DRV421_16 Datasheet, PDF (16/44 Pages) Texas Instruments – Integrated Magnetic Fluxgate Sensor for Closed-Loop Current Sensing

DRV421
SBOS704B – MAY 2015 – REVISED MARCH 2016
7 Detailed Description
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7.1 Overview
The DRV421 is a fully-integrated, magnetic fluxgate sensor, with the necessary sensor conditioning and
compensation circuitry for closed-loop current sensors. The device is inserted into an air gap of an external
ferromagnetic toroid core to sense the magnetic field. A compensation coil wrapped around the magnetic core
generates a magnetic field opposite to the one generated by the current flow to be measured.
At dc and low-frequencies, the magnetic field induced by the current in the primary conductor generates a flux in
the magnetic core. The fluxgate sensor detects the flux in the DRV421. The device filters the sensor output to
provide loop stability. The filter output connects to the built-in H-bridge driver that drives an opposing current
through the external compensation coil. The compensation coil generates an opposite magnetic field that brings
the original magnetic flux in the core back to zero.
At higher frequencies, the inductive coupling between the primary conductor and compensation coil directly
drives a current through the compensation coil.
The compensation current is proportional to the primary current (I_PRIMARY), with a value that is calculated using
Equation 1:
I_ICOMP = I_PRIMARY / N_WINDING
where
• N_WINDING = the number of windings of the compensation coil
(1)
This compensation current generates a voltage drop across a small external shunt resistor, R_SHUNT. An
integrated difference amplifier with a fixed gain of 4 V/V measures this voltage and generates an output voltage
that is referenced to REFIN and proportional to the primary current. The Functional Block Diagram section shows
the DRV421 used as a closed-loop current sensor, for both single-ended and differential primary currents.
7.2 Functional Block Diagram
compensation
coil
magnetic
core
VDD GND
DRV421
Fluxgate Sensor Front-End
ICOMP1
RSHUNT
ICOMP2 AINP
AINN
Shunt
Sense
Amplifier
return current
conductor
(optional)
primary
current
conductor
Fluxgate
H-Bridge
Sensor Integrator and Filter Driver
Device Control and Degaussing
OR ER DEMAG GSEL0 GSEL1
VOUT
REFIN
1.65 V or 2.5 V
Voltage Reference
REFOUT
RSEL0 RSEL1
16
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