Coreless current sensor shrinks footprint

Melexis in Belgium has developed a current sensor that eliminates the need for a ferromagnetic core, reducing the footprint.

The MLX91235 enables the measurement of larger currents flowing through external primary conductors like PCB traces and busbars.

Eliminating the ferromagnetic concentrator cuts the footprint and eliminates hysteresis-related measurement errors. The differential measurement of the magnetic field between two internal sensing elements provides accurate current feedback, as well as ensuring stray field effects are eliminated.

The 500 kHz bandwidth and 2 μs response time means the sensor is suitable for high-speed applications, such as motor control and converter applications, as well as automotive and alternative mobility applications like inverters, battery management systems (BMSs) and low-voltage DC-DC converters.

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The MLX91235’s intelligent digital architecture gives high measurement accuracy and allowing for the incorporation of several key functions that help to enhance development, calibration, and system integration.

When dealing with intrinsic non-linearities, analog architectures often rely on piecewise linear compensations. The digital architecture offers the ability to implement more precise and sophisticated compensation, resulting in a more accurate and smoother output. The fully digital-based thermal compensation further ensures that the MLX91235 provides the highest accuracy measurement across its entire working conditions.

Open calibration via SPI

The device’s ‘open’ calibration is configured through a standard serial peripheral interface (SPI) allowing for configuration in situ via any microcontroller unit (MCU). Compared to analog coreless sensors, digital calibration provides a more direct and significantly quicker experience, enabling easy adjustment of elements like digital gain correction.

The built-in 16-bit over-current detection (OCD) allows for asymmetric thresholds and includes two configurable threshold ranges. It features a configurable detection time with a minimum duration of 2 μs and an optional debounce strategy. This strategy helps to avoid false positives in harsher electromagnetic compatibility (EMC) environments. 

The MLX91235 also includes a 10-bit temperature feedback mechanism, enabling accurate measurement of junction temperature that is transmitted through a high-speed SPI output.

Functional safety & self-test

The coreless current sensor is ISO 26262 compliant as an ASIL B Safety Element out of Context (SEoOC). Going beyond ASIL B requirements, it also incorporates a sophisticated built-in self-test feature that can be triggered via SPI. This functionality allows the sensor to report critical information like temperature, under-voltage, and mechanical stress. Unlike conventional diagnostics, this functionality encompasses the complete signal chain, up to the application MCU that reads the sensor.

“Melexis has a rich history in current sensing technologies and the coreless MLX91235 is our next step,” said Bruno Boury, Product Line Director at Melexis.  “This sensor brings the optimal mix of in-situ fast calibration including ISO26262 compliance and high accuracy owing to the digital core of the sensor.”

Engineering samples of the MLX91235 coreless current sensor and DVK91235 development kits are available now. The product is expected to be fully qualified by mid-2025.