How does a BST pin function?

DC/DC Switching RegulatorsFunction

The BST pin is necessary for DC/DC converters with a bootstrap circuit.

A bootstrap circuit generates the gate drive voltage for a high-side N-channel driver by constructing a boost charge pump circuit utilizing switching operations of a DC/DC converter.*
* An N-channel driver has higher driving capability than a P-channel driver. Therefore, the driver size can be small compared with a P-channel driver.

By connecting a capacitor (CBST) between the BST and Lx pins, the voltage between the two pins is regulated to 5 V typically.
When the Lx voltage is low, the CBST is charged. Then, when the BST voltage exceeds the Lx voltage by 5 V, the N-channel driver can be driven. Any types of capacitors are available as CBST, if the capacitance is equal or more than the recommended value.