slew Rate Control
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Solution Overview
Problem
CMOS imager output drivers experience significant variations in drive strength and slew rate due to fluctuations in power supply voltage, leading to poor simultaneous switching output (SSO) performance, which requires thicker metal tracks and more power supply pads to compensate.
Innovation Solution
The implementation of a power supply independent output driver circuit with two control circuits that regulate the gate-to-source voltage of PMOS and NMOS transistors, ensuring the output is not influenced by power supply voltage variations, using a first control circuit with resistance R1 and current IRp, and a second control circuit with constant power supply VDD and current source IN, to maintain a constant output drive and slew rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a dual power supply strategy is employed in output drivers to provide high output driving capability, then drive strength is improved, but slew rate varies significantly with power supply voltage
Solution Approach 1:
The patent changes the control parameter from direct power supply voltage to gate-to-source voltage controlled by current sources. By controlling the gate voltage of output transistors through regulated current sources rather than directly through power supply voltage, the slew rate becomes independent of power supply variations while maintaining high drive strength capability.
Solution Approach 2:
The patent introduces control circuits with current sources as intermediaries between the power supply and the output transistors. These control circuits regulate the gate-to-source voltage of the output transistors, decoupling the slew rate control from direct power supply dependence and enabling stable performance across varying power supply conditions.
2Adaptability or versatility
If power supply voltage varies to accommodate wide operating range, then adaptability is improved, but output drive and slew rate vary significantly
Solution Approach 1:
The patent employs control circuits that regulate the gate-to-source voltage of output transistors based on current source references. This feedback mechanism ensures that the output drive and slew rate remain consistent even when power supply voltage varies across a wide operating range, as the control circuits actively compensate for power supply variations.
Solution Approach 2:
The patent transforms the control mechanism from direct voltage control to current-based control. By using current sources to regulate gate voltages, the system achieves output drive consistency independent of power supply voltage variations, enabling wide operating range adaptability without sacrificing performance stability.
3Power
If higher supply voltages are used to improve drive capability, then output strength is improved, but simultaneous switching output performance deteriorates
Solution Approach 1:
The patent changes the control parameter from power supply voltage to regulated gate-to-source voltage. By controlling the gate voltage through current sources rather than relying on high power supply voltage, the system achieves high output drive capability while maintaining constant slew rate, thereby improving simultaneous switching output performance without requiring higher supply voltages.
4Speed
If RC time constant circuits are used to control transistor switching, then switching timing is controlled, but slew rate depends on power supply voltage variation
Solution Approach 1:
The patent replaces the RC time constant control mechanism with current source-based gate voltage control. By controlling the gate-to-source voltage through regulated current sources, the system achieves both precise switching timing control and slew rate stability that is independent of power supply voltage variations, overcoming the limitations of RC-based control.
Data Source
AI summary
An output driver having an output that is not dependant on the variation of the voltage level of a variable supply voltage. The output driver, having at least two power supply voltages and which is not influenced by the variation of the voltage level of a variable power supply, leads to a constant output slew rate.