Sense Amplifier Noise Threshold Control for Low Voltage SRAM
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Solution Overview
Problem
Static random access memory (SRAM) cells face challenges in reliable data read and write operations at reduced operation voltages due to increased static noise, leading to false sensing issues, especially in skew corners where bit-cells struggle to overpower keeper circuits at low voltages.
Innovation Solution
The implementation of a sense amplifier circuit with a noise-resistant NAND gate, such as a half-Schmitt trigger, and NMOS transistors configured to lower the trip point voltage, allowing for effective bit-line read operations at lower power supply voltages by making the keeper circuit easier to overcome and reducing noise susceptibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the operation voltage is reduced to scale down integrated circuits, then power consumption is reduced, but read and write margins are reduced leading to increased error possibilities
Solution Approach 1:
The patent changes the voltage threshold parameter of the sense amplifier by using a keeper circuit with a specific voltage threshold that is lower than the bit-cell threshold. This parameter change allows the sense amplifier to operate reliably at reduced supply voltages while maintaining adequate read and write margins despite the scaling effects.
2Ease of operation
If the local bit-line is kept floating during low frequency operation, then leakage from pass gates discharges the bit-line to zero, but this causes false sensing
Solution Approach 1:
The keeper circuit acts as an intermediary element that mediates between the floating bit-line and ground. It provides a controlled discharge path through its transistor, preventing the bit-line from discharging to zero due to pass gate leakage while avoiding false sensing conditions. The keeper circuit thus enables the bit-line to remain in a stable high state during low frequency operations.
3Reliability
If the keeper circuit is made stronger to prevent false sensing, then bit-cell cannot overpower the keeper at low voltages, but this creates a limitation on the lowest desirable power supply voltage
Solution Approach 1:
The patent changes the voltage threshold parameter of the keeper circuit to be lower than that of the bit-cell. This parameter optimization allows the keeper to be sufficiently strong to prevent false sensing while remaining weak enough to be overpowered by the bit-cell during normal read operations, even at reduced supply voltages. This resolves the contradiction by finding an optimal threshold parameter that satisfies both requirements.
4Adaptability or versatility
If conventional sense amplifier is used at low voltages, then circuit operation is possible, but noise susceptibility increases causing errors
Solution Approach 1:
The patent changes the trip point voltage parameter of the sense amplifier to be lower than the precharged bit-line voltage. This parameter adjustment creates a noise margin that allows the sense amplifier to distinguish valid signal transitions from noise-induced fluctuations, thereby reducing noise susceptibility while operating at low voltages.
Data Source
AI summary
A method of maintaining a voltage level of a bit line of a sense amplifier circuit includes providing a power supply voltage at a power supply node, receiving the power supply voltage from the power supply node with an NMOS transistor, and maintaining a voltage level of the bit line by supplying sufficient current with the NMOS transistor to compensate a leakage current of the bit line. The method includes receiving the voltage level of the bit line with a noise threshold control circuit, inverting the voltage level with the noise threshold control circuit, and driving a sense amplifier output with the noise threshold control circuit.