USB Hub
Replicates USB transactions from the upstream port to the active downstream port, generates per-port over-current detection and switched VBUS, and aggregates the tree's connection state into a single device the host enumerates.
An upstream USB transceiver receives the host's traffic into a transaction translator (TT) that buffers low/full-speed traffic to free up the upstream high-speed link. A repeater forwards traffic to the addressed downstream port using each port's own PHY and termination. A per-port load switch (often a TPS2553 or DMG3415) energises VBUS through a current-sense resistor that fires an over-current flag back to the host when a downstream device pulls more than 500 mA.
In plain terms
Like a power strip with smart sockets — a single wall outlet fans into many, each socket independently switched and current-limited so a kettle can't trip the lamp.
Why designers use it
- Expand limited host ports — a laptop's two USB-C ports become a workstation with keyboard, mouse, headset, webcam, and storage off one cable.
- Per-port power isolation — a short on one port's downstream cable shuts only that port without dropping the others.
- Speed mediation — the transaction translator keeps a 1.5 Mb/s mouse from blocking the 480 Mb/s flash drive sharing the upstream port.
- Compliance certification — using a USB-IF certified hub IC saves running an entire compliance test plan against an in-house design.
Best for
- Multi-port docks
- Embedded USB
- Industrial PCs
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- On battery-powered handhelds where every µA matters — even a sleeping hub burns 1–5 mA static current; pull-up the wakeup pin and tolerate the lower port count.
- For USB 3.x SuperSpeed at 10 Gb/s+ where signal integrity is marginal — extra hubs cascade jitter; pick one good hub instead of stacking three cheap ones.
Common mistakes
- Skipping the per-port 120 µF bulk capacitance specified in the USB-IF compliance test — port enumeration passes on the bench but fails when a downstream device draws sudden inrush.
- Wiring all per-port over-current flags together — one fault disables every port, defeating the isolation reason the hub IC was chosen.
Where you will find it
- A CalDigit TS4 Thunderbolt 4 dock uses a Microchip USB5807 7-port USB 3.2 Gen 1 hub IC alongside its TBT4 retimer: each downstream port has an independent TPS25750 PD-aware load switch, so plugging a fast SSD into port 7 doesn't brown out a webcam on port 1.
- Logitech's K780 wireless keyboard's Unifying receiver dongle plugs into a 4-port Anker A7516 hub built around a Cypress (Infineon) HX3 hub controller: the HX3's transaction translator keeps the 1.5 Mb/s HID traffic from delaying the 60 MB/s file copies running on a flash drive sharing the same upstream port.
- An industrial PC inside a Rockwell Allen-Bradley CompactLogix cabinet uses a TI TUSB8044 4-port USB 2.0 hub with a 'PortSwap' feature: when a maintenance technician plugs a tablet into port 1, the IC swaps the upstream and downstream automatically, letting either side enumerate as host without rewiring the box.
A short history
A USB hub expands one host connection into several device connections. Its upstream port faces the host, while downstream ports connect peripherals or additional hubs. This lets devices such as a keyboard and mouse share one host port. The hub's supported USB generation and power arrangement determine its capabilities.