Transient-Compensated Linear Regulator for Back-to-Back Load Steps
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Solution Overview
Problem
Dual-rail linear voltage regulators face challenges in responding quickly to back-to-back load transients, leading to large voltage spikes due to the pass transistor turning fully OFF and the compensation capacitor being charged in an opposite polarity, which prolongs recovery time and increases undershoot amplitude.
Innovation Solution
Incorporating a transient compensation circuit that adjusts the offset of the differential amplifier to prevent the pass transistor from turning fully OFF and maintains the compensation capacitor at a positive polarity, allowing for quicker response to subsequent transients by using a bypass transistor to offset the error signal and maintain regulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the pass transistor is allowed to turn fully OFF during load transients, then the regulator can reduce power consumption and increase output voltage, but the response time to subsequent transients increases and voltage spikes occur
Solution Approach 1:
The transient compensation circuit applies preliminary anti-action by detecting when the pass transistor is fully OFF and injecting a compensation current through the bypass transistor to prevent the compensation capacitor from being fully discharged or charged in opposite polarity. This preliminary action maintains the capacitor in a state that enables faster response to subsequent transients, preventing the harmful effect of prolonged recovery time while allowing the pass transistor to remain OFF for power savings.
Solution Approach 2:
The bypass transistor acts as an intermediary element between the differential amplifier and the compensation capacitor. It provides an alternative current path that prevents the compensation capacitor from being fully discharged when the pass transistor is OFF, thereby maintaining the regulator's ability to respond quickly to subsequent transients without requiring the pass transistor to remain partially ON.
2Measurement precision
If the compensation capacitor is allowed to be charged in opposite polarity during transients, then the differential amplifier can fully respond to voltage changes, but the recovery time increases and undershoot amplitude increases
Solution Approach 1:
The transient compensation circuit applies preliminary anti-action by detecting the state of the pass transistor and preemptively adjusting the offset of the differential amplifier through the bypass transistor. This prevents the compensation capacitor from being charged in opposite polarity, thereby avoiding the harmful effects of increased recovery time and undershoot amplitude while maintaining the ability to accurately respond to voltage changes.
3Speed
If the offset of the differential amplifier is adjusted to prevent the pass transistor from turning fully OFF, then the response time improves, but the power consumption increases and the ability to regulate high output voltage decreases
Solution Approach 1:
The transient compensation circuit implements dynamics by dynamically adjusting the offset of the differential amplifier based on the state of the pass transistor. The bypass transistor is activated only when needed (when the pass transistor is fully OFF), providing a temporary current path to maintain compensation capacitor charge. This dynamic adjustment enables fast response time during transients without maintaining increased power consumption during normal operation.
Solution Approach 2:
The circuit changes the operating parameters of the differential amplifier dynamically. By adjusting the offset voltage through the bypass transistor only during transient conditions, the system achieves fast response time without permanently altering the power consumption characteristics or the ability to regulate high output voltage during normal operation.
4Reliability
If a transient compensation circuit is added to adjust the differential amplifier offset, then the back-to-back transient response improves, but the device complexity increases
Solution Approach 1:
The bypass transistor serves as a simple intermediary element that adds minimal complexity to the circuit. It provides a straightforward current path that activates only during transient conditions, enabling improved back-to-back transient response without significantly increasing overall circuit complexity. The compensation mechanism uses existing circuit nodes and requires only one additional transistor and associated control logic.
Data Source
AI summary
A load coupled to a linear voltage regulator may create a load transient so that an output of the voltage regulator is temporarily raised to an elevated level above a regulated level. Without compensation, the linear voltage regulator may respond by turning a pass transistor completely OFF thereby losing regulation and allowing a compensation capacitor to become charged in a polarization opposite to one required for regulation. If a subsequent load transient (i.e., back-to-back load transient) is generated while the linear voltage regulator is in this condition, a large spike in the output may occur as the voltage regulator recharges the pass transistor turns back ON and as the compensation capacitor recharges. Disclosed herein is a linear voltage regulator with transient compensation circuitry to prevent the scenario described above and reduce the spike in the output.