Class-D Amplifier Load Current Sensing Without Sense Resistors
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Solution Overview
Problem
Existing class-D amplifiers face inefficiencies in load current sensing due to the use of sense resistors, which dissipate power and are sensitive to process and temperature variations, and existing techniques are not effective for inductive loads.
Innovation Solution
A current sensing circuit utilizing replica transistors fabricated at the same process corner as the output transistors, eliminating the need for sense resistors by using operational amplifiers with feedback capacitors and resistors to measure load current through transistor on-resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If sense resistors are used for load current sensing, then load current can be measured, but power is dissipated and sensitivity to process and temperature variations increases
Solution Approach 1:
The patent uses replica transistors that are identical copies of the output transistors, fabricated at the same process corner and located adjacent to them. These replicas sense the load current by experiencing the same on-resistance variations, eliminating the need for separate sense resistors and their associated power dissipation and sensitivity issues.
Solution Approach 2:
The patent introduces operational amplifiers as intermediary devices that measure the voltage drop across the on-resistance of the output transistors during their on-state. This indirect measurement method avoids direct current through sense resistors, thereby reducing power dissipation while maintaining sensing accuracy.
2Measurement precision
If sense resistors are used for load current sensing, then load current can be measured, but sensitivity to process and temperature variations increases
Solution Approach 1:
The replica transistors are fabricated at the same process corner as the output transistors and are located adjacent to them, ensuring they experience identical process and temperature conditions. This copying approach makes the sensing mechanism inherently compensated for process and temperature variations, improving reliability.
Solution Approach 2:
The patent changes the sensing mechanism from using fixed sense resistors with stable resistance values to using transistors whose on-resistance dynamically tracks with process and temperature variations. This parameter change allows the sensing element to adapt to environmental conditions, reducing sensitivity to variations.
3Loss of energy
If class-D amplifier uses rail-to-rail output switching, then efficiency is maximized, but load current sensing becomes more difficult
Solution Approach 1:
The patent performs load current sensing during the brief intervals when output transistors are in their on-state, before they switch off. This preliminary action captures the current information while the transistors are conducting, enabling accurate sensing without compromising the rail-to-rail switching efficiency.
Solution Approach 2:
The sensing operation is performed periodically during each switching cycle, utilizing the on-states of the output transistors. This periodic sensing approach integrates seamlessly with the switching operation, maintaining high efficiency while enabling continuous current measurement through repeated sampling.
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
Accurately senses load current without power dissipation, reducing temperature and process variation sensitivity, and maintaining high efficiency by eliminating the need for sense resistors.
Implementation Method 1
The current sensing circuit includes: an operational amplifier having a positive input terminal and a negative input terminal
Implementation Method 2
a first feedback resistor and a first feedback capacitor connected in parallel between the negative input terminal and the positive output terminal of the operational amplifier
Implementation Method 3
a first feedback resistor and a first feedback capacitor connected in parallel between the negative input terminal and the positive output terminal of the operational amplifier
Implementation Method 4
a positive input replica transistor, where a source of the positive input replica transistor is connected to the positive input terminal of the operational amplifier, a drain of the positive input replica transistor is connected to the drain of the first n-type output transistor
Implementation Method 5
measure load current through transistor on-resistance
Data Source
AI summary
A class-D amplifier includes an output driver stage and a current sensing circuit connected. The current sensing circuit includes: an operational amplifier having a positive input terminal and a negative input terminal; a positive input replica transistor, where a source of the positive input replica transistor is connected to the positive input terminal, a drain and a gate of the positive input replica transistor is connected to the drain and the gate of a first n-type output transistor, respectively; and a negative input replica transistor, where a source of the negative input replica transistor is connected to the negative input terminal, a drain and a gate of the negative input replica transistor is connected to the drain and the gate of the second n-type output transistor, respectively.