Subregulated Amplifier Startup Circuit for Stable Bandgap Bias
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Solution Overview
Problem
Startup circuits for bandgap voltage reference circuits often fail to reliably provide a stable operating voltage, leading to unreliable startup due to fluctuations in the external power supply, which can result in noise coupling and reduced power supply rejection ratio (PSRR).
Innovation Solution
A startup circuit that measures the tail current in a differential amplifier and applies a forcing signal to the output stage when the tail current is below a threshold, ensuring the output stage reaches a nominal voltage level for proper startup of the first stage, thereby isolating fluctuations from the first stage and improving PSRR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the bandgap subcircuit is powered by its own amplified and buffered voltage, then PSRR is dramatically improved, but startup may not occur reliably or at all
Solution Approach 1:
The startup circuit applies a forcing signal to the output of the second stage amplifier before the bandgap subcircuit is fully operational, ensuring the output voltage reaches a nominal level that enables proper startup. This preliminary action establishes the necessary voltage conditions before normal operation begins, resolving the startup reliability issue while maintaining the subregulated power architecture for improved PSRR.
Solution Approach 2:
The startup circuit acts as an intermediary component that temporarily bridges the gap between the second stage output and the bandgap subcircuit during startup. It monitors the tail current and applies a forcing signal when needed, mediating the transition from startup to normal operation. Once startup is complete, the startup circuit is electronically removed, allowing the subregulated architecture to function independently with improved PSRR.
2Reliability
If a startup circuit applies AVDD voltage temporarily to subcircuits, then startup is enabled, but noise on AVDD may couple to the internal reference voltage
Solution Approach 1:
The invention extracts the startup function from the main power supply path by using a separate startup circuit that applies forcing signals only during startup. The startup circuit is electronically removed after startup completes, separating the startup function from the operational power path. This allows the bandgap subcircuit to be powered by the quiet subregulated voltage during normal operation, preventing noise coupling while maintaining startup reliability.
Solution Approach 2:
The startup circuit performs preliminary action by establishing the necessary voltage conditions before normal operation begins. It applies forcing signals to the second stage output during startup, then is removed before the bandgap subcircuit operates with the subregulated supply. This timing separation ensures that noise on AVDD does not couple to the reference voltage during operation, while still enabling reliable startup.
Data Source
AI summary
A multi-stage circuit has a first stage powered by the output voltage of a next stage. A current source within the first stage provides a tail current for a differential amplifier within the first stage. When the first stage has an operating voltage high enough for proper operation, this tail current is at a nominal level; if the voltage is too low for proper operation of the first stage, the tail current is below this nominal level. A comparator, which has one input coupled to a node within this current source, a second input coupled to a threshold voltage, and an output coupled to a control node within the next stage, provides an output indicative of whether or not the tail current is substantially at its nominal level. If tail current is too low, the comparator provides a forcing signal to the control node of the next stage which causes the output of the next stage to be at a substantially nominal level regardless of the voltage at its input, thus providing a suitably high voltage for the first stage to begin normal operation. When the tail current reaches its nominal level, the comparator output changes state to one which has little or no effect on the output voltage of the second stage, and normal operation of the overall circuit begins.