17

INA301

www.ti.com

SBOS713A – SEPTEMBER 2015 – REVISED FEBRUARY 2016

Product Folder Links: INA301

Submit Documentation Feedback

Copyright © 2015–2016, Texas Instruments Incorporated

7.3.4 Selecting a Current-Sensing Resistor

The device measures the differential voltage developed across a resistor when current flows through the

component to determine if the current being monitored exceeds a defined limit. This resistor is commonly

referred to as a current-sensing resistor or a current-shunt resistor, with each term commonly used

interchangeably. The flexible design of the device allows for measuring a wide differential input signal range

across this current-sensing resistor.

Selecting the value of this current-sensing resistor is based primarily on two factors: the required accuracy of the

current measurement and the allowable power dissipation across the current-sensing resistor. Larger voltages

developed across this resistor allow for more accurate measurements to be made. Amplifiers have fixed internal

errors that are largely dominated by the inherent input offset voltage. When the input signal decreases, these

fixed internal amplifier errors become a larger portion of the measurement and increase the uncertainty in the

measurement accuracy. When the input signal increases, the measurement uncertainty is reduced because the

fixed errors are a smaller percentage of the signal being measured. Therefore, the use of larger value current-

sensing resistors inherently improves the measurement accuracy.

However, a system design trade-off must be evaluated through use of larger input signals for improving the

measurement accuracy. Increasing the current sense resistor value results in an increase in power dissipation

across the current-sensing resistor. Increasing the value of the current-shunt resistor increases the differential

voltage developed across the resistor when current passes through the component. This increase in voltage

across the resistor increases the power that the resistor must be able to dissipate. Decreasing the value of the

current-shunt resistor value reduces the power dissipation requirements of the resistor, but increases the

measurement errors resulting from the decreased input signal. Selecting the optimal value for the shunt resistor

requires factoring both the accuracy requirement for the specific application and the allowable power dissipation

of this component.

An increasing number of very low ohmic-value resistors are becoming more widely available with values reaching

down as low as 200 µ

Ω or lower with power dissipations of up to 5 W that enable large currents to be accurately

monitored with sensing resistors.

7.3.4.1 Selecting a Current-Sensing Resistor: Example

In this example, the trade-offs involved in selecting a current-sensing resistor are discussed. This example

requires 2.5% accuracy for detecting a 10-A overcurrent event where only 250 mW is allowable for the

dissipation across the current-sensing resistor at the full-scale current level. Although the maximum power

dissipation is defined as 250 mW, a lower dissipation is preferred to improve system efficiency. Some initial

assumptions are made that are used in this example: the limit-setting resistor (RLIMIT) is a 1% component and the

maximum tolerance specification for the internal threshold setting current source (0.5%) is used. Given the total

error budget of 2.5%, up to 1% of error is available to be attributed to the measurement error of the device under

these conditions.