The FLT pin of UCC21755-Q1 is open drain and can report a fault signal to the DSP/MCU when the fault is detected through the DESAT pin. The FLT pin is pulled down to GND after the fault is detected, and is held low until a reset signal is received from RST/EN. The device has a fault mute time t_FLTMUTE, within which the device ignores any reset signal.

The RST/EN is pulled down internally by a 50-kΩ resistor, and is thus disabled by default when this pin is floating. It must be pulled up externally to enable the driver. The pin has two purposes:

• To reset the FLT pin. If the RST/EN pin is pulled low for more than t_RSTFIL after the mute time t_FLTMUTE, then the fault signal is reset and FLT returns to a high impedance state upon the next rising edge applied to the RST/EN pin.
• To enable and shut down the device. If the RST/EN pin is pulled low for longer than t_RSTFIL, the driver is disabled and OUTL is activated to pull down the gate of the IGBT or SiC MOSFET. The pin must be pulled up externally to enable the part; otherwise, the device is disabled by default.