Low-Voltage Cascode H-Tree Driver for Stable Differential Output
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Solution Overview
Problem
As integrated circuit fabrication advances, the smaller circuit features and lower voltage swings require improved performance from output driver circuits, particularly in maintaining high differential voltage outputs and reducing current source headroom voltage, which is challenging with existing H-tree driver technologies, especially at smaller process nodes like 65 nm.
Innovation Solution
The implementation of low-voltage cascode topology for current sources in H-tree driver circuits, utilizing pairs of transistors connected in series with secondary transistors and bias voltage sources to minimize headroom voltage and achieve high output impedance, thereby enhancing the performance of differential output driver circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional current source topology is used in H-tree driver circuits, then the circuit can operate at standard voltage levels, but the headroom voltage is excessive and differential voltage output is reduced
Solution Approach 1:
The current source is divided into two separate transistors (first and second current source transistors) connected in series, where each transistor contributes to the overall current source function. This segmentation allows the circuit to achieve low voltage operation while maintaining adequate current drive capability, as each transistor operates with a reduced voltage drop compared to a single transistor design.
Solution Approach 2:
The invention changes the voltage operating parameters by using a series configuration of two transistors with specific width-to-length ratios. This parameter change enables the current source to function effectively at lower voltage levels (reduced headroom) while maintaining the required current output for driving differential signals.
2Productivity
If transistor channel length is reduced to increase transistor count density, then more transistors can be packed into the circuit, but voltage threshold control becomes difficult and performance degrades
Solution Approach 1:
The invention applies different width-to-length ratio characteristics to different transistors in the circuit. Specifically, the first current source transistor has a different width-to-length ratio than the second current source transistor, allowing each to be optimized for its specific function. This local differentiation enables precise voltage threshold control even when using short channel lengths for high density.
Solution Approach 2:
By changing the width-to-length parameters of the transistors, the invention compensates for the effects of short channel length. The specific parameter selection allows the transistors to maintain proper voltage threshold characteristics despite the reduced channel length, thereby achieving both high density and precise voltage control.
3Device complexity
If standard H-tree driver topology is used, then the circuit design is simple, but output impedance is insufficient and VOD variation is high
Solution Approach 1:
The current source function is segmented into two transistors working in series, which increases the overall output impedance of the current source. This segmentation creates a higher impedance node that better maintains the differential voltage output, reducing VOD variation without requiring complex additional circuitry.
Data Source
AI summary
An H-tree driver circuit has pull-up and pull-down current sources, each of which is implemented using a low-voltage-cascode topology.