Datasheet

ADM3053 Data Sheet
Rev. E | Page 14 of 18
DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY
The digital signals transmit across the isolation barrier using
iCoupler technology. This technique uses chip-scale transformer
windings to couple the digital signals magnetically from one
side of the barrier to the other.
Digital inputs are encoded into waveforms that are capable of
exciting the primary transformer winding. At the secondary
winding, the induced waveforms are decoded into the binary
value that was originally transmitted.
Positive and negative logic transitions at the isolator input cause
narrow (~1 ns) pulses to be sent to the decoder via the transformer.
The decoder is bistable and is, therefore, either set or reset by the
pulses, indicating input logic transitions. In the absence of logic
transitions at the input for more than 1 µs, periodic sets of refresh
pulses indicative of the correct input state are sent to ensure dc
correctness at the output. If the decoder receives no internal pulses
of more than approximately 5 μs, the input side is assumed to be
unpowered or nonfunctional, in which case, the isolator output
is forced to a default state by the watchdog timer circuit.
This situation must occur in the ADM3053 devices only during
power-up and power-down operations. The limitation on the
ADM3053 magnetic field immunity is set by the condition in
which induced voltage in the transformer receiving coil is
sufficiently large to either falsely set or reset the decoder. The
following analysis defines the conditions under which this
can occur.
The 3.3 V operating condition of the ADM3053 is examined
because it represents the most susceptible mode of operation. The
pulses at the transformer output have an amplitude of >1.0 V. T h e
decoder has a sensing threshold of about 0.5 V, thus establishing a
0.5 V margin in which induced voltages can be tolerated. The
voltage induced across the receiving coil is given by
V = (−dβ/dt)Σπr
n
2; n = 1, 2, … , N
where:
β is magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
is the radius of the n
th
turn in the receiving coil (cm).
Given the geometry of the receiving coil in the ADM3053 and
an imposed requirement that the induced voltage be, at most,
50% of the 0.5 V margin at the decoder, a maximum allowable
magnetic field is calculated as shown in Figure 26.
MAGNETIC FIELD FREQUENCY (Hz)
100
MAXIMUM ALLOWABLE MAGNETIC FLUX
DENSITY (kgauss)
0.001
1M
10
0.01
1k 10k 10M
0.1
1
100M100k
09293-010
Figure 26. 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 of the
worst-case polarity), it reduces the received pulse from >1.0 V to
0.75 V, which is still well above the 0.5 V sensing threshold of
the decoder.
The preceding magnetic flux density values correspond to
specific current magnitudes at given distances from the
ADM3053 transformers. Figure 27 expresses these allowable
current magnitudes as a function of frequency for selected
distances. As shown in Figure 27, the ADM3053 is extremely
immune and can be affected only by extremely large currents
operated at high frequency very close to the component. For the
1 MHz example, a 0.5 kA current must be placed 5 mm away from
the ADM3053 to affect component operation.
MAGNETIC FIELD FREQUENCY (Hz)
MAXIMUM ALLOWABLE CURRENT (kA)
1k
100
10
1
0.1
0.01
1k 10k 100M100k 1M 10M
DISTANCE = 5mm
DISTANCE = 1m
DISTANCE = 100mm
09293-011
Figure 27. Maximum Allowable Current for Various Current-to-ADM3053
Spacings
Note that in combinations of strong magnetic field and high
frequency, any loops formed by the printed circuit board (PCB)
traces can induce error voltages sufficiently large to trigger the
thresholds of succeeding circuitry. Proceed with caution in the
layout of such traces to prevent this from occurring.