ZHCSC46C February 2014 – June 2021 INA300
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.
As shown in Table 7-1, 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 5% maximum total 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 tradeoff for reducing the power dissipation in this scenario by approximately 40%, while still remaining within the defined accuracy region.
|Maximum measurement error||5%|
|PRSENSE||Maximum allowable RSENSE power dissipation||RSENSE × IMAX2||250||mW|
|Initial error||RLIMIT + ILIMIT tolerances||1.5%|
|RSENSE_MAX||Maximum sensing resistor value||PRSENSE / IMAX2||2.5||mΩ|
|VSENSE_MAX||Input sense voltage||RSENSE_MAX × IMAX||25||mV|
|VOS Error||Offset voltage error||(VOS / VSENSE_MAX) × 100||2%|
|Error_Available||Maximum allowable offset error||Maximum Error – Initial Error||3.5%|
|VSENSE_MIN||Minimum input sense voltage||VOS / (Error_Available / 100)||14.3||mV|
|RSENSE_MIN||Minimum sensing resistor value||VSENSE_MIN / IMAX||1.43||mΩ|
|PRSENSE_MIN||Minimum power dissipation||RSENSE_MIN × IMAX2||143||mW|