DRV421_15 Datasheet, PDF (19/44 Pages) Texas Instruments – DRV421 Integrated Magnetic Fluxgate Sensor for Closed-Loop Current Sensing

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DRV421
SBOS704A – MAY 2015 – REVISED JULY 2015
7.3.3 H-Bridge Driver for Compensation Coil
The H-bridge compensation coil driver provides the current for the compensation coil at pins ICOMP1 and
ICOMP2. A fully-differential driver stage maximizes the driving voltage that is needed to overcome the wire
resistance and inductance of the coil with a single 3.3-V or 5-V supply. The low impedance of the H-bridge driver
outputs over a wide frequency range provides a smooth transition between the compensation frequency range of
the integrator-filter stage and the high-frequency range of the primary current that directly couples into the
compensation coil according to the winding ratio (transformer effect).
The common-mode voltage of the H-bridge driver outputs is set by the RSEL pins (see the Voltage Reference
section). Thus, the common-mode voltage of the shunt sense amplifier is matched if the internal reference is
used.
The two compensation driver outputs are protected and accept inductive energy. However, for high-current
sensors, add external protection diodes (see the Protection Recommendations section).
Consider the polarity of the compensation coil connection to the output of the H-bridge driver. If the polarity is
incorrect, the H-bridge output drives to the power supply rails, even at low primary-current levels. In this case,
interchange the connection of pins ICOMP1 and ICOMP2 to the compensation coil.
7.3.4 Shunt Sense Amplifier
The compensation coil current creates a voltage drop across the external shunt resistor, R_SHUNT. The internal
differential amplifier senses this voltage drop. This differential amplifier offers wide bandwidth and a high slew
rate for fast current sensors. Excellent dc stability and accuracy result from an autozero technique. The voltage
gain is 4 V/V, set by precisely-matched and thermally-stable internal resistors.
Both AINN and AINP differential amplifier inputs are connected to the shunt resistor. This resistor, in series with
the internal 10-kΩ resistor, affects the overall gain and causes an additional gain error; this gain error is often
negligible. However, if a common-mode rejection of 70 dB is desired, the match of both divider ratios must be
higher than 1/3000. Therefore, for best common-mode rejection performance, place a dummy shunt resistor (R₅)
with a value higher than the shunt resistor in series with the REFIN pin to restore matching of both resistor
dividers, as shown in Figure 57.
AINN
R1
10 k 
DRV421
R2
40 k 
Compensation
Coil
RSHUNT
AINP
R3
10 k 
_
Shunt Sense
Amplifier
+
R4
40 k 
ICOMP2
ICOMP1
VOUT
RF
500 
optional
CF
10 nF
ADC
REFIN
R5
(Dummy Shunt)
REFIN (compensated)
Figure 57. Internal Difference Amplifier with Example of a Decoupling Filter
For an overall gain of 4 V/V, calculate the value of R₅ using Equation 3:
4 = ^R2 = ^{R4 + R5}
R₁ R_SHUNT + R₃
where:
• R₂ / R₁ = R₄ / R₃ = 4
• R₅ = R_SHUNT × 4
(3)
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