ADUM3480 Datasheet, PDF (18/20 Pages) Analog Devices – Small, 3.75 kV RMS Quad Digital Isolators

ADuM3480/ADuM3481/ADuM3482
The limitation on the magnetic field immunity of the device is set
by the condition in which induced voltage in the receiving coil
of the transformer is sufficiently large to either falsely set or
reset the decoder. The following analysis defines such conditions.
The ADuM3480/ADuM3481/ADuM3482 are examined in a 3 V
operating condition because it represents the most susceptible
mode of operation of these products.
The pulses at the transformer output have an amplitude of
greater than 1.5 V. The decoder has a sensing threshold of
approximately = 1.0 V, thereby establishing a 0.5 V margin
within which induced voltages can be tolerated. The voltage
induced across the receiving coil is given by
V = (−dβ / dt)∑πrn²; n = 1, 2, …, N
where:
β is the magnetic flux density.
rn is the radius of the n^th turn in the receiving coil.
N is the number of turns in the receiving coil.
Given the geometry of the receiving coil in the ADuM3480/
ADuM3481/ADuM3482 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 17.
100
10
1
0.1
0.01
0.001
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 17. Maximum Allowable External Magnetic Flux Density
For example, at a magnetic field frequency of 1 MHz, the
maximum allowable magnetic field of 0.5 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. If such
an event occurs, with the worst-case polarity, during a transmitted
pulse, it reduces the received pulse from >1.0 V to 0.75 V. This is
still well above the 0.5 V sensing threshold of the decoder.
Data Sheet
The preceding magnetic flux density values correspond to
specific current magnitudes at given distances away from the
ADuM3480/ ADuM3481/ADuM3482 transformers. Figure 18
expresses these allowable current magnitudes as a function of
frequency for selected distances. The ADuM3480/ADuM3481/
ADuM3482 are very insensitive to external fields. Only extremely
large, high frequency currents that are very close to the component
are a concern. For the 1 MHz example noted, a 1.2 kA current would
need to be placed 5 mm away from the ADuM3480/ADuM3481/
ADuM3482 to affect component operation.
1000
DISTANCE = 1m
100
10
DISTANCE = 100mm
1
DISTANCE = 5mm
0.1
0.01
1k
10k
100k
1M
10M
MAGNETIC FIELD FREQUENCY (Hz)
100M
Figure 18. Maximum Allowable Current for Various Current to ADuM3480
Spacings
Note that at combinations of strong magnetic field and high
frequency, or any loops formed by PCB traces, can induce
sufficiently large error voltages to trigger the thresholds of
succeeding circuitry. Take care to avoid PCB structures that
form loops.
POWER CONSUMPTION
The supply current at a given channel of the ADuM3480/
ADuM3481/ADuM3482 isolator is a function of the supply voltage,
the data rate of the channel, and the output load of the channel.
Calculating I_DD1 or I_DD2
For each input channel, assuming worst case I/O voltage, the
supply current is given by
IDDI = IDDI (Q)
RD ≤ 2.5 × RR
IDDI = IDDI (D) × (RD−RR) + IDDI (Q)
RD > 2.5 × RR
For each output channel, the supply current is given by
IDDO = IDDO (D) × RD + IDDO (Q)
Rev. 0 | Page 18 of 20