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TVS Diode Application Note
PROTECTION PRODUCTS
Protecting USB Ports from ESD Damage
Since USB is a hot insertion & removal system, USB
components are subject to electrostatic discharge
(ESD). However, ESD protection is not yet a specific
requirement of the USB specification. State-of-the-art
USB ICs are manufactured on high integration CMOS
processes making them extremely sensitive to damage
from the high static voltages associated with an ESD
event. Some USB hub chips are internally protected
from ESD events ranging from 500V to 2kV. This may
provide a false sense of security since IEC 61000-4-2
typically requires commercial equipment pass ESD
immunity tests with voltages up to 15kV for air dis-
charge and 8kV for contact discharge. Ten pulses in
each polarity are required for each test level. The IEC
specification allows all cables to be attached to the
equipment during testing. As such, the equipment may
pass certain regulatory tests with the shielded USB
cable attached. However, by definition USB is a hot
plugging bus. When the socket is open, it is vulnerable
to a potentially hazardous strike. The user may inject
the strike while plugging and unplugging a peripheral
device, or by just reaching for a nearby switch. Physical
contact with the port is not necessary. An air discharge
event can occur several centimeters away from the
conducting surface Damage to the USB interface IC
can occur as a result of the high static potential or
from the conducted ESD currents. The resulting
damage can be catastrophic or latent. Latent failures
manifest themselves long after the ESD event has
occurred.
Protecting vulnerable USB components with devices
designed to handle high energy transients can save
time and money. Protecting the USB port is not a
straight forward task however. The high data transmis-
sion speed and increased sophistication of the USB
controller means a more sophisticated protection
device must be used. Conventional methods used to
protect typical serial ports are rendered useless. An ill
chosen scheme can interfere with the normal opera-
tion of the USB port.
Devices used to protect USB ports must have the
following characteristics :
1. Low capacitance for minimal signal attenuation at
the 12Mbs data rate.
2. Extremely fast response time for responding to the
sub nanosecond rise time of the ESD pulse.
3. Low clamping and operating voltages for optimum
protection of the USB ASIC.
SI96-18
Figure 1 - Single Port USB Protection
Revision 9/2000
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