DRV401-EP Datasheet, PDF (17/33 Pages) Texas Instruments – SENSOR SIGNAL CONDITIONING IC FOR CLOSED-LOOP MAGNETIC CURRENT SENSOR

DRV401-EP
www.ti.com............................................................................................................................................................................................... SBVS104 – JANUARY 2008
COMPENSATION DRIVER
The compensation coil driver provides the driving current for the compensation coil. A fully differential driver
stage offers high signal voltages to overcome the wire resistance of the coil with only +5 V supply. The
compensation coil is connected between I_COMP1 and I_COMP2, both generating an analog voltage across the coil
(see Figure 3) that turns into current from the wire resistance (and eventually from the inductance). The
compensation current represents the primary current transformed by the turns ratio. A shunt resistor is connected
in this loop and the high-precision difference amplifier translates the voltage from this shunt to an output voltage.
Both compensation driver outputs provide low impedance over a wide frequency range to ensure smooth
transitions between the closed-loop compensation frequency range and the high-frequency range, where the
primary winding directly couples the primary current into the compensation coil at a rate set by the winding ratio.
The two compensation driver outputs are designed with protection circuitry to handle inductive energy. However,
additional external protection diodes might be necessary for high current sensors.
For reliable operation, a wire break in the compensation circuit can be detected. If the feedback loop is broken,
the integrating filter drives the outputs I_COMP1 and I_COMP2 to the opposite rails. With one of these pins coming
within 300 mV to ground, a comparator tests for a minimum current flowing between I_COMP1 and I_COMP2. If this
current stays below the threshold current level for at least 100 µs, the ERROR pin is asserted active (low). The
threshold current level for this test is less than 57 mA at 25°C and 65 mA at –40°C, if the I_COMP pins are fully
railed (see the Typical Characteristics). For sensors with high winding resistance (compensation coil resistance +
R_SHUNT) or connected to an external compensation driver, this function should be disabled by pulling the CCdiag
pin low.
R_MAX
+
V _OUT
65mA
Where:
V_OUT equals the peak voltage between I_COMP1 and I_COMP2 at a 65mA drive current.
R_MAX equals the sum of the coil and the shunt resistance.
EXTERNAL COMPENSATION COIL DRIVER
An external driver for the compensation coil can be connected to the I_COMP1 and I_COMP2 outputs. To prevent a
wire break indication, CCdiag has to be asserted low.
An external driver can provide both a higher drive voltage and more drive current. It also moves the power
dissipation to the external transistors, thereby allowing a higher winding resistance in the compensation coil and
more current. Figure 4 shows a block diagram of an external compensation coil driver. To drive the buffer, either
one or both I_COMP outputs can be used. Note, however, that the additional voltage gain could cause instability of
the loop. Therefore, the internal gain can be reduced by approximately 8 dB by asserting the GAIN pin high.
R_SHUNT is connected to GND to allow for a single-ended external compensation driver. The differential amplifier
can continue to sense the voltage, and used for the gain and over-range comparator or ERROR flag.
Copyright © 2008, Texas Instruments Incorporated
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