Fast, Accurate, Isolated Current Sensing with the Allegro ACS37035

Electrified transportation and the AI data center buildout are both pushing power electronics toward higher density. And, to move more power through smaller packages, many engineers are starting to embrace higher switching frequencies and currents. 

However, as transients get faster and systems start generating more heat, current sensing becomes significantly more difficult for designers to implement. For those reasons, there is a pressing industry need for new solutions that provide accurate, isolated sensing at even faster data rates.

Understanding these issues, Allegro MicroSystems offers the ACS37035, an integrated Hall-effect current sensor that pairs high-bandwidth sensing with factory-trimmed accuracy and configurable overcurrent protection. With the ACS37035, engineers have a compact path to improve current measurement and protection in high-density power stages.

Modern Challenges in Current Sensing

A confluence of variables makes current sensing a particularly difficult challenge in modern applications. 

Bandwidth at Fast Switching Speeds

When designing fast-switching power stages, engineers need sensing bandwidth beyond what most current sensors offer. Many integrated sensors roll off somewhere between 100 and 250 kHz, but a fast control loop often needs flat response approaching 1 MHz to track its current accurately.

For example, pulse-width modulation (PWM) motor drives and switch-mode power supplies switch at hundreds of kilohertz, and the newest wide-bandgap designs push fundamental switching frequencies past 1 MHz. Even when the switching frequency remains low, the sharp edges of a GaN or SiC device carry harmonic content that reaches well into the megahertz range, with the fastest edges completing in tens of nanoseconds. 

A sensor that responds only on a microsecond timescale can’t resolve those events. Worse, the added delay appears as phase lag in the control loop, and that lag eats into the phase margin needed to stay stable. As a result, too-slow sensors return incomplete measurements, giving the controller only an approximation of the actual power-stage behavior.

Drift Over Temperature

In addition to speed, power engineers also lose sensing accuracy to temperature. 

• In shunt-based sensing, resistors are prone to changing value as they heat. 

• Unchopped Hall sensors drift as junction temperature climbs

These problems are most significant when sensing at low currents, as offset error can easily dominate the reading and hide small light-load signals. Manufacturers may correct the sensor inaccuracy after assembly with field calibration and programming, but that only raises cost and line time.

How to Implement Isolation?

Power engineers working above a few hundred volts also need galvanic isolation between the power stage and the low-voltage controller.

In traditional shunt-based sensing, a shunt resistor sits directly in the current path, so it dissipates I²R power as heat and provides no isolation on its own. Recovering an isolated measurement means adding an isolation amplifier across the barrier, which brings extra components, cost, and propagation delay. 

Magnetic sensing takes a different route. It measures the field around the conductor rather than a voltage across it, so it stays out of the current path, avoids the I²R loss, and inherently provides isolation. The tradeoff is that a single-ended magnetic sensor also picks up stray fields from nearby conductors and can’t separate that interference from the signal it is trying to measure.

Protection Demands Fast Response

Where current sensing depends on fast response times, no place has a smaller margin for error than protection circuitry. In a hard short circuit, the fault current can rise at hundreds of amperes per microsecond, and wide-bandgap switches tolerate that condition for only a few microseconds before their junction temperature runs away and the device is destroyed. That leaves the sensing and comparison path just microseconds to detect the overcurrent, flag the fault, and command a shutdown before the damage is done. A sensor with slow response, or a fault path with long propagation delay, simply can’t act inside that window.  

Allegro’s ACS37035 Current Sensor

Allegro designed the ACS37035 to solve those challenges specifically for applications such as motor control, switch-mode power supply, and load management. Qualified to AEC-Q100 Grade 1 for ambient operation to 125°C, the line spans bidirectional ranges of ±20, ±40, and ±65 A across 3.3 and 5 V supplies. And, all six variants share the same SOICW-16 footprint, so designers can move up or down the current range as their power stages grow without reworking the board.

1 MHz Sensing Bandwidth

Allegro built the Hall-effect sensor to meet all four criteria in a single SOICW-16 package, opening with 1 MHz of bandwidth and a typical 0.45 µs response. At that speed, the sensor can track PWM and fast-switching waveforms as they happen, resolving the edges that a slower part would blur and feeding the control loop without adding the phase lag that erodes stability.

Chopper-Stabilized Accuracy

To hold accuracy across temperature, Allegro chopper-stabilizes the signal path, continuously canceling the offset that would otherwise climb with die temperature. Sensitivity error stays within ±1.5% from 25 to 125°C and withino ±3% between -40°C and 125°C, while maximum offset voltage stays within ±10 mV from 25 to 125°C. The sensor also reads its Hall plates differentially to reject the common-mode fields that nearby conductors inject, and Allegro reports no magnetic hysteresis.

Reinforced Isolation in the Package

Allegro builds reinforced isolation into the same package, separating the sensing die from the conductor it measures and letting designers place the sensor on the high side at full bus potential. That barrier carries a 500 VRMS reinforced working voltage along with a 4242 VRMS withstand that Allegro rates over a 60-second test. 

The device is also rated to UL 62368-1 (edition 3) with certification pending, and features 8 mm of clearance and creepage. Meanwhile, a typical 1 mΩ internal conductor resistance keeps insertion loss and self-heating low, so the isolation arrives without a power penalty.

Adjustable Overcurrent Protection

The ACES37035 includes an adjustable overcurrent fault on the same die, separate from the analog measurement, so the response never waits on filtering. Designers set the trip point anywhere between 50 and 200% of full-scale current with a resistor divider on the VOC pin, and when the current crosses that threshold, the open-drain FAULT output trips in a typical 0.5 µs. With that fault response, the device can support fast overcurrent detection, helping give the gate-driver protection path time to shut the stage down before damage occurs.

Sensing Faster, Protecting Sooner

As power designers drive up switching speed and current density in their systems, control and protection will only ever be as reliable as the current sensing underneath them. Sensing that keeps up with the switch lets engineers spend design margin on performance instead of on guard bands. With parts like the ACS37035, Allegro is enabling a future for better-performing, safer, and more reliable power electronics.

To explore detailed specifications and learn more about this solution, visit the Allegro ACS37035 page on the Mouser website.