adm2486 Analog Devices, Inc., adm2486 Datasheet - Page 15

• Manufacturer Part Number: adm2486
• Description: High Speed, Half-duplex Icoupler Isolated Rs-485 Transceiver
• Manufacturer: Analog Devices, Inc.

THERMAL SHUTDOWN
The ADM2486 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.
RECEIVER FAIL-SAFE INPUTS
The receiver input includes a fail-safe feature that guarantees a
logic high RxD output when the A and B inputs are floating or
open-circuited.
MAGNETIC FIELD IMMUNITY
Because iCouplers use coreless technology, no magnetic
components are present, and the problem of magnetic
saturation of the core material does not exist. Therefore,
iCouplers have essentially infinite dc field immunity. The
following analysis defines the conditions under which this can
occur. The ADM2486’s 3 V operating condition is examined
because it represents the most susceptible mode of operation.
The limitation on the iCoupler’s ac magnetic field immunity is
set by the condition in which the induced error voltage in the
receiving coil (the bottom coil in this case) is made sufficiently
large, either to 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:
N = 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 27.
β
n
= radius of nth turn in receiving coil (cm).
= magnetic flux density (gauss).
V
=
⎛
⎜
⎝
−
dt
d
β
⎞
⎟
⎠
∑
π
r
n
2
; n = 1, 2, …, N
Rev. D | Page 15 of 20
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. This is well above the 0.5 V sensing threshold
of the decoder.
Figure 28 shows the magnetic flux density values in terms of
more familiar quantities such as maximum allowable current
flow at given distances away from the ADM2486 transformers.
At combinations of strong magnetic field and high frequency,
any loops formed by printed circuit board traces could induce
sufficiently large error voltages to trigger the thresholds of
succeeding circuitry. Care should be taken in the layout of
such traces to avoid this possibility.
1000.00
100.000
100.00
10.000
10.00
1.000
0.100
0.010
0.001
1.00
0.10
0.01
Figure 27. Maximum Allowable External Magnetic Flux Density
1k
1k
DISTANCE = 5mm
Figure 28. Maximum Allowable Current for
Various Current-to-ADM2486 Spacings
DISTANCE = 100mm
10k
10k
MAGNETIC FIELD FREQUENCY (Hz)
MAGNETIC FIELD FREQUENCY (Hz)
100k
100k
DISTANCE = 1m
1M
1M
10M
10M
ADM2486
100M
100M