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 applica-
tions. 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.
SM712
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)
Protects two +12V to -7V lines
Low capacitance
Low clamping voltage
Solid-state silicon avalanche technology
u
u
u
u
Mechanical Characteristics
u
u
u
u
JEDEC SOT23 package
Molding compound flammability rating: UL 94V-0
Marking : 712
Packaging : Tape and Reel per EIA 481
Applications
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Protection of RS-485 transceivers with extended
u
u
u
u
common-mode range
Security systems
Automatic Teller Machines
HFC systems
Networks
Circuit Diagram
Schematic & PIN Configuration
SOT23 (Top View)
Revision 9/2000
1
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SM712
PROTECTION PRODUCTS
Absolute Maximum Rating
R ating
Peak Pulse Pow er (tp = 8/20µ s)
Peak Pulse Current (tp = 8/20µ s)
Lead Sold ering Temp erature
Op erating Temp erature
Storage Temp erature
Symbo l
P
p k
I
PP
T
L
T
J
T
STG
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)
Par ame te r
Reverse Stand -Off Voltage
Reverse Breakd ow n Voltage
Reverse Leakage Current
Clamp ing Voltage
Clamp ing Voltage
Junction Cap acitance
Symbo l
V
RWM
V
BR
I
R
V
C
V
C
C
j
Co nd itio ns
Pin 3 to 1 or Pin 2 to 1
I
PT
= 1mA
V
R =
V
RWM
I
PP
= 5A ,
tp = 8/20µ s
I
PP
= 17A ,
tp = 8/20µ s
V
R
= 0V, f = 1MHz
V
R
= V
RWM
, f = 1MHz
45
13.3
1
20
26
75
45
MI N
T YP
MA X
12
7.5
20
10
12
75
pF
Pins 3 to 1 and
3 to 2
(7V T VS)
MI N
T YP
MA X
7
Units
V
V
nA
V
V
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SM712
PROTECTION PRODUCTS
Typical Characteristics
Non-Repetitive Peak Pulse Power vs. Pulse Time
10
Peak Pulse Power - P
PP
(kW)
110
100
% of Rated Power or I
PP
90
80
70
60
50
40
30
20
10
0.01
0.1
1
10
Pulse Duration - tp (
µ
s)
100
1000
0
0
25
50
75
100
125
o
Power Derating Curve
1
0.1
150
175
Ambient Temperature - T
A
( C)
Pulse Waveform
110
100
90
80
Percent of I
PP
70
60
50
40
30
20
10
0
0
5
10
15
Time (µs)
20
25
30
td = I
PP
/2
e
-t
Clamping Voltage vs. Peak Pulse Current
24
Clamping Voltage - V
C
(V)
Waveform
Parameters:
tr = 8µs
td = 20µs
22
20
18
16
14
12
10
8
6
4
2
0
0
2
4
6
8
10
12
14
16
Peak Pulse Current - I
PP
(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 volt-
ages 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:
l
l
RS-485 Common Mode Voltages
RS-485 Protection Circuit
l
l
l
l
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 over-
shoot 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 con-
nected 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.
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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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