7.3.4.2 Open-Load Detection

In the on state for version A, if the current flowing through the output is less than I_ol,on, the device recognizes an open-load fault. For version B, faults are diagnosed by reading the voltage on the CS pin and judged by the user. A benefit of high-accuracy current sense down to a verylow current range, this device can achieve a very low open-load detection threshold, which correspondingly expands the normal operation region. TI suggests 10 mA as the upper limit for the open-load detection threshold and 25 mA as the lower limit for the normal operation current. In Figure 35, 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 35. 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_S – V_OUT < V_ol,off. For version A, the ST pin goes low to indicate the fault to the MCU. For version B, the CS pin is pulled up to V_CS,h. 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 recommends an external pullup resistor to offset the leakage current. This pullup current should be less than the output load current to avoid false detection in the normal operation mode. To reduce the standby current, TI recommends always to use a switch in series with? the pullup resistor. TI recommends R_pu ≤ 15 kΩ.

Figure 36. Open-Load Detection Circuit