DRV421_15 Datasheet, PDF (23/44 Pages) Texas Instruments – DRV421 Integrated Magnetic Fluxgate Sensor for Closed-Loop Current Sensing

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DRV421
SBOS704A – MAY 2015 – REVISED JULY 2015
Use the inverse of Equation 1 to calculate the current measurement range. For example, if the compensation coil
has 1000 windings, the maximum measurement range is 210 A at a 5-V supply (210-mA minimum compensation
driver capability × 1000 windings). The inductive coupling between primary current and compensation coil initially
provides a correct compensation current. However, over time, the compensation current drops to 210 mA and
the field inside the core increases beyond the measurement range of the fluxgate. Thus, the sensor output
returns to zero because of saturation.
This zero output causes unpredictable behavior in the analog control loop. For example, as a result of an invalid
fluxgate output, the H-bridge drives the wrong compensation current and generates a large magnetic field
through the compensation coil. This magnetic field keeps the fluxgate in saturation and leads to system lockup.
This unpredicatable behavior exists for any fluxgate-based current sensor.
For proper handling of overload currents, the DRV421 features a two-step overload detection and control
function. Firstly, the polarity of the last four fluxgate sensor outputs exceeding a threshold value of approximately
13 µT are internally stored. Secondly, the DRV421 features an additional circuitry that verifies every 4 µs whether
the fluxgate is saturated. If saturation is detected, digital circuitry overrides the fluxgate output and provides a
high output according to the polarity detected during the last valid sensor output. As a result, the H-brigde drives
the outputs to the supply rails, making sure that the magnetic field returns to within the fluxgate range as soon as
the current returns to within the measurement range. After this happens, the fluxgate is no longer saturated, and
normal analog feedback loop operation resumes. During fluxgate saturation, the error pin is pulled low to signal
that the current exceeds the measurement range (see the Error Flag section).
For correct operation of this overload control feature, at least 10 µs are required between the time the field
exceeds the polarity detection threshold (13 µT) and the saturation trip level (1.7 mT). Initially, fast primary
current steps are inductively coupled to the compensation coil (transformer effect); therefore, the primary current
rise time is not limited. Instead, the rise time is determined by the compensation coil inductance; a larger
inductance leads to a slower compensation current decrease. The minimum required inductance is 100 mH; for
optimal robustness, use 300 mH (see the Magnetic Core Design section for detailed requirements).
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