7.2.1.1 Design Requirements

The design requirements for this design are:

• Load current: 0 A to 1 A

• Maximum output voltage: 4.9V

• Maximum shunt voltage: 100mV

7.2.1.2 Detailed Design Procedure

The transfer function of the circuit in Figure 7-1 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 100mV 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 100mΩ. The voltage drop produced by ILOAD and RSHUNT is

amplified by the OPAx310-Q1 to produce an output voltage of approximately 0V to 4.9V. The gain needed by the

OPAx310-Q1 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 49V/V, which is set with resistors RF and RG. Equation 4

sizes the resistors RF and RG, to set the gain of the OPAx310-Q1 to 49V/V.

Gain=1+ RFRG

(4)

Selecting RF as 57.6 kΩ and RG as 1.2 kΩ provides a combination that equals 49V/V. Figure 7-2 shows the

measured transfer function of the circuit shown in Figure 7-1. 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 best for the system

parameters.

7.2.1.3 Application Curve

0

1

2

3

4

5

0

0.2

0.4

0.6

0.8

1

I

LOAD (A)

C219

Figure 7-2. Low-Side, Current-Sense Transfer Function

OPA310-Q1

SBOSAI0A – DECEMBER 2023 – REVISED MAY 2024

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