resistor or a current-shunt resistor, with each term used interchangeably. The flexible design of the device allows

for measuring a wide differential input signal range across this current-sensing resistor, which can extend up to

250 mV.

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 more accurate measurements. This large signal accuracy improvement

results from the fixed internal amplifier errors that are dominated by the inherent input offset voltage of the

device. 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 measured signal.

A system design trade-off for improving the measurement accuracy using larger input signals is the increase

in power 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 low ohmic-value resistors are becoming available with values as low as 200 µΩ, with

power dissipations of up to 5 W that enable large currents to be monitored with sensing resistors.

7.3.1.1 Selecting a Current-Sensing Resistor: Example

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

requires a 5% measurement accuracy for detecting a 10-A overcurrent event at a 50-µs delay setting 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 5%, up to 3.5% of error is available to be attributed to the

internal offset of the device.

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SBOS613C – FEBRUARY 2014 – REVISED JUNE 2021

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