OPA310 datasheet(33/55 Pages) TI

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9.2.1.1 Design Requirements

The design requirements for this design are:

• Load current: 0 A to 1 A

• Maximum output voltage: 4.9 V

• Maximum shunt voltage: 100 mV

9.2.1.2 Detailed Design Procedure

The transfer function of the circuit in Figure 9-1 is given in Equation 1.

VOUT= ILOAD ×RSHUNT× 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

shown using Equation 2 .

RSHUNT= VSHUNT_MAX

ILOAD_MAX = 100 mV1 A=100 mΩ

(2)

Using Equation 2, RSHUNT is calculated to be 100 mΩ. The voltage drop produced by ILOAD and RSHUNT is

amplified by the OPAx310 to produce an output voltage of approximately 0 V to 4.9 V. The gain needed by the

OPAx310 to produce the necessary output voltage is calculated using Equation 3.

Gain=VOUT_MAX − VOUT_MIN

VIN_MAX − VIN_MIN

(3)

Using Equation 3, the required gain is calculated to be 49 V/V, which is set with resistors RF and RG. Equation 4

sizes the resistors RF and RG, to set the gain of the OPAx310 to 49 V/V.

Gain=1+ RFRG

(4)

Selecting RF as 57.6 kΩ and RG as 1.2 kΩ provides a combination that equals 49 V/V. Figure 9-2 shows the

measured transfer function of the circuit shown in Figure 9-1. Notice that the gain is only a function of the

feedback and gain resistors. This gain is adjusted by varying the ratio of the resistors and the actual resistors

values are determined by the impedance levels that the designer wants to establish. The impedance level

determines the current drain, the effect that stray capacitance has, and a few other behaviors. There is no

optimal impedance selection that works for every system; choose an impedance that is ideal for the system

parameters.

9.2.1.3 Application Curve

0.2

0.4

0.6

0.8

LOAD (A)

C219

Figure 9-2. Low-Side, Current-Sense Transfer Function

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OPA310, OPA2310, OPA4310

SBOSAA1C – APRIL 2022 – REVISED SEPTEMBER 2022

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