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PROTECTION PRODUCTS
PROTECTION PRODUCTS
SM712
Asymmetrical TVS Diode for
Extended Common-Mode RS-485
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 = 8/20µs) 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.
Features
u 400 watts peak pulse power (tp = 8/20µs)
u 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 (8/20µs)
u Protects two +12V to -7V lines
u Low capacitance
u Low clamping voltage
u Solid-state silicon avalanche technology
Mechanical Characteristics
u JEDEC SOT23 package
u Molding compound flammability rating: UL 94V-0
u Marking : 712
u Packaging : Tape and Reel per EIA 481
Applications
u Protection of RS-485 transceivers with extended
common-mode range
u Security systems
u Automatic Teller Machines
u HFC systems
u Networks
Circuit Diagram Schematic & PIN Configuration
SOT23 (Top View)
Revision 9/2000
1
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PROTECTION PRODUCTS
PROTECTION PRODUCTS
Absolute Maximum Rating
SM712
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2ã 2000 Semtech Corp.
www.semtech.com
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PROTECTION PRODUCTS
Typical Characteristics
Non-Repetitive Peak Pulse Power vs. Pulse Time Power Derating Curve
SM712
10
(kW)
PP
1
0.1
Peak Pulse Power - P
0.01
0.1 1 10 100 1000
Pulse Duration - tp (µs)
Pulse Waveform Clamping Voltage vs. Peak Pulse Current
110
100
90
80
70
PP
60
50
40
Percent of I
30
20
10
0
0 5 10 15 20 25 30
-t
e
Time (µs)
td = IPP/2
Waveform
Parameters:
tr = 8µs
td = 20µs
110
100
90
PP
80
70
60
50
40
30
% of Rated Power or I
20
10
0
0 25 50 75 100 125 150 175
Ambient T em p er at u re - T
24
22
20
(V)
18
C
16
14
12
10
8
6
Clamping Voltage - V
4
2
0
0 2 4 6 8 10121416
Peak Pulse Current - I
(oC)
A
Pin 1 to 3 & 2 t o 3
Pin 3 to 1 & 3 t o 2
(A)
PP
Capacitance vs. Reverse Voltage
80
70
60
50
40
30
Capacitance (pF)
20
10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13
ã 2000 Semtech Corp.
Reverse Volt age (V)
3
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PROTECTION PRODUCTS
PROTECTION PRODUCTS
Applications Information
SM712
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.
RS-485 Common Mode Voltages
RS-485 Protection Circuit
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:
l Place the SM712 near the input terminals or
connectors to restrict electromagnetic coupling.
l Minimize the path length between the SM712 and
the protected line. This minimizes voltage overshoot due to parasitic inductance of board traces.
l Use ground planes whenever possible.
l 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.
l Minimize all conductive loops including power and
ground loops.
l Never run critical signals near board edges.
4ã 2000 Semtech Corp.
www.semtech.com
Page 5
PROTECTION PRODUCTS
Outline Drawing - SOT23
SM712
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.
ã 2000 Semtech Corp.
5
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PROTECTION PRODUCTS
PROTECTION PRODUCTS
Ordering Information
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SM712
Contact Information
Semtech Corporation
Protection Products Division
652 Mitchell Rd., Newbury Park, CA 91320
Phone: (805)498-2111 FAX (805)498-3804
6ã 2000 Semtech Corp.
www.semtech.com
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