Synthesized Current Sense Resistor for Wide Dynamic Range
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Solution Overview
Problem
Current sense resistor circuits face inefficiencies due to high power wastage at high currents and high demands on comparators at low currents, as they require significant voltage across the sense resistor and are limited by offset and noise, making it difficult to achieve accurate and stable current sensing over a wide dynamic range.
Innovation Solution
A feedback circuit synthesizes a resistor using a MOSFET or active component, employing two matched devices and a precision resistor to establish a stable and predictable sense resistance, allowing for a constant voltage threshold and high efficiency by scaling the resistance with the current command signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high resistance sense resistor is used to improve accuracy at low current, then measurement precision is improved, but power loss increases excessively at high current
Solution Approach 1:
The patent applies dynamics by making the sense resistance variable rather than fixed. The sense resistor circuit adjusts its resistance value dynamically based on the commanded current level - using high resistance at low currents for accurate sensing and low resistance at high currents to minimize power loss. This is achieved through the control device that modifies the effective sense resistance according to operating conditions.
Solution Approach 2:
The patent changes the resistance parameter of the sense resistor based on operating conditions. By varying the resistance value according to the commanded current, the system optimizes both measurement precision and power efficiency across different operating ranges. The resistance parameter is adjusted to match the sensing requirements at each current level.
2Loss of energy
If a low resistance sense resistor is used to reduce power loss at high current, then loss of energy is reduced, but measurement precision deteriorates at low current
Solution Approach 1:
The sense resistor circuit dynamically adjusts its resistance value based on the commanded current level. At high currents, it presents low resistance to minimize power loss, while at low currents, it increases resistance to maintain adequate voltage for accurate sensing. This dynamic adaptation resolves the contradiction between power efficiency and measurement precision.
Solution Approach 2:
The resistance parameter is varied according to operating conditions to optimize both power efficiency and sensing accuracy. The system changes the sense resistance parameter to match the requirements of the current operating point, achieving low power loss at high currents and high precision at low currents.
3Measurement precision
If a very accurate current sense comparator with very low offset is used to improve measurement precision, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The system performs preliminary action by pre-conditioning the sensing signal through dynamic resistance adjustment. By optimizing the sense resistance value before the comparison operation based on the commanded current, the system ensures adequate signal levels and characteristics for the comparator, reducing the need for ultra-low offset comparators and simplifying the overall device complexity.
Solution Approach 2:
The dynamic sense resistor circuit acts as an intermediary between the current being measured and the comparator. It conditions the sensing signal by adjusting its resistance to provide optimal voltage levels for the comparator, thereby reducing the performance demands on the comparator itself and simplifying the overall system.
4Ease of operation
If a linear regulator controls load current based on voltage input, then ease of operation is improved, but productivity decreases due to weak control signal at low current
Solution Approach 1:
The sense resistor circuit dynamically adjusts its resistance to maintain optimal control signal strength across the full current range. By increasing resistance at low commanded currents, it ensures adequate voltage development for strong control signals, while maintaining ease of voltage-based operation throughout the operating range.
Data Source
AI summary
A circuit has a first sense resistor circuit having components including a first-circuit active element to provide a sense resistance to sense a current in a load in series therewith, the sense resistance being established by an input command voltage. A second sense resistor circuit has components replicating the components of the first sense resistor circuit including a replicated active element, a resistance of the replicated active element also being established by the input command voltage. A precision resistor is coupled to the replicated active element to provide a load thereto. When the input command voltage establishes a voltage across the replicated active element, a voltage is established across the first-circuit active element in proportion thereto to command a desired current in the load.