DATA-LINE INTERFACE
On the data-line side, the three most common interfaces are the Universal Serial Bus (USB), Ethernet, and the Peripheral Component Interface (PCI). USB and Ethernet typically connect to external equipment, which might include an ADSL modem, while the PCI bus generally connects to internal plug-in boards. Because PCI is internal to the end-user equipment, it's less susceptible to environmentally generated electrical stresses. For this reason, only the protection of USB and Ethernet lines is considered here.
USB, arguably the most popular interface for home computer equipment, moves data at rates of 1.5 Mbits/s or 12 Mbits/s. To ensure that EMI and RFI are suppressed at USB's high data rate, U.S. equipment must meet FCC part 15 requirements and, internationally, International Special Committee on Radio Interference (CISPR) publication 22. These requirements are in addition to meeting ESD standards like IEC61000-4-2 (8-kV contact, 15-kV air discharge) and Mil-Std 883E, Method 3015-7 (25 kV).
Fortunately, as with the line interface, protection components that meet all of these requirements are commercially available. One example is the USBUF01W6, an integrated passive and active device comprising ESD protection, as well as EMI filters and line terminations for upstream USB ports. The USBUF01W6, which integrates bypass capacitors and series-termination (pull-up) resistors, accommodates both high and low baud rates. Specifically, the data rate is selected by connecting a 1.5-kΩ resistor to data line D+ or D− (Fig. 2).
Ethernet local-area networks, used largely for communications within a building, transmit data over two twisted-wire pairs at data rates of 10 Mbits/s (10BaseT) or 100 Mbits/s (100BaseT). In either case, one wire pair carries received data while the other carries transmitted data. Both wire pairs connect to an Ethernet transceiver IC through a line transformer that serves to isolate the end equipment.
Generally, the surge protection required for Ethernet lines is similar to that for standard telephone lines (Table 4). Consequently, the protection devices are also similar. Specifically, a crowbar device is placed across each wire pair, before the line transformer. Beyond protecting equipment in accordance with required specifications, a key requirement for the crowbar device is a low capacitance to accommodate Ethernet's high data rates. Also, the device should work at Ethernet's low-signal voltages, maintaining open-circuit characteristics in the absence of a surge.
Primary protection is commonly achieved with a tripolar crowbar device like the TPN3021, which contains three crowbar devices in one package. Presenting a typical capacitance of only 16 pF, the device handles surge currents from 30 A (10/1000 µs) to 200 A (2/10 µs). Minimum holding current is a low 30 mA. Alternatively, protection can be provided by two separate crowbar devices, like the SMP100LC (Fig. 3).
In both cases, the need for low breakover voltage applies. Moreover, multiple-diode packages like the DALC208SC6 afford secondary-side spike protection. That device is a diode array that features rail-to-rail clamping and a capacitance of less than 5 pF per diode. Protecting up to four lines, the array carries leakage current and peak-reverse-voltage ratings of less than 1 µA and 9 V per diode, respectively. Housed in a space-saving six-lead SOT-23, the DALC208SC6 integrates the eight diodes necessary to connect all four wires to both the power-supply rail and ground.
Philippe Rabier is a product marketing engineer for the telecommunications sector at STMicroelectronics (www.st.com), Tours, France. He has an engineering degree from the Conservatoire National des Arts et Metiers, Tours. Rabier can be reached via e-mail at philippe.rabier@st.com.
Stephane Serrier is a strategic marketing engineer for the telecommunications sector at STMicroelectronics. He has an engineering degree in electronics from the Ecole Superieur d'Electronique de l'Ouest. Serrier can be reached via e-mail at stephane.serrier@st.com.