DRV421_16 Datasheet, PDF (17/44 Pages) Texas Instruments – Integrated Magnetic Fluxgate Sensor for Closed-Loop Current Sensing

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DRV421
SBOS704B – MAY 2015 – REVISED MARCH 2016
7.3 Feature Description
7.3.1 Fluxgate Sensor
The fluxgate sensor of the DRV421 is uniquely suited for closed-loop current sensors because of its high
sensitivity, low noise, and low offset. The fluxgate principle relies on repeatedly driving the sensor in and out of
saturation; therefore, the sensor is free of any significant magnetic hysteresis. The feedback loop accurately
drives the magnetic flux inside the core to zero.
The DRV421 package is free of any ferromagnetic materials in order to prevent magnetization by external fields
and to obtain accurate and hysteresis-free operation. Select nonmagnetizable materials for the printed circuit
board (PCB) and passive components in the direct vicinity of the DRV421; see the Layout Guidelines section for
more details.
Figure 52 shows the orientation of the fluxgate sensor and the direction of magnetic sensitivity inside of the
package. This orientation is marked by a straight line on top of the package.
D421
TI Date
Code
Figure 52. Orientation and Magnetic Sensitivity Direction of the Integrated Fluxgate Sensor
7.3.2 Integrator-Filter Function and Compensation Loop Stability
The DRV421 and the magnetic core are components of the system feedback loop that compensates the
magnetic flux generated by the primary current. Therefore, the loop properties and stability depend on both
components. Four key parameters determine the stability and effective loop gain at high frequencies:
GSEL[1:0] Filter gain setting pins of the DRV421
G_CORE
Open-loop, current-to-field transfer of the magnetic core
Amount of magnetic field generated by 1 A of uncompensated primary current (unit is T/A).
N_WINDING
L
Number of compensation coil windings
Compensation coil inductance
A minimum inductance of 100 mH is required for stability. Higher inductance improves
overload current robustness (see the Overload Detection and Control section).
To properly select the filter gain of the DRV421, combine these three parameters into a modified gain factor
(G_MOD) using Equation 2:
G_MOD
G_CORE u N_WINDING
L
(2)
The effective loop gain is proportional to the current-to-field transfer of the magnetic core (larger field means
larger gain) and number of compensation coil windings (larger number of windings means larger compensation
field for a given input current). The compensation coil inductance adds a low-frequency pole to the system, thus
a larger inductance reduces the effective loop gain at higher frequencies. A more detailed review of system loop
stability is provided in application report SLOA224, Designing with the DRV421: Control Loop Stability.
For stable operation with a wide range of magnetic cores, the DRV421 features an adjustable loop filter
controlled with pins GSEL1 and GSEL0. Table 1 lists the different filter settings and the related core properties.
For standard closed-loop current transducer modules with medium inductance and small shunt resistor value,
use gain setting 10. Gain setting 01 features a higher integrator-filter crossover frequency of 3.8 kHz, and is
recommended for fault-current sensors with a large shunt resistor and medium inductance.
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