The high-accuracy current-sense function is internally implemented, which allows real-time monitoring 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. 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 3.3V, the maximum output on the SNS pin is approximately 3.3V. However, if the voltage at DIAG_EN is above 3.3V, then the fault SNS voltage, V_SNSFH, tracks that voltage up to 5V. 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 minimum R_SNS value is usually 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 maximum R_SNS value is chosen 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 maximum R_SNS value can 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. In most cases the following boundary equation can be used to determine the R_SNS value.

In some applications, where there is a higher load current range the above boundary equation can only satsify either the lower or upper bound. In these cases, more emphasis can be put on the lower measurable current values which increases R_SNS. Likewise, if the higher currents are of more interest the R_SNS can be decreased.

Figure 7-1 Voltage Indication on the Current-Sense Pin

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 can be up to the maximum ILIM value but the current sense output accuracy is not specified above the maximum rated value in the Current Sense Characteristics.

Figure 7-2 Current-Sense and Current-Limit Block Diagram

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.