DC/DC Converter Current Sensing with DCR Calibration
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Solution Overview
Problem
Current DC/DC converters face challenges in accurately sensing output current due to variations in inductor DC Resistance and PCB resistances, which affect power efficiency and require external reference currents that may be unavailable or inaccurate.
Innovation Solution
A current sensing circuit that performs calibration using a reference voltage and resistance, with active ripple cancellation and thermal monitoring, to determine a calibration gain that compensates for these variations, allowing for accurate current sensing without relying on external reference currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If external reference currents are used for current sensing calibration, then current sensing accuracy may be improved, but system complexity and dependency on external components increases
Solution Approach 1:
The system performs self-calibration by injecting a known test current through the inductor and measuring the resulting voltage, then using this measurement to automatically determine and store the calibration gain. This eliminates the need for external reference currents or manual calibration procedures, making the system self-sufficient while maintaining high measurement precision.
Solution Approach 2:
The calibration process is performed automatically during system initialization or startup before normal operation begins. By pre-determining the calibration gain through an automated test sequence, the system prepares the accurate sensing parameters in advance, eliminating the need for continuous external reference currents during operation.
2Measurement precision
If inductor DC Resistance variations are compensated, then current sensing precision is improved, but additional calibration circuitry and processing is required
Solution Approach 1:
The calibration function is integrated directly into the existing current sensing circuitry by reusing the differential amplifier and ADC resources. The same hardware components that sense current during normal operation are utilized during calibration by switching in a test current source, eliminating the need for separate dedicated calibration hardware and reducing overall device complexity.
Solution Approach 2:
The system changes the operating parameter from normal load current to a known test current during calibration mode. By injecting a precisely controlled test current and measuring the corresponding voltage, the system determines the actual inductor resistance and compensates for variations without requiring additional sensing hardware.
3Measurement precision
If PCB resistance variations are compensated, then measurement accuracy improves, but calibration complexity increases
Solution Approach 1:
The calibration path is merged with the existing current sensing path by using the same differential amplifier inputs and ADC. During calibration, switches redirect the test current through the inductor and PCB traces, allowing the system to measure and compensate for PCB resistance variations using the same hardware infrastructure, thereby avoiding increased calibration complexity.
4Speed
If ripple cancellation is implemented, then current sensing speed is improved, but circuit complexity increases
Solution Approach 1:
The ripple cancellation function is integrated into the differential amplifier circuit by injecting an inverted ripple signal through the same amplifier that senses the current. This shared hardware approach eliminates the need for separate ripple cancellation circuitry while achieving faster, cleaner current measurements by simultaneously performing sensing and ripple rejection.
Solution Approach 2:
The differential amplifier acts as an intermediary that performs dual functions: it amplifies the small voltage signal from the inductor for current sensing, and simultaneously rejects the large ripple voltage by subtracting an inverted copy of the ripple signal. This intermediary role allows ripple cancellation without requiring separate dedicated circuitry.
Data Source
AI summary
A method for sensing an output current of a Direct Current-to-Direct Current (DC/DC) converter having an external power stage configured to supply a converted current to an external inductor. During a calibration phase at a first start-up of the DC/DC converter: the method includes injecting a calibration current through a switching node of the power stage and through the inductor; and determining a calibration gain of the DC/DC converter to compensate for DC Resistance (DCR) variation by comparing a gain-adjusted voltage across the inductor with a reference voltage. During a measurement phase, the method includes reducing ripple voltage of a switching voltage at the switching node to generate a ripple-reduced switching voltage; and sensing the output current based on a DCR-compensated voltage across the inductor, which is a difference between the ripple-reduced switching voltage and an output voltage of the DC/DC converter with compensation for the DCR variation based on the calibration gain.