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Asymmetrical TVS Diode for Extended Common-Mode RS-485
PROTECTION PRODUCTS Description
The SM712 transient voltage suppressor (TVS) diode is designed for asymmetrical (12V to -7V ) protection in multi-point data transmission standard RS-485 applications. The SM712 may be used to protect devices from transient voltages resulting from electrostatic discharge (ESD), electrical fast transients (EFT), and lightning. The SM712 features 400 Watts (tp = 8/20s) of power handling capability to accommodate the higher transient voltage levels which may be expected in extended common mode applications. This provides higher equipment reliability and eliminates the "guess work" required when using zener diodes that are not rated to handle such transient conditions. The SM712 replaces four discrete components by integrating two 12V and two 7V TVS diodes in a single package. The integrated design aids in reducing voltage over-shoot associated with trace inductance. The low clamping voltage of the SM712 minimizes the stress on the protected transceiver. The SOT23 package allows flexibility in the design of "crowded" circuit boards.
SM712
Features
400 watts peak pulse power (tp = 8/20s) Transient protection for asymmetrical data lines to IEC 61000-4-2 (ESD) 15kV (air), 8kV (contact) IEC 61000-4-4 (EFT) 40A (t5/50ns) IEC 61000-4-5 (Lightning) 12A (8/20s) Protects two +12V to -7V lines Low capacitance Low clamping voltage Solid-state silicon avalanche technology
Mechanical Characteristics
JEDEC SOT23 package Molding compound flammability rating: UL 94V-0 Marking : 712 Packaging : Tape and Reel per EIA 481
Applications
Protection of RS-485 transceivers with extended common-mode range Security systems Automatic Teller Machines HFC systems Networks
Circuit Diagram
Schematic & PIN Configuration
SOT23 (Top View)
Revision 11/2001 1 www.semtech.com
SM712
PROTECTION PRODUCTS Absolute Maximum Rating
R ating Peak Pulse Power (tp = 8/20s) Peak Pulse Current (tp = 8/20s) Lead Soldering Temp erature Op erating Temp erature Storage Temp erature Symbol Pp k IP P TL TJ TSTG Value 400 17 260 (10 sec.) -55 to +125 -55 to +150 Units Watts A C C C
Electrical Characteristics
SM712 Pins 1 to 3 and 2 to 3 (12V T VS) Parameter Reverse Stand-Off Voltage Reverse Breakdown Voltage Reverse Leakage Current Clamp ing Voltage Clamp ing Voltage Junction Cap acitance Symbol VRWM V BR IR VC VC Cj Conditions Pin 3 to 1 or Pin 2 to 1 IPT = 1mA VR = VRWM IPP = 5A, tp = 8/20s IPP = 17A, tp = 8/20s VR = 0V, f = 1MHz VR = VRWM, f = 1MHz 45 13.3 1 20 26 75 45 MI N T YP MAX 12 7.5 20 10 12 75 Pins 3 to 1 and 3 to 2 (7V T VS) MI N T YP MAX 7 Units V V A V V pF pF
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SM712
PROTECTION PRODUCTS Typical Characteristics
Non-Repetitive Peak Pulse Power vs. Pulse Time
10 Peak Pulse Power - PPP (kW) 110 100 % of Rated Power or PP I 90 80 70 60 50 40 30 20 10 0.01 0.1 1 10 Pulse Duration - tp (s) 100 1000 0 0 25 50 75 100 125 150 175 Ambient Temperature - TA (oC)
Power Derating Curve
1
0.1
Pulse Waveform
110 100 90 80 Percent of IPP 70 60 50 40 30 20 10 0 0 5 10 15 Time (s) 20 25 30 td = IPP/2 e
-t
Clamping Voltage vs. Peak Pulse Current
Waveform Parameters: tr = 8s td = 20s 24 22 Clamping Voltage - V (V) C 20 18 16 14 12 10 8 6 4 2 0 0 2 4 6 8 10 12 14 16 Peak Pulse Current - IPP (A) Pin 3 to 1 & 3 to 2 Pin 1 to 3 & 2 to 3
Capacitance vs. Reverse Voltage
80 70 60 Capacitance (pF) 50 40 30 20 10 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Reverse Voltage (V)
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SM712
PROTECTION PRODUCTS Applications Information
Device Connection for Protection of Two RS-485 Data Lines EIA RS-485 specifies a 7V ground difference between devices on the bus. This permits the bus voltage to range from +12V (5V + 7V) to -7V (0 - 7V). The SM712 is designed to protect two RS-485 data lines in extended common mode applications. The SM712 may be used to protect devices from transient voltages resulting from ESD, EFT, and lightning. The device is designed with asymmetrical operating voltages for optimum protection. The TVS diodes at pins 1 & 2 have a working voltage of 12 volts. These pins are connected to the differential data line pairs. The TVS diodes at pin 3 have a working voltage of 7 volts. Pin 3 is connected to ground. The internal TVS diodes of the SM712 will protect the transceiver input from positive transient voltage spikes greater than 12V and negative spikes greater than 7V. A series current limiting resistor may be added in applications requiring enhanced surge immunity. Circuit Board Layout Recommendations. Good circuit board layout is critical for the suppression of fast rise-time transients such as ESD. The following guidelines are recommended: Place the SM712 near the input terminals or connectors to restrict electromagnetic coupling. Minimize the path length between the SM712 and the protected line. This minimizes voltage overshoot due to parasitic inductance of board traces. Use ground planes whenever possible. Long, single trace ground conductors should be avoided. The ground pin (Pin 3) should be connected directly to a ground plane on the circuit board for best results. Minimize all conductive loops including power and ground loops. Never run critical signals near board edges. RS-485 Common Mode Voltages
RS-485 Protection Circuit
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SM712
PROTECTION PRODUCTS Outline Drawing - SOT23
Land Pattern - SOT23
Note 1 : Grid placement courtyard is 8 x 8 elements (4mm x 4mm) in accordance with the international grid detailed in IEC Publication 97.
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SM712
PROTECTION PRODUCTS Ordering Information
Part Number SM712.TC SM712.TG Qty per R eel 3,000 10,000 R eel Size 7 Inch 13 Inch
Contact Information
Semtech Corporation Protection Products Division 652 Mitchell Rd., Newbury Park, CA 91320 Phone: (805)498-2111 FAX (805)498-3804
www.semtech.com
2001 Semtech Corp.
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