Band-gap Reference Circuit Chopping for Low-Voltage Stability
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Solution Overview
Problem
Conventional current-mode bandgap reference (BGR) circuits face challenges in maintaining reference voltage stability at low supply voltages, particularly due to mismatched output currents and low-frequency noise, which require significant silicon area and compromise accuracy.
Innovation Solution
The implementation of voltage and current chopping techniques, along with a low-frequency filter, to alternately swap temperature-dependent voltages and currents, reducing input offset and 1/f noise, thereby enhancing reference voltage stability without increasing chip area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional current-mode BGR circuits are used to generate reference voltage, then the circuit can operate with standard supply voltages, but the output current mismatch and low-frequency noise significantly degrade reference voltage stability
Solution Approach 1:
The patent applies periodic action by introducing a chopping circuit that periodically switches the current paths at a high frequency. The switching circuit alternately connects different current mirrors to the output, effectively averaging out the current mismatches and reducing low-frequency noise through time-domain modulation. This periodic switching transforms the DC current mismatch problem into an AC signal that can be filtered out.
Solution Approach 2:
The patent introduces an intermediary switching circuit between the current mirrors and the output node. This switching circuit acts as a mediator that periodically routes different current paths to the output, allowing the system to average multiple current sources and reduce the impact of individual current mirror mismatches on the final reference voltage.
2Reliability
If current mirror devices are enlarged to reduce threshold voltage variations and 1/f noise, then noise performance improves, but the silicon area occupied by the BGR circuit increases significantly
Solution Approach 1:
Instead of enlarging current mirror devices to reduce noise, the patent uses periodic switching to achieve noise reduction. By rapidly switching between multiple current mirrors at a frequency much higher than the 1/f noise corner frequency, the system averages out the noise contributions without requiring larger device areas, thus maintaining compact silicon footprint while improving noise performance.
Solution Approach 2:
The patent changes the operational parameters by moving from static DC current mirroring to dynamic time-varying current switching. This parameter change allows the system to exploit frequency-domain separation, where the useful DC reference voltage is maintained while the noise components at lower frequencies are averaged out through high-frequency switching.
3Measurement precision
If multiple current mirrors are used to compensate for output current mismatch, then current matching accuracy improves, but the device complexity and chip area increase
Solution Approach 1:
The patent uses periodic switching to manage multiple current mirrors in a controlled manner. Instead of permanently connecting multiple current mirrors to the output (which would increase complexity), the switching circuit periodically connects different current mirrors in sequence, achieving current matching through time-averaging while keeping the circuit structure manageable and systematic.
Solution Approach 2:
The patent introduces dynamics into the current mirror system by using time-varying switching control. This dynamic approach allows the system to adaptively manage multiple current mirrors, switching between them based on a periodic control signal, thereby achieving improved current matching without requiring complex static compensation circuits.
4Reliability
If voltage chopping technique is applied to reduce input offset and 1/f noise, then reference voltage stability improves, but additional circuit components are required
Solution Approach 1:
The patent merges the voltage chopping function with the existing current mirror structure by integrating the switching circuit directly into the current path control. This combination allows the chopping action to be performed using the same switching elements that control the current mirrors, thereby achieving noise reduction without adding completely separate voltage chopping circuitry.
Solution Approach 2:
The switching circuit in the patent serves multiple functions simultaneously: it acts as the current mirror selection switch, performs the voltage chopping action to reduce offset and noise, and provides the periodic modulation needed for noise averaging. This multi-functionality reduces the need for separate dedicated circuits for each function.
Data Source
AI summary
A BGR circuit for sub-1V ICs utilizes a voltage chopping circuit and/or a current chopping circuit and a low-frequency filter to stabilize the output reference voltage that is generated by an op-amp, a current mirror circuit, a CTAT stage, a PTAT stage, and an output stage. The voltage chopping circuit reduces input offset and 1/f noise by periodically alternating (time-averaging) the negative temperature dependent and positive temperature dependent voltages supplied by the CTAT and PTAT stages to the op-amp's input terminals. The current chopping circuit minimizes current variations caused by process-related differences in the current mirror devices by periodically alternating (time-averaging) three balanced currents generated by the current mirror circuit such that each current is transmitted equally to each of the CTAT, PTAT and output stages. The filter serves to maintain loop stability and remove the low frequency noise generated by the applied voltage and/or current chopping operations.