ALERT

Alert

Output

13

INA381

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SBOS848 – DECEMBER 2017

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7.3.7.1 Selecting a Current-Sensing Resistor: Example

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

requires 5% accuracy for detecting a 10-A overcurrent event under 20 µs 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. Given the total

error budget of 5%, the INA381 total error is less than 1%. The INA381 is well suited for this application as up to

1% of error is available to be attributed to the measurement error of the device under these conditions.

As shown in Table 2, the maximum value calculated for the current-sensing resistor with these requirements is

2.5 mΩ. Although this value satisfies the maximum power dissipation requirement of 250 mW, headroom is

available from the 2.5% maximum total overcurrent detection error to reduce the value of the current-sensing

resistor and reduce the power dissipation further. Selecting a 1.5-mΩ, current-sensing resistor value offers a

good tradeoff for reducing the power dissipation in this scenario by approximately 40% and still remaining within

the accuracy region.

PARAMETER

EQUATION

VALUE

UNIT

Maximum current

10

A

Maximum allowable power dissipation

250

mW

2

2.5

mΩ

500

µV

Initial offset voltage error

2%

Gain error

1%

Total measurement error

G

2.23%

Allowable current threshold accuracy

5%

Total system overcurrent response time

10

µs

Allowable overcurrent response

20

µs

7.3.8 Hysteresis

The on-board comparator in the INA381 is designed to reduce the possibility of oscillations in the alert output

when the measured signal level is near the overlimit threshold level as a result of noise. When the voltage

voltage must drop below the CMPREF pin threshold voltage, as shown in Figure 6, by the hysteresis level of 50

mV for the ALERT pin to de-assert and return to the nominal high state. The INA381 is designed with a

hysteresis of 50 mV.

Figure 6. Typical Comparator Hysteresis