Load Current Sense Circuit with Sensing Loop for Light Load Accuracy
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Solution Overview
Problem
Current current sensing methods in integrated circuits, such as measuring voltage drop across a sense resistor, are inefficient and fail to provide accurate current measurements under light load conditions due to voltage offsets in sense amplifiers.
Innovation Solution
A current sensing circuit with a sensing loop that maintains a predetermined channel resistance for the pass transistor, using a second sense transistor and associated sense circuit to ensure accurate current measurements by equalizing voltage across the pass and sense transistors, even under light loads, and reducing no-load dropout quiescent current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If voltage drop across a sense resistor is measured to sense current, then current measurement is provided, but efficiency deteriorates and accuracy worsens under light load conditions due to voltage offsets
Solution Approach 1:
The patent introduces a sense transistor as an intermediary element that generates a sense current proportional to the load current. This sense current serves as a mediator that provides accurate current information without requiring direct voltage measurement across a sense resistor, thereby improving both measurement accuracy and efficiency
Solution Approach 2:
The patent replaces the traditional voltage-based current sensing method (measuring voltage drop across a sense resistor) with a current-based method (generating and measuring sense current through a sense transistor). This substitution eliminates the efficiency losses and voltage offset problems associated with resistive sensing
2Measurement precision
If a sense amplifier is used to measure voltage drop, then current sensing is enabled, but measurement precision deteriorates under light load conditions due to voltage offsets
Solution Approach 1:
The patent changes the operating parameter from voltage measurement to current measurement. By generating a sense current that is proportional to the load current and measuring this current directly, the circuit achieves accurate light load measurements without being affected by voltage offsets in sense amplifiers
Solution Approach 2:
The sense transistor creates a copy of the load current as a sense current. This copied current signal preserves the proportional relationship with the original load current while being easier to measure accurately, especially under light load conditions
3Measurement precision
If channel resistance of pass transistor is not controlled, then circuit simplicity is maintained, but measurement accuracy deteriorates because voltage across pass transistor may be less than sense amplifier offset
Solution Approach 1:
The patent implements a sensing loop that provides feedback control of the pass transistor's channel resistance. The loop monitors the voltage across the pass transistor and adjusts the resistance to ensure it remains above the sense amplifier offset voltage, thereby guaranteeing accurate current measurements
Solution Approach 2:
The sensing loop proactively controls the channel resistance before measurement errors can occur. By maintaining the voltage across the pass transistor above the offset threshold through preliminary resistance adjustment, the system prevents measurement inaccuracies rather than correcting them
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides accurate and proportional sense currents under all operational conditions, including light loads and dropout regions, enhancing measurement accuracy and reducing quiescent current in low dropout regulators and other applications.
Implementation Method 1
a sensing loop for controlling the channel resistance of a pass transistor... The channel resistance is maintained at a value that provides a voltage across the pass transistor that is greater than the offset of the sense amplifier
Data Source
AI summary
A current sensing circuit includes a pass transistor, a first sense transistor, a second sense transistor, a driver circuit, and sense circuitry. The driver circuit coupled to, and configured to generate a drive signal to control, the pass transistor, the first sense transistor, and the second sense transistor. The sense circuitry coupled to the pass transistor, the first sense transistor, and the second sense transistor. The sense circuitry includes a first sense circuit and a second sense circuit. The first sense circuit is configured to generate an output current proportional to a current flowing in the pass transistor. The second sense circuit is coupled to the driver circuit and is configured to set the drive signal to a predetermined voltage responsive to a voltage across the pass transistor being less than a threshold voltage.