New Hall Effect Current Sensing IC from Melexis is Optimized for E-Mobility ApplicationsDecember 08, 2020 by Nicholas St. John
Melexis, a microelectronics company based in Belgium, has released a second generation medium-speed current sensor, the MLX91211 IC line.
The MLX91211 IC is a Hall-Effect current sensing circuit that outputs a voltage proportional to the magnetic field sensed from a current flowing through a conductor such as a cable, wire, bus bar or PCB trace, according to the product flyer.
There are two versions of the IC offered, one being the MLX91211UA-ABT with multi-temperature trimming of the sensitivity and optimized for high accuracy, and the other being the MLX91211UA-ABA version which has default trimming, lower bandwidth and standard accuracy, giving a lower cost option for less stringent requirements.
According to the press release, this IC is ideal for e-mobility applications such as belt-driven starter generators and traction inverters for e-scooters, as well as industrial applications such as forklifts and pallet trucks.
Bruno Boury, the Product Line Manager for Current Sensors at Melexis, explains how these new sensor ICs are allowing designers of e-mobility traction solutions of low-speed electric vehicles to have an option that does not sacrifice accuracy, as the sensitivity thermal drift of the MLX91211 IC is better than 1.5%, which is crucial for automotive applications, while tailoring the features for the economy and reliability of the device.
Furthermore, for automotive applications, both versions of the device are qualified to AEC-Q100 Grade 0, and have an operating temperature range of -40oC to 150oC, allowing the device to be used even under the hood of an automobile. The series is currently in full production and is offered in both through hole (TO92 UA 3L) and surface mount (SOT-23 3L) packages.
According to the datasheet of the MLX91211, the device is a Linear Hall sensor with the capability of measuring currents from DC to 40kHz in frequency, and the response time of the sensor is less than 10 microseconds. The device can also be operated at a supply voltage ranging from approximately 3.3V to 5V, which affects the sensitivity of the IC, with the minimum sensitivity for the lower cost option still being rated at 17.32 mV/mT. With the high sensitivity, Melexis says it is best to have the device within a ferromagnetic core to minimize interference from unwanted noise. The block diagram of the device is shown below:
Image Courtesy of MLX91211 Datasheet
From this, we see that the Hall Effect sensor output, being a voltage signal, is demodulated in order to extract the signal level from the Hall output. This then feeds through a Low-Pass filter and put through a gain amplifier in order to output a DC voltage proportional to the current originally sensed by the Hall Sensor.
To support the signal path, there is a “Sensitivity Trim” and “Offset Trim” block which operates depending on the EEPROM memory, containing the calibration data. These blocks are put in the calibrated correction factors needed for maximum accuracy at the given temperature, which is determined via the “Temperature Compensation” block.
The output voltage of this sensor is determined by the following equation: VOUT = VDQ + S, where VDQ is the measured quiescent output voltage at B=0, and S is the magnetic sensitivity at the specific supply voltage that the device is being operated at. The datasheet also contains the following two application diagrams, one for typical usage and one for exceptionally noisy environments.
Images Courtesy of MLX91211 Datasheet
Here, noisy environments require an additional resistor (RS) and capacitors (C2-C4) for filtering whose values should match the following criteria:
- $$R_S \geq 50$$
- $$1nF \leq C_2 \leq 10nF$$
- $$1nF \geq C_3 \leq 4.7nF$$
- $$1nF \geq C_3 \leq 10nF$$
The MLX91211 IC is a very capable tool for designers as we continue to get deeper and deeper into the realm of e-mobility applications. This chip provides an astounding capability to sense a large bandwidth of current signals with high accuracy, while being robust enough to survive automobile applications and do all of these things at minimal cost.