DRV421_16 Datasheet, PDF (24/44 Pages) Texas Instruments – Integrated Magnetic Fluxgate Sensor for Closed-Loop Current Sensing

DRV421
SBOS704B – MAY 2015 – REVISED MARCH 2016
www.ti.com
7.3.8 Magnetic Core Demagnetization
Ferromagnetic cores can have a significant remanence (residual magnetism in the absence of any currents).
This core magnetization is caused by strong external magnetic fields, overcurrent conditions in the system, or if a
significant primary current flows when the sensor is not powered. This remaining magnetic field is
indistinguishable from an actual primary current, and creates a magnetic offset error. This magnetic offset error
limits the precision and the dynamic range of the current sensor, and is independent of the fluxgate sensor front-
end offset specified in this data sheet.
To reduce errors caused by core magnetization, the DRV421 features a unique closed-loop demagnetization
feature. Conventional open-loop demagnetization techniques rely on driving a fixed ac waveform through the
compensation coil. Instead, the DRV421 demagnetization feature first measures the magnetic offset using its
integrated fluxgate sensor, and then drives a controlled ac waveform to reduce the measured magnetization.
This method results in significantly better results. Moreover, any fluxgate offset is part of the closed-loop
demagnetization measurement, and therefore removed along with core magnetization, leaving only fluxgate
offset drift over temperature as an error source.
Start the demagnetization feature on demand by pulling the DEMAG pin high for at least 25 µs. This process
starts a 500-ms demagnetization cycle. During this time, the error pin (ER) is pulled low to indicate that the
output is not valid. When DEMAG is high during power up, the demagnetization cycle initiates immediately after
the supply voltage crosses the power-up threshold. Hold DEMAG low to avoid this cycle during start up. To abort
the demagnetization cycle, pull DEMAG low for longer than 25 µs. Figure 61 shows the ICOMPx output behavior
during a demagnetization sequence. Figure 62 shows the reduced error resulting from core demagnetization.
6
V_DD
V_OUT
5
V_ICOMP1
V_ICOMP2
4
3
2
1
0
-0.1
0
0.1 0.2 0.3 0.4 0-.85 0.6
Time (ms)
D052
Figure 61. Demagnetization Sequence
1000 mA
100 mA
10 mA
Error Current Level
before Demagnetization
Repeatable Error Current
Level after Demagnetization
1 mA
Figure 62. Impact of Demagnetization on Error Current
During a demagnetization cycle, the primary current must be zero because the resulting magnetic field cannot be
distinguished from the remanence of the core. A demagnetization cycle in the presence of primary current (or
any other sources of magnetic field) leads to residual errors because the demagnetization feature attempts to
reduce the primary-generated field to zero, but significantly magnetizes the core instead of demagnetizing the
core. If a primary current is present that is large enough to saturate the fluxgate sensor during start up, the
DRV421 skips demagnetization (regardless of the level on the DEMAG pin), and the search function starts
instead (see the Search Function section for more details).
To reduce effects from the earth's magnetic field, degauss in the same orientation as nominal operation of the
system.
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