adm2491e Analog Devices, Inc., adm2491e Datasheet - Page 13

• Manufacturer Part Number: adm2491e
• Description: High Speed, Esd-protected, Half-/full-duplex, Icoupler Isolated Rs-485 Transceiver
• Manufacturer: Analog Devices, Inc.

THERMAL SHUTDOWN
The ADM2491E contains thermal shutdown circuitry that
protects the part from excessive power dissipation during fault
conditions. Shorting the driver outputs to a low impedance
source can result in high driver currents. The thermal sensing
circuitry detects the increase in die temperature under this
condition and disables the driver outputs. This circuitry is
designed to disable the driver outputs when a die temperature
of 150°C is reached. As the device cools, the drivers are
re-enabled at a temperature of 140°C.
FAIL-SAFE RECEIVER INPUTS
The receiver inputs include a fail-safe feature that guarantees a
logic high on the RxD pin when the A and B inputs are floating
or open circuited.
MAGNETIC FIELD IMMUNITY
Because iCoupler devices use a coreless technology, no magnetic
components are present and the problem of magnetic saturation
of the core material does not exist. Therefore, iCoupler devices
have essentially infinite dc field immunity. The following analysis
defines the conditions under which this may occur. The 3 V
operating condition of the ADM2491E is examined because it
represents the most susceptible mode of operation.
The limitation on the ac magnetic field immunity of the
iCoupler is set by the condition that induced an error voltage in
the receiving coil (the bottom coil in this case) that was large to
either falsely set or reset the decoder. The voltage induced
across the bottom coil is given by
where (if the pulses at the transformer output are greater than
1.0 V in amplitude):
β is the magnetic flux density (gauss).
N is the number of turns in receiving coil.
r
The decoder has a sensing threshold of about 0.5 V; therefore,
there is a 0.5 V margin in which induced voltages can be
tolerated.
Given the geometry of the receiving coil and an imposed
requirement that the induced voltage is, at most, 50% of the
0.5 V margin at the decoder, a maximum allowable magnetic
field is calculated, as shown in Figure 26.
n
is the radius of n
V
=
−
⎛ −
⎜
⎝
dt
dβ
⎞
⎟
⎠
∑
th
turn in receiving coil (cm).
π
r
n
2
;
n
=
1
2 ,
. ,
, . .
N
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For example, at a magnetic field frequency of 1 MHz, the
maximum allowable magnetic field of 0.2 kgauss induces a
voltage of 0.25 V at the receiving coil. This is about 50% of the
sensing threshold and does not cause a faulty output transition.
Similarly, if such an event occurs during a transmitted pulse and
is the worst-case polarity, it reduces the received pulse from
>1.0 V to 0.75 V—still well above the 0.5 V sensing threshold of
the decoder.
Figure 27 shows the magnetic flux density values in terms of
more familiar quantities, such as maximum allowable current
flow, at given distances away from the ADM2491E transformers.
With combinations of strong magnetic field and high frequency,
any loops formed by printed circuit board traces could induce
error voltages large enough to trigger the thresholds of succeeding
circuitry. Care should be taken in the layout of such traces to
avoid this possibility.
0.001
1000
Figure 26. Maximum Allowable External Magnetic Flux Density
0.01
0.01
100
100
0.1
0.1
10
10
1
1
1k
1k
DISTANCE = 5mm
Figure 27. Maximum Allowable Current for
Various Current-to-ADM2491E Spacings
DISTANCE = 100mm
10k
10k
MAGNETIC FIELD FREQUENCY (Hz)
MAGNETIC FIELD FREQUENCY (Hz)
100k
100k
DISTANCE = 1m
1M
1M
10M
10M
ADM2491E
100M
100M