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.

The maximum voltage out on the SNS pin is clamped to V_SNSFH, which is the fault voltage level. This voltage is made a function of the DIAG_EN voltage to ensure that it is not higher than what the system can tolerate. 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, choose the sense resistor value, R_SNS, to maximize the range of currents needed to be measured by the system. 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. Choose the minimum acceptable R_SNS value so that the V_SNS voltage is less than the V_SNSFH value, allowing for the system to correctly determine faults.

The clamp on the SNS pin is enabled when DIAG_EN in low as well. So, if the application has tied SNS pins of multiple devices together, external multiplexers can be used to read individual SNS pin voltages. The multiplexers can be controlled by the DIAG_EN signal of individual devices.

In case a GND network is used for reverse polarity protection, the voltage drop across the GND network has to be taken into account to ensure that the SNS pin voltage does not exceed the maximum acceptable ADC voltage. For example, if the microcontroller is running at 3.3V and a GND network is used, the effective DIAG_EN voltage seen by the device will be lowered by the amount of offset induced by the GND network, typically about 0.7V. As a result, the SNS pin will clamp to about 3.3V plus the GND offset. External clamp diodes can be used to restrict the SNS pin voltage in such cases.

The difference between the maximum readable current through the SNS pin, I_LOAD,max × R_SNS / K_SNS, and the V_SNSFH is called the headroom voltage, V_HR. The headroom voltage is determined by the system, the supply voltage and whether a ground network is used. The current sensing output voltage limitation due to V_HR starts at VBB ~ 2.6V + V_SNS when a GND network is used. In the case of a fault level, the voltage drop is smaller (~1.5V), because when there is a fault, the internal SENSE FET is byapssed, which leads to lower headroom requirement. Without a ground network, the SNS pin voltage limitation due to V_HR starts at a lower VBB (by about 0.7V).

It is important to maintain a headroom 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 set the boundary equation -

Figure 7-2 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.

Figure 7-3 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.