In the inverting buck-boost topology, the voltage on the SW pin switches from VIN to VOUT, instead of from VIN to GND. As the high-side MOSFET turns on, the SW node sees the input voltage; as the low-side MOSFET turns on, the SW node sees the device ground, which is the output voltage. During startup, VIN rises to achieve the desired input voltage. VOUT starts ramping down after the EN pin voltage exceeds the threshold level and VIN exceeds the UVLO threshold. As VOUT continues to ramp down, the SW node low level follows VOUT down. Figure 2-5 shows the resulting normal and smooth startup of the output voltage.

Figure 2-5 SW Node Voltage During Startup