Friday, September 12, 2008

Bypass Capacitor Connection to USB Port

Capacitors should be placed as close as possible to the port and the power-carrying traces should be as wide as possible, preferably, a plane. There should also be double vias on power and ground nets and the trace lengths should be kept as short as possible. Standard bypass routing and design methods should be used at all times to minimize inductance and resistance between bypass bulk storage capacitors and the USB connectors.

VBUS Trace Width

The trace width for the VBUS current path from the VBUS source to the bypass bulk storage capacitor, over current protection device, and USB connector power and ground pins should be at least 0.050-in.-wide, with 1.5-oz. to 2-oz. copper on outer layer, to ensure adequate current carrying capability.

It is essential to make the power-carrying traces wide enough that the system over current protection will trip instead of fusing the board traces in an overload event. Depending on the rating of the over current protection device, a good “rule of thumb” is to ensure the power-carrying traces are wide enough to carry at least twice the amperage rating of the over current protection device.

Most motherboards use a long route for VBUS, as seen in Figure 4, that must be sufficiently wide in order to support the number of USB ports on the back panel. The power-handling capacity of a printed circuit trace depends mostly on its cross sectional area and the allowable temperature rise.

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