DRV421_15 Datasheet, PDF (20/44 Pages) Texas Instruments – DRV421 Integrated Magnetic Fluxgate Sensor for Closed-Loop Current Sensing

DRV421
SBOS704A – MAY 2015 – REVISED JULY 2015
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If the input signal is large, the amplifier output drives close to the supply rails. The amplifier output is able to drive
the input of a successive approximation register (SAR) analog-to-digital converter (ADC). For best performance,
add an RC low-pass filter stage between the shunt sense amplifier output and the ADC input. This filter limits the
noise bandwidth and decouples the high-frequency sampling noise of the ADC input from the amplifier output.
For filter resistor R_F and filter capacitor C_F values, refer to the specific converter recommendations in the
respective product data sheet.
The shunt sense amplifier output drives 100 pF directly and shows 50% overshoot with a 1-nF capacitance. Filter
resistor R_F extends the capacitive load range. 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.
Reference input REFIN is the common-mode voltage node for output signal VOUT. Use the internal voltage
reference of the DRV421 by connecting the REFIN pin to reference output REFOUT. To avoid mismatch errors,
use the same reference voltage for REFIN and the ADC. Alternatively, use an ADC with a pseudodifferential
input, with the positive input of the ADC connected to the VOUT and the negative input connected to REFIN of
the DRV421.
7.3.5 Overrange Comparator
High peak current across the shunt resistor can generate a voltage drop that overloads the shunt sense amplifier
input. The open-drain, active-low output overrange pin (OR) indicates an overvoltage condition of the amplifier.
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 resistor is required to return the
OR pin to high.
This OR output can be used as a window comparator to actively shut 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 comparator. The trip level (I_MAX) of this window comparator is calculated using
Equation 4:
I_MAX = Input Voltage Swing / R_SHUNT
where
• Input Voltage Swing = Output Voltage Swing / Gain
(4)
For 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 voltage swing of ±0.6125 V.
The resulting trip level is I_MAX = 0.6125 V / R_SHUNT.
See Figure 32 and Figure 33 in the Typical Characteristics section for details.
Common window comparators use a preset level to detect an overrange condition. The DRV421 internally
detects an overrange condition as soon as the amplifier exceeds the linear operating range, not just at a preset
voltage level. Therefore, the error is reliably indicated in faults such as output-short, low-load, or low-supply
conditions. This configuration is a safety improvement if compared to a standard voltage-level comparator.
The internal resistance of the compensation coil may prevent high compensation current flow because of H-
bridge driver overload; therefore, the shunt sense amplifier might not overload. However, a fast rate of change of
the primary current transmitted through transformer effect safely triggers the overload flag.
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