© 2006 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FSUSB31 Rev. 1.1.0

10

Application Guidance: Meeting USB 2.0 Vbus Short Requirements

In section 7.1.1 of the USB 2.0 specification, it notes that

USB devices must be able to withstand a Vbus short to

D+ or D- when the USB devices is either powered off or

powered on. The FSUSB31 can be successfully config-

ured to meet both these requirements.

Power-Off Protection

For a Vbus short circuit, the switch is expected to with-

stand such a condition for at least 24 hours. The

FSUSB31 has specially designed circuitry which pre-

vents unintended signal bleed through as well as guar-

anteed system reliability during a power-down, over-

voltage condition. The protection has been added to the

common pins (D+, D-).

Power-On Protection

The USB 2.0 specification also notes that the USB

device should be capable of withstanding a Vbus short

during transmission of data. Fairchild recommends add-

ing a 100Ω series resister between the switch VCC pin

and supply rail to protect against this case. This modifi-

cation works by limiting current flow back into the VCC

rail during the over-voltage event so current remains

within the safe operating range. In this application, the

switch passes the full 5.25V input signal through to the

selected output, while maintaining specified off isolation

on the un-selected pins.

D+ = 5.25V

100 Ohms

FSUSB31

D- = 5.25V

HSD+

HSD-

short when powered up

For more information, see Applications Note AN-6022 — Using the FSUSB30/FSUSB31 to Comply with