PASSIVE SOLUTIONS

High-Speed Ports Need Protection

As the TV and computing industries converge around interactive TV (iTV) appliances, it is becoming increasingly important to protect costly equipment from circuit damage caused by customer misuse, environmental hazards or power variations. In particular, USB 2.0, IEEE 1394, Digital Visual Interface (DVI) and High Definition Multimedia Interface (HDMI) hardware supports high data rates, plug and play operation and hot swapping, and therefore requires protection from ElectroStatic Discharge (ESD) pulses from the operating environment and peripherals.



At the high data rates of USB 2.0, DVI and HDMI, the parasitic impedance of traditional protection devices can affect signal integrity. Such applications require low capacitance alternatives such as Tyco Electronics’ PESD devices. With capacitance typically 0.25pF, these devices perform well in Transmission Line Pulse (TLP) testing, as well as IEC61000-4-2 testing, especially after multiple hits (up to 1000).

USB connections are prone to voltage/current transients each time a peripheral is connected or disconnected. Protection against these events is best provided by PolySwitch™ Polymeric Positive Temperature Coefficient (PPTC) devices. The low trigger and clamping voltages of a PESD may also be advantageous in these applications.

When specifying a PESD suppressor, there are several key selection and design decisions. The first is to ensure that the device will survive a minimum of 100 pulses, and the typical 500 pulses as described in IEC61000-4-2, Level 4 contact method. The device must suppress transients on both data signal ground and the Vbus line. Decoupling capacitors must be included between Vbus and the equipment chassis, to reduce EMC problems, while the two grounds should, for preference, be kept separate to reduce the propagation of transients between them. The PESD device should also be installed as close as possible, physically, to the source of the transient.

Consumer electronics also needs protection from home chargers, car chargers, USB chargers, docking stations and a variety of connecting audio and video cables that increase the risk of overcurrent and overvoltage damage. Solutions include custom-keyed connectors to prevent mis-connections, high voltage-capable silicon solutions, discrete solutions and traditional clamping diodes.

The polymer protected Zener diode is the latest generation of clamping diode. It helps provide protection against damage caused by improper power supplies, as well as transient suppression, reverse bias protection and overcurrent protection, in a compact micro-assembly.

Tyco Electronics’ PolyZen™ device consists of a low resistance, precision Zener diode that is thermally coupled to a PPTC thermal switch. In operation, extended overvoltage or reverse bias conditions will cause the PPTC to trip as the diode begins to heat up. A trip event causes the PPTC to transition from a low to a high resistance state, helping protect downstream electronics by generating a series-element voltage drop.


Typical HDMI Circuit Schematic with PESD Devices (based on HDMI Specification Version 1.2)

Features

• Transient, ESD, overvoltage and overcurrent protection
• PESD devices
  • Low capacitance: typically 0.25pF
  • Accommodate high data rates
  • Up to 1000 hits
  • Tested for IEC61000-4-2 conformance
  • TLP tested
  • Low trigger and clamping voltages
• Polymeric Positive Temperature Coefficient (PPTC) devices provide resettable protection
• PolyZen Zener devices
  • Combine diode and PPTC protection
  • PPTC trips to form a series element in the event of a fault

Applications

• USB, DVI and HDMI ports
• iTV STBs
• Consumer electronics
• Computers and peripherals
• Portable electronics

For further information, including Datasheets for Tyco's PolyZen, ESD protection and PolySwitch resettable devices, please click here