The voltage on the switch node switches from the input voltage to the negative output voltage in an inverting topology. During start-up, V_IN rises to achieve the set input voltage, then the device begins switching, which causes the V_OUT to start ramping down after the enable pin voltage exceeds its threshold and V_IN exceeds its UVLO threshold. As V_OUT continues to ramp down, the switch node low-level follows V_OUT downward. Figure 3-2 displays a typical behavior of the switch node at start-up.