ZHCSC46C February 2014 – June 2021 INA300
The typical approach for setting the limit threshold voltage is to connect a resistor from the LIMIT terminal to ground. The value of this resistor, RLIMIT, is chosen to create a corresponding voltage at the LIMIT terminal equivalent to the voltage, VTRIP, developed by the load current flowing through the current-sensing resistor. An internal 20-µA current source is present at the LIMIT terminal that creates the corresponding voltage depending on the value of RLIMIT. In the equations from Table 7-2, VTRIP represents the overcurrent threshold the device is programmed to monitor for and VLIMIT is the programmed signal set to detect the VTRIP level. The term noise adjustment factor (NAF) is included in the VLIMIT equation for the 10-µs delay setting. This value is equal to 500 µV and adjusts the operating point for the internal noise in this delay setting. The 50-µs and 100-µs delay settings do not use the NAF term in calculating the VLIMIT threshold. See Section 7.3.7 for more details on the noise adjustment factor.
In Table 7-2, the process for calculating the required value for RLIMIT to set the appropriate threshold voltage, VLIMIT, is shown. This calculation is based on the 10-µs delay setting so the NAF term is included in the calculation. For a delay setting of 50 µs or 100 µs, the NAF term is omitted.
|VTRIP||Desired current trip value||ILOAD × RSENSE|
|VLIMIT||Programmed threshold limit voltage||VLIMIT = VTRIP|
|VLIMIT (1)||Threshold voltage||(ILIMIT × RLIMIT) – NAF|
|RLIMIT (1)||Threshold limit setting resistor||(VLIMIT + NAF) / ILIMIT|
|RLIMIT (1)||Limit setting resistor||(VLIMIT + 500 µV) / 20 µA|
TI recommends using NAF in calculating the value for VLIMIT and RLIMIT at the 10-µs delay setting. Removing NAF from the VLIMIT and RLIMIT calculation at the 10-µs delay setting lowers the trigger point of the alert output. Lowering the trigger point results in the device issuing an overcurrent alert prior to reaching the corresponding VTRIP threshold. The averaging effect included with the 50-µs and 100-µs delay settings inherently eliminates the effect internal noise has on the threshold voltage.