OPA2991IDDFR datasheet(36/77 Pages) TI

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8.2.1.2 Detailed Design Procedure

The transfer function of the circuit in Figure 8-1 is given in Equation 1:

OUT

LOAD

SHUNT

Gain

(1)

The load current (ILOAD) produces a voltage drop across the shunt resistor (RSHUNT). The load current is set from

0 A to 1 A. To keep the shunt voltage below 100 mV at maximum load current, the largest shunt resistor is

defined using Equation 1:

SHUNT

MAX

SHUNT

LOAD

MAX

100mV

100m

(2)

Using Equation 1, RSHUNT is calculated to be 100 mΩ. The voltage drop produced by ILOAD and RSHUNT is

amplified by the OPA991 to produce an output voltage of 0 V to 4.9 V. The gain needed by the OPA991 to

produce the necessary output voltage is calculated using Equation 1:

OUT

MAX

OUT

MIN

MAX

MIN

Gain

(3)

Using Equation 1, the required gain is calculated to be 49 V/V, which is set with resistors RF and RG. Equation 1

is used to size the resistors, RF and RG, to set the gain of the OPA991 to 49 V/V.

Gain

(4)

Choosing RF as 360 kΩ, RG is calculated to be 7.5 kΩ. RF and RG were chosen as 360 kΩ and 7.5 kΩ because

they are standard value resistors that create a 49:1 ratio. Other resistors that create a 49:1 ratio can also be

used. Figure 8-2 shows the measured transfer function of the circuit shown in Figure 8-1.

8.2.1.3 Application Curves

ILOAD (A)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Figure 8-2. Low-Side, Current-Sense, Transfer Function

OPA991, OPA2991, OPA4991

SBOS969D – OCTOBER 2019 – REVISED JULY 2020

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