ADM3053_12 AD [Analog Devices], ADM3053_12 Datasheet - Page 14

• Manufacturer Part Number: ADM3053_12
• Description: Signal and Power Isolated CAN Transceiver
• Manufacturer: AD [Analog Devices]

ADM3053
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 should 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.
The 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
where:
β is magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
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.
n
is the radius of the n
0.001
0.01
100
0.1
V = (−dβ/dt)Σπr
10
1
Figure 26. Maximum Allowable External Magnetic Flux Density
1k
10k
MAGNETIC FIELD FREQUENCY (Hz)
n
2; n = 1, 2, … , N
th
turn in the receiving coil (cm).
100k
1M
10M
100M
Rev. A | Page 14 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 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.
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.
Figure 27. Maximum Allowable Current for Various Current-to-ADM3053
0.01
100
0.1
1k
10
1
1k
DISTANCE = 100mm
10k
DISTANCE = 5mm
MAGNETIC FIELD FREQUENCY (Hz)
100k
Spacings
1M
DISTANCE = 1m
Data Sheet
10M
100M