The high-accuracy current-sense function is internally implemented, which allows a better real-time monitoring effect and more-accurate diagnostics without further calibration. A current mirror is used to source 1 / K_SNS of the load current, flowing out to the external resistor between the SNS pin and GND, and reflected as voltage on the SNS pin.

K_SNS is the ratio of the output current and the sense current. The accuracy values of K_SNS quoted in the electrical characteristics do take into consideration temperature and supply voltage. Each device was internally calibrated while in production, so post-calibration by users is not required in most cases.

Figure 8-1 Current-Sense Accuracy

The maximum voltage out on the SNS pin is clamped to V_SNSFH, which is the fault voltage level. To make sure that this voltage is not higher than the system can tolerate, TI has correlated the voltage coming in on the DIAG_EN pin with the maximum voltage out on the SNS pin. If DIAG_EN is between V_IH and 3.3 V, the maximum output on the SNS pin is approximately 3.3 V. However, if the voltage at DIAG_EN is above 3.3 V, then the fault SNS voltage, V_SNSFH, tracks that voltage up to 5 V. Tracking is done because the GPIO voltage output that is powering the diagnostics through DIAG_EN is close to the maximum acceptable ADC voltage within the same microcontroller. Therefore, the sense resistor value, R_SNS, can be chosen to maximize the range of currents needed to be measured by the system. The R_SNS value must be chosen based on application need. The maximum usable R_SNS value is bounded by the ADC minimum acceptable voltage, V_ADC,min, for the smallest load current needed to be measured by the system, I_LOAD,min. The minimum acceptable R_SNS value has to ensure the V_SNS voltage is below the V_SNSFH value so that the system can determine faults. This difference between the maximum readable current through the SNS pin, I_LOAD,max × R_SNS, and the V_SNSFH is called the headroom voltage, V_HR. The headroom voltage is determined by the system but is important so that there is a difference between the maximum readable current and a fault condition. Therefore, the minimum R_SNS value has to be the V_SNSFH minus the V_HR times the sense current ratio, K_SNS divided by the maximum load current the system must measure, I_LOAD,max. Use the following equation to see the boundary equation.

The maximum current the system wants to read, I_LOAD,max, must be below the current-limit threshold because after the current-limit threshold is tripped the V_SNS value goes to V_SNSFH. Additionally, currents being measured must be below 6 A to ensure that the current sense output is not saturated.

Because this scheme adapts based on the voltage coming in from the MCU, there is no need to have a Zener diode on the SNS pin to protect from high voltages.