DRV401AMDWPREP datasheet(18/33 Pages) TI

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DRV401

External

Buffer

V+

−

Compensation

Coil

SHUNT

COMP1

COMP2

SHUNT SENSE AMPLIFIER

4 +

) R5

SHUNT

) R3

DRV401-EP

SBVS104 – JANUARY 2008............................................................................................................................................................................................... www.ti.com

Figure 4. DRV401 With External Compensation Coil Driver and RSHUNT Connected to GND

The differential (H-bridge) driver arrangement for the compensation coil requires a differential sense amplifier for

the shunt voltage. This differential amplifier offers wide bandwidth and a high slew rate for fast current sensors.

Excellent dc stability and accuracy result from an auto-zero technique. The voltage gain is 4 V/V, set by precisely

matched and stable internal SiCr resistors.

Both inputs of the differential amplifier are normally connected to the current shunt resistor. This resistor adds to

the internal (10 k

Ω) resistor, slightly reducing the gain in this leg. For best common-mode rejection (CMR), a

dummy shunt resistor (R5) is placed in series with the REFIN pin to restore matching of both resistor dividers, as

shown in Figure 5a.

For gains of 4 V/V:

With R2/R1 = R4/R3 = 4; R5 = RSHUNT × 4

Typically, the gain error resulting from the resistance of RSHUNT is negligible; for 70 dB of common-mode

rejection, however, the match of both divider ratios needs to be better 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 DRV401 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 RF and CF values, refer to the specific converter recommendations in the specific

product data sheet. Empirical evaluation may be necessary to obtain optimum results.

The output can drive 100 pF directly and shows 50% overshoot with approximately 1 nF capacitance. Adding RF

allows much larger capacitive loads, as shown in Figure 5b and Figure 5c. Note that with RF of only 20 Ω, the

load capacitor should be either smaller than 1 nF or larger than 33 nF to avoid overshoot; with RF 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 of 2.5 V. Using the same reference for REFIN

and the ADC avoids mismatch errors that exist between two reference sources.

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