Preliminary Technical Data
ADM2485
100
10
THERMAL SHUTDOWN
The ADM2485 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
1
0.1
temperature of 150°C is reached. As the device cools, the drivers
are re-enabled at a temperature of 140°C.
0.01
RECEIVER FAIL-SAFE INPUTS
0.001
1k
10k
100k
1M
10M
100M
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 FREQUENCY (Hz)
Figure 27. Maximum Allowable External Magnetic Flux Density
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.
MAGNETIC FIELD IMMUNITY
Because iCouplers use a 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
analysis below defines the conditions under which this may
occur. The ADM2485’s 3 V operating condition is examined
because it represents the most susceptible mode of operation.
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 ADM2485 transformers.
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
1000
DISTANCE = 1m
100
− dβ
dt
⎛
⎜
⎝
⎞
⎟
⎠
πr2
; n =1, 2, . . . , N
V =
∑
DISTANCE = 5mm
10
n
where, if the pulses at the transformer output are greater than
1.0 V in amplitude:
DISTANCE = 100mm
1
β = magnetic flux density (gauss)
N = number of turns in receiving coil
rn = radius of nth turn in receiving coil (cm)
0.1
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.
0.01
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 28. Maximum Allowable Current for
Various Current-to-ADM2485 Spacings
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.
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.
Rev. PrK | Page 13 of 15
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