USB on a 16-bit core — what that buys the design
Its standout feature is integrated USB 2.0 — unusual for a 16-bit ultra-low-power part — which lets a single chip handle the USB protocol stack alongside sensor acquisition or control loops without an external bridge. The 24 KB Flash and 6 KB RAM are enough for a composite HID device or a simple CDC virtual COM port, but you'll want to budget the firmware footprint carefully if you're adding a full USB host stack or multiple endpoints.
Housed in a 48-LQFP (7×7 mm) package with a 0.5 mm pitch, it's a hand-solderable surface-mount part — no special reflow profile beyond standard lead-free.
TI lists the MSP430F5509IPT as Active. The base product number MSP430F5509 covers the whole density variant; if you need more Flash or RAM later, the MSP430F5xx family has pin-compatible siblings (e.g., the MSP430F5510 with 32 KB Flash) that share the same 48-LQFP footprint.