I have designed in the TS3USB221A USB MUX on a circuit board with a 5" 'USB cable' on it. What is the best layout and termination for such a 'cable'?
The product has two USB connectors. It uses USB 2.0 and 1.1. It has a standard through-hole B connector 1.5" away from the TS3USB221A and a standard SMD A connector 5" away from it. The mux output sits very close to an Atmel AT32UC3A3 CPU in TQFP144 package. The 1.5" track runs straight into the MUX while the 5" track also has to feature a 180^0 bend in order to connect with the mux. On the board the track looks like a J with the A connector at the end of the long straight leg and the MUX at the end closest to the bend.
The MCU has some peculiar USB pins, one pair for USB 1.1 and another for USB 2.0, 0R series from connector to Hi-Speed pins, 39R series from connector to Full-Speed pins. I use that resistor structure to the MUX output and no resistors between MUX inputs and connectors. Resistors may be an issue, and I believe I have some issues with the 5" cable on the board as well. The application works when the external USB cable at the A connector is very short (3-5") but not when it is longer (3ft). USB 1.1 works over both a long and a short cable, the A connector and the internal PCB cable.
In addition to the routing I worry a little bit about ESD protection. The TS3USB221A features ESD protection diodes, but it will sit quite a distance away from the connector it protects. However, that part of the PCB is only a USB cable and does not feature other parts or nets.
Thanks,
Borge
