The FLT pin of the UCC21750-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. Pull up externally to enable the driver. The pin has two purposes:

• To reset the FLT pin: to reset, then RST/EN pin is pulled low; if the pin is set and held in low state for more than t_RSTFIL after the mute time t_FLTMUTE, then the fault signal is reset and FLT is reset back to the high impedance status at the rising edge of the input signal at RST/EN pin.
• Enable and shutdown the device: if the RST/EN pin is pulled low for longer than t_RSTFIL, the driver disables 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.