When the main channel is enabled faults are diagnosed by reading the voltage on the SNS or FLT pin and evaluated by the user. A benefit of high-accuracy current sense is that this device can achieve a very low open-load detection threshold, which correspondingly expands the normal operation region. TI suggests 5 mA as the upper limit for the open-load detection threshold and 15 mA as the lower limit for the normal operation current. In Figure 7-13, the recommended open-load detection region is shown as the dark-shaded region and the light-shaded region is for normal operation. As a guideline, do not overlap these two regions.

Figure 7-13 On-State Open-Load Detection and Normal-Operation Diagram

In the off state, if a load is connected, the output voltage is pulled to 0 V. In the case of an open load, the output voltage is close to the supply voltage, V_BB – V_OUT < V_ol,off. The FLT pin goes low to indicate the fault to the MCU, and the SNS pin is pulled up to I_SNSFH. There is always a leakage current I_ol,off present on the output, due to the internal logic control path or external humidity, corrosion, and so forth. Thus, TI implemented an internal pullup resistor to offset the leakage current. This pullup current must be less than the output load current to avoid false detection in the normal operation mode. To reduce the standby current, TI implemented a switch in series with the pullup resistor controlled by the DIAG_EN pin. The pull up resistor value is R_pu ≤ 150 kΩ.

Figure 7-14 Open-Load Detection Circuit