Current Limiting Circuitry for Pass Transistors
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Solution Overview
Problem
Existing circuits for linear voltage regulators and operational amplifiers face challenges in accurately limiting current through pass transistors due to variations in manufacturing process parameters, voltage, and temperature, leading to reduced accuracy and reliability, especially in safety-critical applications.
Innovation Solution
A circuit and method that utilize a reference current sensing transistor and a buffer to accurately limit the maximum current flowing through a pass transistor without using a current limiting resistor, ensuring precise control of the gate voltage to prevent excessive current flow, thereby maintaining system reliability and safety.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a current limiting resistor is used to limit the maximum current through the pass transistor, then the current can be limited, but the accuracy is reduced due to variations in manufacturing process parameters, voltage, and temperature
Solution Approach 1:
The patent uses a sensing transistor that copies the electrical characteristics of the pass transistor to create a scaled-down replica of the current flow. This replica current is then compared against a reference current to determine when limiting is needed, eliminating the need for imprecise current limiting resistors while maintaining accuracy across PVT variations
Solution Approach 2:
The patent implements a feedback mechanism where the sensing transistor continuously monitors the current through the pass transistor and feeds this information back to the control circuit. When the current exceeds the predetermined limit, the feedback signal triggers the limiting action, ensuring accurate and dynamic current control that adapts to PVT variations
2Reliability
If the size of the pass transistor is reduced to limit current, then the maximum current is limited, but the current conducting capability is reduced
Solution Approach 1:
The patent introduces a sensing transistor as an intermediary element that provides current sensing and control information without being part of the main current conduction path. This allows the pass transistor to maintain its full current conducting capability while the sensing transistor, operating in parallel, provides the limiting function through electrical signal monitoring
3Reliability
If a current limiting resistor is added to the circuit, then the maximum current can be limited, but the device complexity increases
Solution Approach 1:
The sensing transistor serves multiple functions simultaneously: it acts as a current sensor, a current limiter, and a signal generator for the control circuit. This multi-functionality eliminates the need for separate current limiting resistors and additional control components, reducing overall circuit complexity while maintaining reliable current limiting
Data Source
AI summary
Circuitry (10-2) for limiting the maximum amount of current (IREF) flowing through a first electrode (DRAIN) of a first transistor (T1) includes an amplifier (14) having an output coupled by a conductor (19) to a control electrode of the first transistor and limiting circuitry (17) including reference current sensing circuitry (22,TSENSE) having a reference current source (IREF<sub2>—</sub2>SENSE). A reference current sensing transistor (TSENSE) has a control electrode coupled to the control electrode of the first transistor, a first electrode coupled to a terminal (20) of the reference current source, and a second electrode (SOURCE) coupled to a second electrode of the first transistor. A buffer (T2) has an input coupled to the terminal of the reference current source. The maximum amount is limited in accordance with the reference current source to prevent an increase in magnitude of voltage applied by the amplifier to the first transistor.