In this example, the trade-offs involved in selecting a current-sensing resistor are described. This example requires 2.5% accuracy for detecting a 10-A overcurrent event, with only 250 mW of allowable power 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 in order to improve system efficiency. Some initial assumptions are made that are used in this example:

• the limit-setting resistor (R_LIMIT) is a 1% component
• 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.

As shown in Table 8-1, 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 in order 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% while still remaining within the accuracy region.