adum5400 Analog Devices, Inc., adum5400 Datasheet - Page 15

• Manufacturer Part Number: adum5400
• Description: Quad-channel Isolator With Integrated Dc-to-dc Converter
• Manufacturer: Analog Devices, Inc.

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 18).
The propagation delay to a logic low output may differ from the
propagation delay to a logic high.
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.
Channel-to-channel matching refers to the maximum amount
the propagation delay differs between channels within a single
ADuM5400 component.
Propagation delay skew refers to the maximum amount the
propagation delay differs between multiple ADuM540x
components operating under the same conditions.
EMI CONSIDERATIONS
The dc-to-dc converter section of the ADuM5400 component
must, of necessity, operate at very high frequency to allow
efficient power transfer through the small transformers. This
creates high frequency currents that can propagate in circuit board
ground and power planes, causing edge and dipole radiation.
Grounded enclosures are recommended for applications that use
these devices. If grounded enclosures are not possible, good RF
design practices should be followed in layout of the PCB. See
www.analog.com
recommendations specifically for the ADuM5400.
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 10) by the watchdog
timer circuit. This situation should occur in the ADuM5400
device only during power-up and power-down operations.
INPUT (V
OUTPUT (V
Preliminary Technical Data
Ix
)
Ox
)
Figure 18. Propagation Delay Parameters
for the most current PCB layout
t
PLH
t
PHL
50%
50%
Rev. PrA | Page 15 of 21
The limitation on the ADuM5400 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 ADuM5400 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 ADuM5400, 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 19.
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.
n
is the radius of the n
V = (−dβ/dt)
0.001
0.01
Figure 19. Maximum Allowable External Magnetic Flux Density
100
0.1
10
1
1k
∑
10k
πr
MAGNETIC FIELD FREQUENCY (Hz)
n
2
th
; n = 1, 2, … , N
turn in the receiving coil (cm).
100k
1M
ADuM5400
10M
100M