Look-Ahead Pull-Up Assist in High-Speed Level Shifters
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Solution Overview
Problem
Conventional level shifters introduce significant delay due to the struggle between NMOS and PMOS transistors, leading to large timing windows and setup/hold issues, particularly when operating in dual voltage rail systems with large voltage differentials, which negatively affects memory designs at higher speeds.
Innovation Solution
The implementation of a high-speed level shifter that includes a look-ahead module and a pull-up assist module to enhance the rising speed of output signals by precharging and assisting the transition from a low to a high voltage level, using a cross-coupled pair of semiconductor devices and additional PMOS transistors to reduce delay.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If traditional level shifters are used, then voltage level shifting from low-voltage domain to high-voltage domain is achieved, but the output transition time is slow causing large timing windows and setup/hold issues
Solution Approach 1:
The look-ahead circuit detects the falling edge of the inverted output signal in advance and activates the pull-up assist circuit before the actual voltage transition occurs. This preliminary action prepares the pull-up path by turning on the additional PMOS transistors ahead of time, so when the main PMOS transistor turns on, the voltage can rise much faster without waiting for the gradual charging process
Solution Approach 2:
The pull-up assist circuit introduces additional PMOS transistors as intermediary elements that work in parallel with the main PMOS transistor. These intermediary transistors provide an extra conduction path for charging the output node, effectively mediating the voltage transition process to achieve faster rise times while maintaining the basic level shifting function
2Productivity
If conventional level shifting approaches are used, then voltage domain translation is achieved, but delay is excessive at higher operating speeds
Solution Approach 1:
The look-ahead circuit performs preliminary detection of the input signal transition and pre-activates the pull-up assist path before the main level shifting transition occurs. This eliminates the propagation delay through the inverter by preparing the assist circuit in advance, allowing the output to transition faster and supporting higher operating speeds
Solution Approach 2:
The level shifting function is segmented into multiple parallel paths: the main PMOS transistor path and the additional PMOS assist path. By dividing the single transition path into multiple concurrent paths, the overall transition time is reduced, enabling faster operation at higher speeds
3Adaptability or versatility
If cross-coupled pair of semiconductor devices is used, then voltage level shifting between domains is achieved, but the struggle between NMOS and PMOS transistors causes significant delay
Solution Approach 1:
The pull-up assist circuit with additional PMOS transistors acts as an intermediary that reinforces the PMOS action during the voltage transition. When the PMOS transistor needs to pull up the output node, the assist transistors provide additional conduction channels, mediating the NMOS-PMOS struggle by strengthening the PMOS side without affecting the NMOS operation
Solution Approach 2:
The pull-up assist circuit provides counterbalancing strength to offset the delay caused by the NMOS-PMOS struggle. By adding extra PMOS transistors in parallel, the overall pull-up strength increases, counteracting the resistive effect of the NMOS transistor and reducing the net delay in the voltage transition
Data Source
AI summary
A circuit including an output node and a cross-coupled pair of semiconductor devices configured to provide, at the output node, an output signal in a second voltage domain based on an input signal in a first voltage domain is described herein. The circuit further includes a pull-up assist circuit coupled to the output node; and a look-ahead circuit coupled to the pull-up assist circuit, wherein the look-ahead circuit is configured to cause the pull-up assist circuit to assist in increasing a voltage level at the output node when there is a decrease in a voltage level of an inverted output signal in the second voltage domain from a high voltage level of the second voltage domain to a low voltage level of the second voltage domain.