adum5401w Analog Devices, Inc., adum5401w Datasheet - Page 16

• Manufacturer Part Number: adum5401w
• Description: Quad-channel Isolators With Integrated Dc-to-dc Converter
• Manufacturer: Analog Devices, Inc.

ADuM5401W/ADuM5402W/ADuM5403W
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 21).
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
ADuM5401W/ADuM5402W/ADuM5403W component.
Propagation delay skew refers to the maximum amount the
propagation delay differs between multiple ADuM5401W/
ADuM5402W/ADuM5403W components operating under
the same conditions.
EMI CONSIDERATIONS
The dc-to-dc converter section of the ADuM5401W/
ADuM5402W/ADuM5403W components must, of necessity,
operate at a 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, follow good RF design
practices in the layout of the PCB. See
most current PCB layout recommendations specifically for the
ADuM5401W/ADuM5402W/ADuM5403W.
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 not powered or nonfunctional, in which case,
the isolator output is forced to a default high state by the watchdog
timer circuit. This situation should only occur during power-up
and power-down operations.
INPUT (V
OUTPUT (V
Ix
)
Ox
)
Figure 21. Propagation Delay Parameters
t
PLH
t
PHL
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The limitation on the ADuM5401W/ADuM5402W/ADuM5403W
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 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 the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
Given the geometry of the receiving coil in the ADuM5401W/
ADuM5402W/ADuM5403W, 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 22.
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
Figure 22. Maximum Allowable External Magnetic Flux Density
0.01
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
10M
100M