DRV425 Datasheet, PDF (20/37 Pages) Texas Instruments – DRV425 Fluxgate Magnetic-Field Sensor

DRV425
SBOS729 – OCTOBER 2015
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Feature Description (continued)
7.3.1.4 Magnetic Field Range, Overrange Indicator, and Error Flag
The measurement range of the DRV425 is determined by the amount of current driven into the compensation coil
and the output voltage range of the shunt-sense amplifier. The maximum compensation current is limited by the
supply voltage and the series resistance of the compensation coil and the shunt.
The magnetic field range is adjusted with the external shunt resistor. The DRV425 System Parameter Calculator,
SLOC331 provides the maximum shunt resistor values depending on the supply voltage (VDD) and the selected
reference voltage (V_REFIN) for various magnetic field ranges.
For proper operation at a maximum field (B_MAX), choose a shunt resistor (R_SHUNT) using Equation 3
  _RSHUNT
d
min
  VDD  V_REFIN ,V_REFIN  0.085 V
B_MAX u12.2 A/T u 4 V/V
where
• VDD = minimum supply voltage of the DRV425 (V),
• V_REFIN = common-mode voltage of the shunt-sense amplifier (V), and
• B_MAX = desired magnetic field range (T)
(3)
Alternatively, to adjust the output voltage of the DRV425 for a desired maximum voltage (V_VOUTMAX), use
Equation 4:
R_SHUNT
@
V_VOUTMAX V_REFIN
B_MAX K12.2 A/T K 4 V/V
where
• V_VOUTMAX = desired maximum output voltage at VOUT pin (V), and
• B_MAX = desired magnetic field range (T)
(4)
To avoid railing of the compensation coil driver, assure that Equation 5 is fulfilled:
    BMAX u (RCOIL  RSHUNT ) u12.2A / T  0.1V d min
2
VDD  V_REFIN , V_REFIN
where
• B_MAX = desired magnetic field range (T),
• R_COIL = compensation coil resistance (Ω),
• VDD = minimum supply voltage of the DRV425 (V), and
• V_REFIN = selected internal reference voltage value (V)
(5)
The DRV425 System Parameter Calculator, SLOC331 is designed to assist with selecting the system
parameters.
The DRV425 offers two diagnostic output pins to detect large fields that exceed the measurement range of the
sensor: the overrange indicator (OR) and the ERROR flag.
In normal operation, the DRV425 sensor feedback loop compensates the magnetic field inside the fluxgate to
zero. Therefore, a large field inside the fluxgate indicates that the feedback loop is not properly working and the
sensor output is invalid. To detect this condition, the ERROR pin is pulled low if the internal field exceeds 1.6
mT. The ERROR output is suppressed for 4 µs to 6 µs to prevent an undesired reaction to transients or noise.
For static and slowly varying ambient fields, the ERROR pin triggers when the ambient field exceeds the sensor
measurement range by more than 1.6 mT. For dynamic magnetic fields that exceed the sensor bandwidth as
specified in the Specifications section, the feedback loop response is too slow to accurately compensate the
internal field to zero. Therefore, high-frequency fields can trigger the ERROR pin, even if the ambient field does
not exceed the measurement range by 1.6 mT.
In addition, the low-active overrange pin (OR) indicates railing of the output of the shunt-sense amplifier. The OR
output is suppressed for 2.5 µs to 3.5 µs to prevent an undesired reaction to transients or noise. The OR pin trip
level refers to the output voltage value of the shunt-sense amplifier as specified in the Specifications section. Use
Equation 3 and Equation 4 to adjust the OR pin behavior to the specific system-level requirements.
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