SI96-18

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.

USB 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 discharge 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 of USB 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 transmission 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 operation 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.