TPS2596: Fault Indicator for Short Circuit Protection

Hi Itoh,

When a short circuit occurs, the current through the device increases very rapidly. As the current exceeds 1.5 x ILIM, the device engages a fast current clamping circuit to regulate down the current faster than the nominal overcurrent response time (tLIM). The device does not completely turn off the power FET to ensure uninterrupted power in the event of transient overcurrents or supply transients. As shown in figure 50 of the datasheet, the device starts limiting the current to a  foldback value ( value less than current limit setting). The output voltage drops in the current limiting state, resulting in increased power dissipation in the internal FET and might lead to thermal shutdown if the condition persists for an extended period of time.

As you rightly said, the time taken for the device to reach Thermal Shutdown depends on various parameters like foldback current value, Short circuit impedance, input voltage, RthetaJA (Board Layout dependent), Ambient Temperature. The Power dissipation across the FET and RthetaJA are very important factors among them. Figure 26 and 27 in the datasheet shown the time taken for the device to reach thermal shutdown vs power dissipation across the FET for two different layouts.  

For example, if the Vin = 3.3V and the current limit setting is low the power dissipation in the FET may not be high enough to increase the internal die temperature to Thermal Shutdown Threshold value. In this case, the FLT/ may not be asserted.