USB 2.0 Transmitter Cascode Output Stage for 2.5V CMOS
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Solution Overview
Problem
Conventional USB transmitters using 3.3V devices are not portable in leading edge CMOS fabrication processes and replacing them with 2.5V devices subjects transistors to stress, increasing production costs and risk of damage during 5V short conditions.
Innovation Solution
A 3.3V output stage design utilizing a cascode approach with modified pullup/pulldown signal levels and cascode biasing that maintains transistor potentials at 2.5V or less, using standard 2.5V transistors and shutting off the pullup path during 5V short conditions to prevent damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If 3.3V devices are used in USB transmitters, then the output signal level requirement is met, but the devices are not portable in leading edge CMOS fabrication processes and add production costs
Solution Approach 1:
The patent changes the voltage parameter from 3.3V to 2.5V by introducing cascode transistors that transform the output voltage level while allowing the use of standard 2.5V CMOS devices in leading edge fabrication processes, thus resolving the contradiction between meeting output signal requirements and fabrication process compatibility
2Ease of manufacture
If 2.5V devices replace 3.3V devices, then fabrication process compatibility is improved, but transistors are subjected to stress and risk of damage during 5V short conditions
Solution Approach 1:
The cascode transistors serve as intermediary elements between the 2.5V devices and the 5V potential, limiting the voltage across the 2.5V transistors to safe levels even during 5V short conditions, thus protecting the devices while maintaining fabrication process compatibility
Solution Approach 2:
The cascode configuration provides beforehand protection by structurally limiting the maximum voltage that can appear across the 2.5V transistors, cushioning them against stress and potential damage before such conditions occur
3Ease of manufacture
If 2.5V devices are used without cascode protection, then production costs are reduced, but accurate models accounting for device degradation are required
Solution Approach 1:
The cascode transistors act as protective intermediaries that eliminate the need for complex degradation models by physically preventing excessive voltage stress on the 2.5V devices, thus reducing both production costs and modeling complexity
Data Source
AI summary
A USB transmitter 3.3V output stage includes a PMOS cascode transistor connected between a PMOS pullup transistor and a USB port data pin, an NMOS cascode transistor connected between an NMOS pulldown transistor and the data pin, and an output driver circuit that generates a pullup signal range of 0.8V to 3.3V, and a pulldown signal range of 0V to 2.5V, whereby the pullup and pulldown transistors are subjected to 2.5V gate-to-source potentials. A protection/bias circuit biases the PMOS cascode transistor during normal operation such that the pullup resistance matches the pulldown resistance, and turns off the PMOS cascode transistor to shut off the pullup path during a 5V short condition. N-wells of the PMOS pullup and cascode transistors are connected to the 3.3V supply via a resistor.