Dual-Loop LDO Startup Control for External Pass Transistor Priority
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Solution Overview
Problem
Low dropout (LDO) voltage regulators face challenges in determining whether an external pass device is present at startup, which affects thermal management and current distribution between internal and external pass devices.
Innovation Solution
A voltage regulator design that employs two concurrent control loops, one for an external pass transistor and another for an internal pass transistor, with a prioritized first control loop to ensure the external pass transistor supplies most of the load current at startup.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If an external pass device is used to supply high load current, then heat dissipation outside the integrated circuit increases, but the presence of the external pass device is unknown at startup causing control uncertainty
Solution Approach 1:
The patent segments the control system into two independent control loops: a first control loop for the external pass device and a second control loop for the internal pass device. Each loop operates autonomously with its own error amplifier and feedback path, allowing the system to manage the uncertainty of external device presence by treating it as a separate controllable entity rather than assuming its state.
Solution Approach 2:
The patent changes the feedback voltage parameter by providing different feedback voltages to the two control loops through a voltage divider circuit with multiple taps. The first control loop receives a first feedback voltage while the second control loop receives a second feedback voltage, allowing differential control of the two pass devices based on their respective operational states and priorities.
2Productivity
If both control loops run concurrently at startup, then current distribution can be controlled, but thermal stress on the internal pass transistor increases without prioritization
Solution Approach 1:
The patent applies preliminary action by establishing a default priority scheme where the first control loop (external pass device) is given higher priority than the second control loop (internal pass device) at startup. This prioritization is built into the control architecture before operation begins, ensuring that the external device assumes the primary load-carrying role and the internal device remains in a reduced-conduction state, thereby preventing thermal stress accumulation.
Solution Approach 2:
The patent implements feedback mechanisms in both control loops where error amplifiers continuously monitor the respective feedback voltages and adjust the control signals to the pass devices. The first error amplifier receives a first feedback voltage from a first tap of the voltage divider circuit, while the second error amplifier receives a second feedback voltage from a second tap, enabling dynamic adjustment of current distribution to maintain thermal safety.
Data Source
AI summary
A voltage regulator utilizes an external pass transistor coupled between a supply voltage node and a regulated node. The voltage regulator includes an internal pass transistor that is coupled between the supply voltage node and the regulated node. A two-tap voltage divider between the regulated node and a ground node provides two feedback voltages. A first control loop that controls the external pass transistor receives the first feedback voltage and a second control loop that controls the internal pass transistor receives the second feedback voltage. At startup the control loops run concurrently but the first control loop is prioritized to reduce load current provided by the internal pass transistor at the end of startup. The voltage difference between the first and second feedback voltages ensures prioritization of the first control loop and the external pass transistor supplies most or all of the load current.