Level Shifter Bias Control for Stable Duty Cycle Across PVT
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Solution Overview
Problem
Level shifters face challenges in maintaining a constant duty cycle across process, voltage, and temperature variations, leading to inefficiencies and potential damage to transistors due to thicker gate oxides required for higher voltage resistance.
Innovation Solution
A level shifter design incorporating a current mirror circuit with coupling transistors receiving a variable bias voltage from a bias voltage generator, controlled by a digital controller to maintain a predetermined duty cycle, such as 50%, through adjustment of bias voltages in response to measured duty cycle changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If transistors are implemented with thicker gate oxide to withstand higher voltage levels, then transistor reliability is improved, but transistor drive capability deteriorates and circuit area increases
Solution Approach 1:
The patent applies different gate oxide thicknesses to different transistors based on their specific voltage exposure requirements. Transistors 22 and 24 that handle lower voltage signals use thinner gate oxide for better drive capability, while transistors 18 and 20 that may expose to higher voltages use thicker gate oxide for protection. This localized differentiation resolves the contradiction by optimizing each transistor's gate oxide thickness to its actual operational needs rather than uniformly thickening all transistors.
Solution Approach 2:
The patent dynamically adjusts the voltage level applied to transistors 18 and 20 through control signal LVI_B_HV. When the input signal is valid, these transistors are enabled with appropriate voltage levels. When the input signal is invalid or during power-up, control signal LVI_B_HV forces these transistors into a safe state, preventing overstress. This dynamic voltage control allows the use of thinner gate oxide transistors while maintaining reliability through active voltage management.
2Reliability
If transistors 18 and 20 are biased using lower voltage level signals to protect transistors 26 and 28, then transistor reliability is improved, but drive capability is lost
Solution Approach 1:
The patent employs control signal LVI_B_HV that provides feedback about the validity of input signals to adjust the biasing of transistors 18 and 20. When input signals are valid, the control signal enables full drive capability. When input signals are invalid or during abnormal conditions, the control signal reduces the voltage level to protect the transistors. This feedback mechanism resolves the contradiction by dynamically balancing protection and performance based on actual operating conditions.
Solution Approach 2:
The patent applies preliminary protective action through control signal LVI_B_HV that preemptively lowers the voltage level to transistors 18 and 20 when invalid input conditions are detected or during power-up sequences. This preliminary protection prevents damage before it can occur, while still allowing full drive capability when conditions are normal. The protection is applied in advance based on signal validation rather than waiting for damage to occur.
3Reliability
If transistors 22 and 24 are implemented using thicker gate oxide, then resistance to higher voltage damage is improved, but IC surface area increases
Solution Approach 1:
The patent implements local quality by assigning different gate oxide thicknesses to different transistor groups based on their specific voltage exposure. Transistors 22 and 24 that operate at lower voltage levels use thinner gate oxide, reducing their area requirements. Only transistors that actually need to withstand higher voltages use thicker gate oxide. This selective approach resolves the contradiction by minimizing total IC area while maintaining necessary voltage resistance where required.
4Reliability
If level shifter uses thicker gate oxide transistors, then protection against voltage damage is improved, but duty cycle stability across PVT variations deteriorates
Solution Approach 1:
The patent applies local quality by using thinner gate oxide for the differential input transistors (22, 24) that directly process the input signals. Thinner gate oxide transistors have better voltage-controlled current characteristics, which improves duty cycle stability across PVT variations. The thicker gate oxide is selectively applied only to transistors (18, 20) that need voltage protection, not to all transistors. This selective differentiation resolves the contradiction by optimizing duty cycle stability in the signal path while maintaining protection where needed.
Data Source
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AI summary
A level shifter includes a level shifting circuit, a variable bias voltage generator, and a bias voltage generator controller. The level shifting circuit is configured to level shift an input signal at a first voltage level to an output signal having a second voltage level. The second voltage level is higher than the first voltage level. The level shifting circuit includes a current mirror, an input circuit for receiving the differential input signals, and a coupling circuit for coupling the current mirror to the input circuit in response to a variable bias voltage. The variable bias voltage generator is configured to provide the variable bias voltage at one of a plurality of voltage levels. The bias voltage generator controller provides a select signal to select the voltage level from the plurality of voltage levels in response to measuring the duty cycle of the output signal to maintain the duty cycle of the output signal at a predetermined duty cycle.