ADuM4470/ADuM4471/ADuM4472/ADuM4473/ADuM4474
Data Sheet
The limitation on the ADuM447x 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.
THERMAL ANALYSIS
The ADuM447x consist of two internal die attached to a split
lead frame with two die attach paddles. For the purposes of
thermal analysis, the die are treated as a thermal unit, with the
highest junction temperature reflected in the θJA from Table 5.
The value of θJA is based on measurements taken with the parts
mounted on a JEDEC standard, 4-layer board with fine width traces
and still air. Under normal operating conditions, the ADuM447x
operate at a full load across the full temperature range without
derating the output current. However, following the recom-
mendations in the Printed Circuit Board (PCB) Layout section
decreases thermal resistance to the PCB, allowing increased
thermal margins in high ambient temperatures. The ADuM447x
has an thermal shutdown circuit that shuts down the dc-to-
dc converter and the outputs of the ADuM447x when a die
temperature of about 160°C is reached. When the die cools
below about 140°C, the ADuM447x dc-to-dc converter and
outputs turn on again.
The 3.3 V operating condition of the ADuM447x 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
2
V = (−dβ/dt)∑πrn ; n = 1, 2, … , N
where:
β is magnetic flux density (gauss).
N is the number of turns in the receiving coil.
rn is the radius of the nth turn in the receiving coil (cm).
Given the geometry of the receiving coil in the ADuM447x
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 53.
100
PROPAGATION DELAY-RELATED PARAMETERS
Propagation delay is a parameter that describes the time it takes
a logic signal to propagate through a component (see Figure 52).
The propagation delay to a logic low output may differ from the
propagation delay to a logic high output.
10
1
INPUT (V
)
50%
Ix
tPLH
tPHL
OUTPUT (V
)
50%
Ox
0.1
Figure 52. Propagation Delay Parameters
Pulse width distortion is the maximum difference between these
two propagation delay values and is an indication of how
accurately the input signal timing is preserved.
0.01
0.001
Channel-to-channel matching refers to the maximum amount
the propagation delay differs between channels within a single
ADuM447x component.
1k
10k
100k
1M
10M
100M
MAGNETIC FIELD FREQUENCY (Hz)
Figure 53. Maximum Allowable External Magnetic Flux Density
Propagation delay skew refers to the maximum amount
the propagation delay differs between multiple ADuM447x
components operating under the same conditions.
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.
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 ADuM447x
devices only during power-up and power-down operations.
Rev. 0 | Page 30 of 36
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