MicroNote™ 122 Transient Voltage Protection Across High Data Rate & RF Lines Early systems using RS-232 ambling along at 19.6kbps were compatible with the capacitance of silicon transient voltage suppressor (TVS) devices of that era. No significant signal attenuation was observed because of the relative low data transmission rates; however, with today's signal rates pushing into the Gbps range, TVS capacitance becomes a significant issue. It often becomes a challenge for the designer to find protective devices compatible with high data rates such as those used on Universal Serial Bus lines at 10Mbps, IEEE -1394 (FireWire) at >100Mbps, and recent CATV innovations at rates up to 1GHz. Microsemi Scottsdale now offers a broad spectrum of LoCAP™ low capacitance silicon TVS devices specifically for protection across data lines to prevent signal attenuation at these high transmission rates. Electrostatic discharge (ESD) is the most significant threat with induced lightning and load switching also contributing to upset or failure of I/O port components. Don't overlook the possibility of latch-up or latent failures that can occur weeks or months after the electrical overstress event. DESIGNING LOCAP TVS DEVICES For high data rates, low capacitance is achieved by inserting a high voltage rectifier chip, which inherently has a low capacitance value in series with and also in opposite polarity to the TVS chip. Proper selection of the diode chip will provide the required capacitance and sufficient cross sectional area to withstand rated surge current. Higher powered LoCap TVSs are inherently higher in capacitance from the larger chip sizes required to withstand the associated higher surge currents. The rules for diode capacitance reduction are basically those governing capacitors in series and parallel as shown in Figure 1. www. Copyright © Microsemi Corp. Rev. 1.0 9/99 .com TVSs are attributed to the high doping level of the starting silicon material to produce lower breakdown voltage devices. Figure 1a illustrates typical capacitance values for a 500W, 10V TVS and with an appropriate rectifier chip for fabricating a 10 V low capacitance silicon TVS, while figure 2b illustrates their polarity relationship. With more than an order of magnitude between the value of the series capacitance of the two chips, the total value is calculated to be slightly less than the smallest value, which is 14.6pf for this example. Figure 1c illustrates the V/I curve of the combined low capacitance rectifier chip with the TVS. Note that clamping protection is provided only in one direction, the third quadrant, with the first quadrant containing the reverse breakdown of the rectifier. Hence, it becomes necessary to place two of the rectifier / TVS strings in antiparallel, forming a functionally bidirectional LoCap, low capacitance element as shown in Figure 2a. Figure 2b illustrates the resultant electrical characteristics of the symmetrical V/I curve with clamping protection for both positive and negative transient voltage excursions. The bidirectional LoCap TVS is bilaterally symmetrical, having the same electrical characteristics in both the 1st and 3rd quadrants as depicted in figure 2b. This feature accommodates signals having both positive and negative excursions. For most LoCap devices, the "legs" are connected externally to the package. by Mel Clark Microsemi - Scottsdale, AZ [email protected] Microsemi - Scottsdale has the broadest offerings of LoCap silicon TVS devices in the industry. This includes the following types listed below in Table A. Many of the Microsemi's devices have very conservative values listed for capacitance. For example, the SAC5 and the SMBJSAC types typically measure 13-17 pf and the USB series typically ranges from 1.8-2.0 pf. The USB0805C (5 V operating voltage) has been configured as shown in Figure 3 to reduce the capacitance to approximately 1pf for use across rf amplifiers up to 750 MHz with no noticeable attenuation. Figure 1a C1 = 500pf C2 = 15pf Ct = Figure 1b Q2 1 1/C1 +1/C 2 = 14.6pf +I Q1 -V Figure 1c +V Q3 Q4 Figure 1. Capacitance of TVS & Low Capacitance Rectifier Chips Table A Device Series Surge Power LC6.5 LCE6.5 SAC5 SMCJLCE5.0 SMBJSAC5.0 SMP6LC6.5 SM8LC03 SM16LC03 USB0403C USB0803C 1.5KW 1.5KW 500W 1.5KW 500W 600W 300W 300W 300W 300W Waveform (microseconds) 10/1000 10/1000 10/1000 10/1000 10/1000 10/1000 8/20 8/20 8/20 8/20 Capacitance Package Voltage Range 50pf 50pf 25pf 50pf 25pf 30pf 25pf 25pf 5pf 5pf DO-13 Axial lead Axial lead SMT/DO-214AB SMT/DO-214AA SO-16 SO-8 SO-16 SO-4 SO-8 6.5 V - 170 V 6.5 V - 170 V 5.0 V - 50 V 5.0 V - 50 V 5.0 V - 50 V 6.5V 3.0 V- 24 V 3.0 V - 24 V 3.0V - 24 V 3.0V - 24 V 1 MicroNote is a trademark of Microsemi Corporation. MicroNote122 APPLICATIONS Most RF and data I/O signal inputs are sensitive to electrical overstress. During operation "Desert Watch", the inputs on solid state receivers reportedly failed at an alarming rate. This was attributed to static electricity generated when wind blown desert sand blasted external antennas. For data rates > 50kbps, low capacitance TVSs are often necessary to minimize signal attenuation while still providing overvoltage protection. The earlier standards calling out maximum bit rates no longer apply as maximum operating limits, e.g., RS-232 originally specified a max bit rate of 19.6kbps, but some users are demanding (and getting) up to 300kbps operating capability. Typical data transmission/reception specifications are listed below: Signal Type EIA - 232 EIA - 422 EIA - 423 EIA - 485 USB (universal serial bus) Telecom, modems IEEE-1394 (fire wire) CATV Data Rate 19.6kbps 10Mbps 100kbps 5Mbps 12.5Mbps 60kb/1.5Mbps 125Mbps up to 1Gbps The maximum rates listed above are with 10 m or less of interconnecting cable representing minimum load capacitance. Speeds are significantly reduced with increasing lengths of interconnecting cable. IEEE-485 is Side of Package C = 2 x Ct Q2 subject to a number of boundary conditions governing its maximum data rate. USB runs at either 1.5 or 10 Mbps depending on the signal type transmitted. Cable lengths are normally less than 3 meters. Computer modems normally transmit at rates of 60 kbps or 1.5 Mbps depending on the modem and its capability. Illustrating the advantage of the higher speed, if a program requires 10 minutes to download at 60 kbps, this time would be reduced to less than one-half minute at 1.5 Mbps. Applications for one of the developing transmission protocols, "FireWire" is still over the horizon as this technology has not matured sufficiently to determine the specifics for protection requirements. Internet access is being offered on a few selected CATV locations at about 100Mbps, almost two orders of magnitude greater than the fast telecom modems. Some new generation computers, now in the development stages, reportedly operate at data rates well into the Gbps region. Their sensitive interfacing I/O ports will require external protection for their submicron on-chip components. Q1 +I 8 -V +V from Fig. 1b Q3 Q4 CAPACITANCE- pf The 5 V, USB0805C elements have been wired in series to reduce the capacitance by one-half, which is normally about 1pf. Figure 4 depicts the capacitance values for a USB0805C as configured in Figure 3b, from 0V through -4V bias. In this same graph, an equivalent competitive device is compared with Microsemi's LoCap TVS. Note the significant difference with Microsemi's part having the lowest value of these two devices, both of which have the same data sheet specifications. The competitive device is 2pf over its limit of 5pf per line; however, the Microsemi LoCap TVS has a capacitance well within its upper specified limit, about 3pf below its maximum limit of 5pf per protected line. COMPETITIVE PRODUCT 6 4 MICROSEMI USB0805C 2 Figure 2b Figure 2a Figure 2. Configuration of Microsemi's Bidirectional LoCap TVS 0 -1 -2 -3 -4 REVERSE BIAS - VOLTS TO LINE TO LINE 1 4 3 2 1 TO LINE 2 4 USB0805C 5 6 7 TO GROUND 3 2 1 Figure 4. Capacitance Values of LoCap TVS Under Reverse Bias USB0805C 8 5 6 7 8 TO GROUND PINS 5,6,7,8 CONNECTED TOGETHER Figure 3a. USB Capacitance (5pf max/line) 2 Figure 3b. Reduced Capacitance Config. www. .com MicroNote 122 PROTECTION GUIDELINES SUMMARY The following Microsemi-Scottsdale TVS devices are recommended for protection in the applications listed in Table B. The information in this table may not be applicable for some circuits depending on the amount of signal distortion the system will tolerate. High internal capacitance is inherent in low voltage TVS devices due to the low resistivity silicon substrate required to produce low voltage breakdown pn junctions. This high capacitance is due to the very thin region of space charge in low voltage pn junctions. Effective capacitance can be reduced by orders of Table B UPPER LIMITS RECOMMENDED TVS PRIMARY THREATS SURGE bit/s ESD Load Sw itch Lightning Fam ily Pow er 250 kb 250 kb 250 kb 1.5 Mb 1.5 Mb 1.5 Mb 5 Mb 5 Mb 12.5 Mb 12.5 Mb 125 Mb 125 Mb 1 Gb * * * * * * * * * * * * * * * * * * * * * * * * * LC6.5 LCE6.5 SMCJLC5.0 SAC5.0 SMBJSAC5.0 SMP6LC6.5 SM8LC03 SM16LC03C USB0403C USB0803C USB0403C USB0403C USB0803C (1) 1.5 kW 1.5 kW 1.5 kW 600 W 600 W 600 W 300 W 300 W 300 W 300 W 300 W 300W 300 W magnitude by placing a rectifier chip, which inherently has low capacitance, in series but in opposite polarity with the TVS chip. Microsemi - Scottsdale has the broadest selection of LoCap silicon TVS devices for virtually all applications with data rates up through 1Gbps. The tables above describe the data rates for most signal types and provides guidelines for selecting the most effective LoCap TVS for EOS protection. In harsh lightning environments the addition of a gas discharge tube along with the TVS may be required to provide high surge withstand capability. For additional information in selecting the most effective LoCap TVS, please contact Mel Clark or Kent Walters at Microsemi - Scottsdale, Phone: (480) 941 6300 or Fax: (480) 947-1503. (1) only when both elements of the TVS are in series for reduced capacitance as illustrated in figure 3b. www. .com 3