Datasheet

ISO
GND2 N
ISO IN
Z
v = v
Z + Z
´
9
GN D2 ISO
9 4
N ISO IN
v R 10
= =
v R + R
10 + 6 10
1
16
GND2 ISO
ISO
N
ISO IN
IN
1
v C
1 1
= = = = 0.94
1 1 C
v
+ 1 +
1 +
C C
C
V
N
R
IN
C
IN
R
ISO
C
ISO
SystemGround(GND1)
BusReturn (GND2)
16 V
A, B, Y, orZ
ISO3086T
SLLSE27C –JANUARY 2011– REVISED JULY 2011 www.ti.com
APPLICATION INFORMATION
REFERENCE DESIGN
ISO3086T Reference design (SLUU469) and miniature evaluation boards are available.
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 and the transient ratings of the ISO3086T 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 easily exceed insulation ratings. ESD generators simulate static
discharges that may occur during device or equipment handling with low-energy but very high voltage transients.
Figure 32 models the ISO3086T bus IO connected to a noise generator. C
IN
and R
IN
is 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 ISO308x plus those of any other insulation (transformer, etc.), and
we assume stray inductance negligible. From this model, the voltage at the isolated bus return is
and will always be less than 16 V from V
N
. If the ISO3086 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.
spacer for space between the paragraphs
Note from Figure 32 that the resistor ratio determines
spacer
the voltage ratio at low frequency and it is the inverse
capacitance ratio at high frequency. In the
spacer
stand-alone case and for low frequency,
or essentially all of noise appears across the barrier.
At very high frequency,
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, as it should.
We recommend the reader not test equipment
transient susceptibility with ESD generators or
consider product claims of ESD ratings above the
Figure 32. Noise Model
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.
20 Copyright © 2011, Texas Instruments Incorporated