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 FSUSB30 can be successfully config-
ured to meet both these requirements.
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 100 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.
Power-Off Protection
For a Vbus short circuit, the switch is expected to with-
stand such a condition for at least 24 hours. The
FSUSB30 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-).
VCC= 3.6 V
100 Ohms
HSD+
HSD+
D+ = 5.25V
D- = 5.25V
FSUSB30
HSD-
HSD-
Figure 17. Adding 100 resistor in series with the VCC supply allows the FSUSB30 to withstand a
Vbus short when powered up
For more information, see Applications Note AN-6022 Using the FSUSB30 to Comply with USB 2.0 Fault
Condition Requirements at www.fairchildsemi.com
© 2006 Fairchild Semiconductor Corporation
FSUSB30 Rev. 1.1.6
www.fairchildsemi.com
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