The UCC21738-Q1 implements the internal UVLO protection feature for both input and output power supplies VCC and VDD. When the supply voltage is lower than the threshold voltage, the driver output is held as LOW. The output only goes HIGH when both VCC and VDD are out of the UVLO status. The UVLO protection feature not only reduces the power consumption of the driver itself during low power supply voltage condition, but also increases the efficiency of the power stage. For SiC MOSFET and IGBT, the on-resistance reduces while the gate-source voltage or gate-emitter voltage increases. If the power semiconductor is turned on with a low VDD value, the conduction loss increases significantly and can lead to a thermal issue and efficiency reduction of the power stage. The UCC21738-Q1 implements a 12-V threshold voltage of VDD UVLO, with 800-mV hysteresis. This threshold voltage is suitable for both SiC MOSFET and IGBT.

The UVLO protection block features with hysteresis and deglitch filter, which help to improve the noise immunity of the power supply. During the turn-on and turn-off switching transient, the driver sources and sinks a peak transient current from the power supply, which can result in a sudden voltage drop of the power supply. With hysteresis and UVLO deglitch filter, the internal UVLO protection block ignores small noises during the normal switching transients.

The timing diagrams of the UVLO feature of VCC and VDD are shown in Figure 7-8 and Figure 7-9. The RDY pin on the input side is used to indicate the power-good condition. The RDY pin is open drain. During UVLO condition, the RDY pin is held in low status and connected to GND. Normally the pin is pulled up externally to VCC to indicate the power good.