The 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.

The maximum voltage out on the SNS pin is clamped to V_SNSFH which is the fault voltage level. In order 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 will be ~3.3 V. However, if the voltage at DIAG_EN is above 3.3 V, then the fault SNS voltage, V_SNSFH, will track that voltage up to 5 V. This is done because the GPIO voltage output that is powering the diagnostics through DIAG_EN, will be 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 should 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 RSNS 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 needs to measure, I_LOAD,max. This boundary equation can be seen in Equation 1.

Figure 8-2 Voltage Indication on the Current-Sense Pin

The maximum current the system wants to read, I_LOAD,max, needs to be below the current limit threshold because once the current limit threshold is tripped the V_SNS value will go to V_SNSFH. Additionally, currents being measured should be below 7 A to ensure that the current sense output is not saturated.

Figure 8-3 Current-Sense and Current-Limit Block Diagram

Since 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.