Data Sheet ADM2486
Rev. E | Page 15 of 20
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
∑
π
β−
=
2
n
r
dt
d
V
; n = 1, 2, …, N
where if the pulses at the transformer output are greater than
1.0 V in amplitude:
= magnetic flux density (gauss).
N = number of turns in receiving coil.
r
n
= radius of nth turn in receiving coil (cm).
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.
04604-016
MAGNETIC FIELD FREQUENCY (Hz)
1k 10k 100k 100M1M 10M
100.000
10.000
1.000
0.100
0.010
0.001
MAXIMUM
ALLOWABLE MAGNETIC
FLUX DENSITY (kGAUSS)
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. 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.
04604-017
MAGNETIC FIELD FREQUENCY (Hz)
1k 10k 100k 100M1M 10M
DISTANCE = 1m
DISTANCE = 100mm
DISTANCE = 5mm
1000.00
100.00
0.10
1.00
10.00
0.01
MAXIMUM
ALLOWABLE CURRENT (kA)
Figure 28. Maximum Allowable Current for
Various Current-to-ADM2486 Spacings
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.
