DRV421_15 Datasheet, PDF (25/44 Pages) Texas Instruments – DRV421 Integrated Magnetic Fluxgate Sensor for Closed-Loop Current Sensing

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DRV421
SBOS704A – MAY 2015 – REVISED JULY 2015
7.3.9 Search Function
Closed-loop current sensors usually require primary current to be applied only after the sensor is powered up.
This requirement allows the feedback loop to start from zero current operation; the magnetic core is maintained
at zero flux at all times, thus preventing magnetization. Moreover, the DRV421 integrated fluxgate has a limited
measurement range of 1.7 mT. As a result, the presence of a significant primary current at power up saturates
the fluxgate, and the system feedback loop does not work; similar to the presence of an overload current (see
the Overload Detection and Control section).
The DRV421 search function allows for a power up in presence of primary dc current. If the fluxgate is saturated
at power up, the digital logic of the DRV421 connects ICOMP1 to VDD and COMP2 to GND for 30 ms. Because
of the compensation coil inductance, the compensation current slowly increases during this time, and depending
on the primary current polarity, may at some point compensate the primary current. In this case, the fluxgate
sensor desaturates and normal operation initiates. If the fluxgate sensor is still saturated after 30 ms, the voltage
polarity on ICOMPx pins is inverted (ICOMP1 = GND, ICOMP2 = VDD) and the process repeats for opposite
primary current polarity. If the fluxgate remains saturated after 60 ms, the error state persists and the error pin
ER remains active low. Figure 63 shows a search sequence starting with the wrong polarity.
VDD
Search Function
with Wrong Polarity
VICOMP1
VICOMP2
VER
Inverted
Polarity
Normal
Operation
Figure 63. Search Sequence Starting with Wrong Polarity
The search funciton cannot be used for primary ac currents. Moreover, the presence of primary current before
the sensor is powered up may lead to core magnetization, and thus offset shift. Therefore, for robust operation,
do not power up in the presence of primary currents.
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