Current-Starved Relaxation Oscillator for Low-Power Sharp Clock Edges
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Solution Overview
Problem
Existing low-frequency relaxation oscillators in wireless sensor circuits face high power dissipation due to long edge times in Schmitt triggers and inverters, which limits battery life in IoT applications.
Innovation Solution
The relaxation oscillator circuit employs current mirroring to starve the Schmitt trigger and inverters, limiting current supply and reducing power dissipation, along with an inverter switching module to manage current flow and prevent short currents, and a Schmitt trigger switching module to reset the Schmitt trigger output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional relaxation oscillators are used for low-frequency operation, then the circuit can trigger sensor readout events, but the Schmitt trigger and inverters draw excessive current during long edge transitions, resulting in high power dissipation
Solution Approach 1:
The patent pre-charges and pre-discharges the capacitor to rail voltages using switching modules before the Schmitt trigger needs to switch. This preliminary action ensures that when the Schmitt trigger does switch, the transitions are sharp and fast, minimizing the time spent in the high-current threshold region. The switching modules S1-S4 are controlled to charge/discharge the capacitor C in advance, preparing the voltage at node TOP for the next oscillation cycle.
Solution Approach 2:
The patent dynamically controls the current supply to the Schmitt trigger and inverters using switching modules. During portions of the oscillation cycle when the Schmitt trigger output is stable away from thresholds, the switching modules disconnect or reduce current supply to these components. This dynamic current management maintains sharp edges when needed while minimizing power dissipation during stable periods.
2Use of energy by moving object
If the oscillator operates at low frequency (1-100 Hz), then the clock period is long (10-1000 ms), but the Schmitt trigger remains close to threshold voltages for extended periods, causing continuous current draw and excess power dissipation
Solution Approach 1:
The switching modules S1-S4 perform preliminary charging and discharging of the capacitor C to the supply rail voltages before the Schmitt trigger needs to transition. This ensures that when the Schmitt trigger does switch, it does so quickly from a well-defined voltage state, minimizing the duration spent near the threshold voltages where excessive current is drawn.
Solution Approach 2:
The patent uses the switching modules to rapidly charge or discharge the capacitor C directly to rail voltages, effectively skipping through the intermediate voltage regions where the Schmitt trigger would draw excessive current. This rushing through of the voltage transition minimizes the time the Schmitt trigger spends in the high-current state, even during long clock periods at low frequencies.
3Productivity
If current sources continuously charge or discharge the capacitor, then the oscillator maintains low-frequency operation, but the constant current flow results in high power dissipation
Solution Approach 1:
The patent replaces continuous current flow with periodic action. The switching modules S1-S4 are controlled to charge or discharge the capacitor C only during specific portions of the oscillation cycle, rather than maintaining continuous current flow. The current sources I_RN and I_RP provide current only when needed for capacitor charging/discharging phases, while the switching modules interrupt current flow during other phases, significantly reducing average power dissipation while maintaining continuous oscillation.
Solution Approach 2:
The patent dynamically controls the current paths using switching modules S1-S4, which are turned on and off based on the oscillation phase. This dynamic switching allows the circuit to maintain oscillation continuity while minimizing the time that current sources are actively charging or discharging the capacitor, thereby reducing overall power dissipation compared to continuous current flow approaches.
Data Source
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AI summary
A relaxation oscillator circuit includes a current mirror configured to receive the input current from the and generate a plurality of starved currents, a Schmitt trigger configured to be current starved by a first starved current of the plurality of starved currents and a plurality of inverters configured to receive a Schmitt trigger output signal and generate an output clock signal, the inverters including a plurality of current starved inverters that are current starved by a second starved current of the plurality of starved currents, the plurality of current starved inverters receiving the Schmitt trigger output signal and generating a first inverter output signal, upon which an output clock signal is based. The relaxation includes a capacitor configured to charge or discharge in response to the output clock signal and a switching module configured to provide current from the current source based on the output clock signal.