Datasheet

ISO
GND2 N
ISO IN
Z
V = V
Z Z+
x
9
GND2 ISO
9 4
N ISO IN
V R
10
= =
V R R
10 6 10
+
+
ISO
ISO
ISO IN
IN
1
C
GND2
C
1 1 1
N
C C 16
C
V
1 1
= = = = 0.94
V 1
1
+ +
+
V
N
C
IN
R
ISO
R
IN
A,B, Y,orZ
SystemGround (GND1)
BusReturn (GND2)
16V
C
ISO
ISO15, ISO35
ISO15M, ISO35M
www.ti.com SLOS580F – MAY 2008– REVISED JANUARY 2012
APPLICATION INFORMATION
Transient Voltages
Isolation of a circuit insulates it from other circuits and earth so that noise develops across the insulation rather
than circuit components. The most common noise threat to data-line circuits is voltage surges or electrical fast
transients that occur after installation. The transient ratings of the ISO15 and ISO35 are sufficient for all but the
most severe installations. However, some equipment manufacturers use their ESD generators to test transient
susceptibility of their equipment, and can exceed insulation ratings. ESD generators simulate static discharges
that may occur during device or equipment handling with low-energy but high voltage transients.
Figure 21 models the ISO15 and ISO35 bus IO connected to a noise generator. C
IN
and R
IN
is capacitance or
resistance across the device and any other stray or added capacitance or resistance across the A or B pin to
GND2. C
ISO
and R
ISO
is the capacitance and resistance between GND1 and GND2 of the ISO15 and ISO35 plus
those of any other insulation (transformer, etc.). The stray inductance is assumed to be negligible. From this
model, the voltage at the isolated bus return is,
(1)
and will always be less than 16 V from V
N
. If the ISO15 and ISO35 are tested as a stand-alone device, R
IN
= 6 ×
10
4
Ω, C
IN
= 16 × 10
–12
F, R
ISO
= 10
9
Ω and C
ISO
= 10
–12
F.
Note from Figure 21 that the resistor ratio determines the voltage ratio at low frequency and it is the inverse
capacitance ratio at high frequency. In the stand-alone case and for low frequency,
(2)
or essentially all of noise appears across the barrier. At high frequency,
(3)
and 94% of V
N
appears across the barrier. As long as R
ISO
is greater than R
IN
and C
ISO
is less than C
IN
, most of
transient noise appears across the isolation barrier.
It is not recommend for the user to test equipment transient susceptibility with ESD generators, or consider
product claims of ESD ratings above the barrier transient ratings of an isolated interface. ESD is best managed
through recessing or covering connector pins in a conductive connector shell and installer training.
Figure 21. Noise Model
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Product Folder Link(s): ISO15 ISO35 ISO15M ISO35M