When a DESAT fault is detected, the UCC218915-Q1 initiates a soft shutdown (SSD) to protect the power semiconductor device. When an overcurrent or short circuit event happens, the SSD feature slowly discharges the gate voltage to limit the overshoot voltage on the switching device created by high dI/dt due to the channel current. There is a tradeoff that needs to be made between the overshoot voltage and the short circuit energy. The turn-off speed must be slow enough to limit the overshoot voltage, but the shutdown time must not be too long that the large energy dissipation in the device can cause breakdown. The UCC218915-Q1 provides an internal pulldown FET that can be used with an external resistor to optimize the discharge current of the gate. When a DESAT fault is detected, OUTP is driven high and OUTN is driven low so that both the PMOS and NMOS buffer FETs are held off. The internal pulldown MOSFET on the SSD/GATE pin is turned on for a fixed time, t_SSD. After this time, OUTN is driven high to turn on the NMOS and hold the power semiconductor device off. A simplified block diagram is shown in Figure 6-10 and timing diagrams are shown in Figure 6-9.

Figure 6-10 Soft Shutdown