An external fault input signal may be applied to the controller SD (shutdown) pin. This signal forces the controller into fault mode. To trigger the fault, the voltage on this pin should be pulled below the fault trip threshold. A typical application is shown in Figure 37, where this pin is used to shut down the controller in the event of an over-temperature event as detected by a NTC (negative temperature coefficient) thermistor. The device pulls up the SD pin internally using a current source. As temperature rises, the external NTC resistance decreases, reducing the voltage on the pin. When the pin voltage drops to the fault trip threshold, the controller enters fault mode. The fault response (latching or recovery) depends on the device variant, per Table 3.
The required trip resistance can be calculated from the internal trip voltage and pull-up current source. Nominally, this is 9.5 kΩ. Choose the NTC should so that it can achieve this value of resistance at the desired hot-spot trip temperature. If the NTC resistance is too low at the required trip temperature, connect a standard chip resistor in series to bring the total resistance up to 9.5 kΩ.
The device internally filters the SD pin with persistence delay as listed in Table 3. An external filter capacitor is not normally necessary. However, if an application uses an external filter capacitor, the value should be limited to 1 nF maximum. A larger value may impact the useful life of the controller.