DRV411_15 Datasheet, PDF (18/34 Pages) Texas Instruments – Sensor Signal Conditioning IC for Closed-Loop Magnetic Current Sensors

DRV411
SBOS693B – AUGUST 2013 – REVISED DECEMBER 2013
www.ti.com
Compensation
Coil
IAN1
R1
10 k 
R2
40 k 
RSHUNT
ICOMP
IAIN2
R3
10 k 
_
Differential
Amplifier
+
R4
40 k 
DRV411
VOUT
RF
500 
optional
CF
10 nF
ADC
REFIN
R5
(Dummy Shunt)
REFIN (compensated)
Figure 52. Internal Difference Amplifier with Example of a Decoupling Filter
Typically, the gain error resulting from the resistance of R_SHUNT is negligible; for 70 dB of common-mode
rejection, however, the match of both divider ratios must be higher than 1/3000.
The amplifier output can drive close to the supply rails, and is designed to drive the input of a SAR-type ADC;
adding an RC low-pass filter stage between the DRV411 and the ADC is recommended. This filter not only limits
the signal bandwidth but also decouples the high-frequency component of the converter input sampling noise
from the amplifier output. For R_F and C_F values, refer to the specific converter recommendations in the
respective product data sheet. Empirical evaluation may be necessary to obtain optimum results.
The output drives 100 pF directly and shows 50% overshoot with approximately 1-nF capacitance. Adding R_F
allows for much larger capacitive loads. Note that with an R_F of only 20 Ω, the load capacitor must be either less
than 1 nF or more than 33 nF to avoid overshoot; with an R_F of 50 Ω, this transient area is avoided.
The reference input (REFIN) is the reference node for the exact output signal (VOUT). Connecting REFIN to the
reference output (REFOUT) results in a live zero reference voltage that is user-selectable. Use the same
reference for REFIN and the ADC to avoid mismatch errors that exist between the two reference sources.
OVERRANGE COMPARATOR
High peak current can overload the differential amplifier connected to the shunt. The OR pin, an open-drain
output, indicates an overvoltage condition for the differential amplifier by pulling low. The output of this flag is
suppressed for 3 μs, preventing unwanted triggering from transients and noise. This pin returns to high as soon
as the overload condition is removed (an external pull-up is required to return the pin high).
This error flag not only provides a warning about a signal-clipping condition, but is also a window comparator
output for actively shutting off circuits in the system. The value of the shunt resistor defines the operating window
for the current and sets the ratio between the nominal signal and the trip level of the overrange flag. The trip
current of this window comparator is calculated using the following example:
With a 5-V supply, the output voltage swing is approximately ±2.45 V (load and supply voltage-dependent).
The gain of 4 V/V enables an input swing of ±0.6125 V.
Thus, the clipping current is IMAX = 0.6125 V / R_SHUNT.
See Figure 13 and Figure 14 in the Typical Characteristics section for details.
The overrange condition is internally detected as soon as the amplifier exceeds its linear operating range, not
just a preset voltage level. Therefore, the error of the overrange comparator level is reliably indicated in fault
conditions such as output shorts, low load, or low-supply conditions. As soon as the output cannot drive the
voltage higher, the flag is activated. This configuration is a safety improvement over a voltage-level comparator.
Note that the internal resistance of the compensation coil may prevent high compensation current from flowing
because of ICOMP driver overload. Therefore, the differential amplifier may not overload with this current.
However, a fast rate of change of the primary current is transmitted through transformer action and safely
triggers the overload flag.
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