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Data Sheet
Output ringing can be reduced by adding a series gate resistor
to dampen the response. For applications that use a load of 1 nF
or less, it is recommended that a series gate resistor of about 5 ?
be added. As shown in Figure 7, R GATE is 5 ?, which yields a
calculated Q factor of about 0.3. Figure 7 illustrates a damped
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
100
10
1
0.1
0.01
ADuM3220/ADuM3221
transitions of more than 1 μs at the input, a periodic set of refresh
pulses indicative of the correct input state is sent to ensure dc
0.001
1k
10k 100k 1M 10M
MAGNETIC FIELD FREQUENCY (Hz)
100M
correctness at the output.
If the decoder receives no internal pulses for more than about
3 μs, the input side is assumed to be unpowered or nonfunc-
tional, in which case the isolator output is forced to a default
low state by the watchdog timer circuit. In addition, the outputs
are in a low default state while the power is rising before the
UVLO threshold is crossed.
The ADuM3220 / ADuM3221 are immune to external magnetic
fields. The limitation on the ADuM3220 / ADuM3221 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 V operating
condition of the ADuM3220 / ADuM3221 is examined because
it represents the most susceptible mode of operation. The pulses
at the transformer output have an amplitude greater than 1.0 V.
The decoder has a sensing threshold at about 0.5 V, therefore
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 ) ∑ π r n 2 ; n = 1, 2, ... , N
Figure 22. Maximum Allowable External Magnetic Flux Density
For example, at a magnetic field frequency of 1 MHz, the maxi-
mum 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. Simi-
larly, if such an event were to occur during a transmitted pulse
(and had the worst-case polarity), the received pulse is reduced
from >1.0 V to 0.75 V, still well above the 0.5 V sensing thresh-
old of the decoder.
The preceding magnetic flux density values correspond to
specific current magnitudes at given distances away from the
these allowable current magnitudes as a function of frequency
are immune and can be affected only by extremely large currents
operated at a 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 ADuM3220 / ADuM3221 to affect the operation
of the component.
1000
where:
β is the magnetic flux density (gauss).
r n is the radius of the nth turn in the receiving coil (cm).
N is the number of turns in the receiving coil.
100
10
DISTANCE = 100mm
DISTANCE = 1m
ADuM3221 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
1
0.1
0.01
DISTANCE = 5mm
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 23. Maximum Allowable Current for Various
Current-to- ADuM3220 / ADuM3221 Spacings
Rev. C | Page 13 of 16
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