adum3471 Analog Devices, Inc., adum3471 Datasheet - Page 29

• Manufacturer Part Number: adum3471
• Description: Pwm Controller And Transformer Driver With Quad-channel Isolators Adum3470/adum3471/adum3472/adum3473/adum3474
• Manufacturer: Analog Devices, Inc.

DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY
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 (see Table 17) by the watchdog
timer circuit. This situation should occur in the ADuM347x
devices only during power-up and power-down operations.
The limitation on the ADuM347x 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 ADuM347x 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 estab-
lishing 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 ADuM347x 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 43.
n
is the radius of the n
V = (−dβ/dt)∑πr
0.001
0.01
Figure 43. Maximum Allowable External Magnetic Flux Density
100
0.1
10
1
1k
10k
MAGNETIC FIELD FREQUENCY (Hz)
n
2
th
; n = 1, 2, … , N
turn in the receiving coil (cm).
100k
1M
10M
ADuM3470/ADuM3471/ADuM3472/ADuM3473/ADuM3474
100M
Rev. 0 | Page 29 of 32
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 ADuM347x
transformers. Figure 44 expresses these allowable current magnitudes
as a function of frequency for selected distances. As shown in
Figure 44, the ADuM347x are 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 needs to be placed 5 mm away from the ADuM347x to
affect component operation.
In combinations of strong magnetic field and high frequency,
any loops formed by PCB traces can induce error voltages
sufficiently large to trigger the thresholds of succeeding circuitry.
Care should be taken in the layout of such traces to avoid this
possibility.
Figure 44. Maximum Allowable Current for Various Current-to-ADuM347x
0.01
100
0.1
1k
10
1
1k
DISTANCE = 100mm
10k
DISTANCE = 5mm
MAGNETIC FIELD FREQUENCY (Hz)
100k
Spacings
1M
DISTANCE = 1m
10M
100M