ADM2484E
100
10
THERMAL SHUTDOWN
The ADM2484E 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 ±10°C is reached. As the device cools, the drivers re-enable at
a temperature of ±40°C.
1
0.1
0.01
TRUE FAIL-SAFE RECEIVER INPUTS
0.001
The receiver inputs have a true fail-safe feature ensuring that
the receiver output is high when the inputs are open or shorted.
During line-idle conditions, when no driver on the bus is enabled,
the voltage across a terminating resistor at the receiver input decays
to 0 k. With traditional transceivers, receiver input thresholds
specified between −200 mk and +200 mk mean that external
bias resistors are required on the A and B pins to ensure that the
receiver outputs are in a ꢀnown state. The true fail-safe receiver
input feature eliminates the need for bias resistors by specifying
the receiver input threshold between −30 mk and −200 mk.
The guaranteed negative threshold means that when the voltage
between A and B decays to 0 k; the receiver output is guaran-
teed to be high.
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 26. Maximum Allowable External Magnetic Flux Density
For example, at a magnetic field frequency of ± MHz, the
maximum allowable magnetic field of 0.2 ꢀgauss induces a
voltage of 0.21 k at the receiving coil. This is about 10% 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
>±.0 k to 0.71 k, still well above the 0.1 k 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 ADM2484E transformers.
1000
MAGNETIC FIELD IMMUNITY
The limitation on the magnetic field immunity of the iCoupler
is set by the condition in which an induced voltage in the
receiving coil of the transformer is large enough to either falsely
set or reset the decoder. The following analysis defines the
conditions under which this may occur. The 3 k operating
condition of the ADM2484E is examined because it represents
the most susceptible mode of operation.
DISTANCE = 1m
100
DISTANCE = 5mm
10
DISTANCE = 100mm
1
The pulses at the transformer output have an amplitude greater
then ± k. The decoder has a sensing threshold of about 0.1 k,
thus establishing a 0.1 k margin in which induced voltages can
be tolerated.
0.1
0.01
The voltage induced across the receiving coil is given by
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
−dβ
dt
⎛
⎜
⎝
⎞
⎟
⎠
V =
where:
πr2
; n =±, 2, . . . , N
∑
n
Figure 27. Maximum Allowable Current for
Various Current-to-ADM2484E Spacings
With combinations of strong magnetic field and high frequency,
any loops formed by PCB traces can induce error voltages large
enough to trigger the thresholds of succeeding circuitry. Care
should be taꢀen in the layout of such traces to avoid this
possibility.
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
rn is the radius of the nth turn in the receiving coil (cm).
Given the geometry of the receiving coil and an imposed
requirement that the induced voltage is, at most, 10% of the
0.1 k margin at the decoder, a maximum allowable magnetic
field can be determined using Figure 26.
Rev. 0 | Page 13 of 16
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