LDO Regulator Error Amplifier Powered by Output Voltage
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Solution Overview
Problem
Conventional low drop-out (LDO) voltage regulators have a relatively low power supply rejection ratio (PSRR) due to unstable input voltage, which affects the stability of the output voltage and requires a high gain error amplifier to compensate for input voltage variations.
Innovation Solution
A power regulator design where the error amplifier and reference signal circuit are powered by the output voltage, incorporating a start-up circuit that uses the input voltage during startup and transitions to output voltage power during normal operation, forming a negative feedback loop with the pass device to maintain stability and improve PSRR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the error amplifier and reference voltage circuit are powered by the input voltage, then the regulator can operate during start-up, but the power supply rejection ratio is low and output voltage stability deteriorates
Solution Approach 1:
The start-up circuit is enabled before the main regulator operation to provide initial drive current to the pass device, ensuring the regulator can start up properly. Once the output voltage reaches a sufficient level, the start-up circuit is disabled and the error amplifier switches to being powered by the output voltage, thereby achieving both reliable start-up and high PSRR during normal operation.
Solution Approach 2:
The power supply for the error amplifier is dynamically switched between input voltage (during start-up) and output voltage (during normal operation). This dynamic switching allows the system to adapt to different operational phases, using the appropriate power source for each phase to optimize both start-up capability and power supply rejection ratio.
2Measurement precision
If the error amplifier gain is increased to compensate for input voltage variations, then output voltage stability improves, but device complexity and noise increase
Solution Approach 1:
The error amplifier is extracted from the input voltage domain and relocated to the output voltage domain by powering it from the output voltage instead of the input voltage. This extraction removes the coupling between input voltage variations and error amplifier operation, allowing the error amplifier to maintain stable operation with moderate gain without being affected by input voltage fluctuations.
3Speed
If the pass device is driven directly by input voltage variations, then response speed is fast, but output voltage precision deteriorates
Solution Approach 1:
A feedback loop is implemented where the error amplifier continuously monitors the output voltage and adjusts the control signal to the pass device accordingly. This feedback mechanism ensures that the pass device responds quickly to maintain output voltage precision, as the error amplifier can rapidly adjust the control signal in response to any deviations, thereby achieving both fast response and high precision.
Data Source
AI summary
A power regulator for converting an input voltage to an output voltage includes an error amplifier, a start-up circuit, and a pass device. The error amplifier is powered by the output voltage and provides a control current according to a difference between a reference signal and a feedback signal indicative of the output voltage. The start-up circuit is powered by the input voltage and provides a start-up current. The pass device receives the input voltage, provides the output voltage at an output terminal of the power regulator, generates an output current flowing through the output terminal according to the start-up current during a start-up duration of the power regulator, and generates the output current through the output terminal according to the control current during a normal operation of the power regulator.