The driver of the power switch (HS switch) requires bias higher than VIN when the HS switch is ON. The capacitor connected between CBOOT and SW works as a charge pump to boost voltage on the CBOOT terminal to (SW + VCC). The boot diode is integrated on the LM6x4xx-Q1 die to minimize the physical solution size. TI recommends a 100-nF capacitor rated for 10 V with X7R or better dielectric for the CBOOT capacitor. The boot (CBOOT) rail has a UVLO to protect the chip from operation with too little bias. This UVLO has a threshold of V_{BOOT_UVLO} and is typically 2.1 V. If the CBOOT capacitor voltage drops below V_{BOOT_UVLO}, then the device initiates a charging sequence using the low-side FET before attempting to turn on the high-side device.