The UCC21521 has an internal under voltage lock out (UVLO) protection feature on the supply circuit blocks between the VDD and VSS pins for both outputs. When the VDD bias voltage is lower than V_{VDD_ON} at device start-up or lower than V_{VDD_OFF} after start-up, the VDD UVLO feature holds the effected output low, regardless of the status of the input pins (INA and INB).

When the output stages of the driver are in an unbiased or UVLO condition, the driver outputs are held low by an active clamp circuit that limits the voltage rise on the driver outputs (Illustrated in Figure 8-1). In this condition, the upper PMOS is resistively held off by R_Hi-Z while the lower NMOS gate is tied to the driver output through R_CLAMP. In this configuration, the output is effectively clamped to the threshold voltage of the lower NMOS device, typically less than 1.5 V, when no bias power is available.

Figure 8-1 Simplified Representation of Active Pulldown Feature

The VDD UVLO protection has a hysteresis feature (V_{VDD_HYS}). This hysteresis prevents chatter when there is ground noise from the power supply. Also this allows the device to accept small drops in bias voltage, which is bound to happen when the device starts switching and operating current consumption increases suddenly.

The input side of the UCC21521 also has an internal under voltage lock out (UVLO) protection feature. The device isn't active unless the voltage, VCCI, is going to exceed V_{VCCI_ON} on start up. And a signal will cease to be delivered when that pin receives a voltage less than V_{VCCI_OFF}. Also, like the UVLO for VDD, there is hysteresis (V_{VCCI_HYS}) to ensure stable operation.

All versions of the UCC21521 can withstand an absolute maximum of 30 V for VDD, and 20 V for VCCI.