Datasheet

ADuM1400/ADuM1401/ADuM1402
Rev. D | Page 19 of 24
APPLICATION INFORMATION
PC BOARD LAYOUT
The ADuM140x digital isolator requires no external interface
circuitry for the logic interfaces. Power supply bypassing is
strongly recommended at the input and output supply pins
(
Figure 17). Bypass capacitors are most conveniently connected
between Pins 1 and 2 for V
DD1
and between Pins 15 and 16 for
V
DD2
. The capacitor value should be between 0.01 μF and 0.1 μF.
The total lead length between both ends of the capacitor and
the input power supply pin should not exceed 20 mm.
Bypassing between Pins 1 and 8 and between Pins 9 and 16
should also be considered unless the ground pair on each
package side is connected close to the package.
V
DD1
GND
1
V
IA
V
IB
V
IC/OC
V
ID/OD
V
E1
GND
1
V
DD2
GND
2
V
OA
V
OB
V
OC/IC
V
OD/ID
V
E2
GND
2
0
3786-017
Figure 17. Recommended Printed Circuit Board Layout
In applications involving high common-mode transients, care
should be taken to ensure that board coupling across the isolation
barrier is minimized. Furthermore, the board layout should be
designed such that any coupling that does occur equally affects
all pins on a given component side. Failure to ensure this could
cause voltage differentials between pins exceeding the device’s
Absolute Maximum Ratings, thereby leading to latch-up or
permanent damage.
PROPAGATION DELAY-RELATED PARAMETERS
Propagation delay is a parameter that describes the time it takes
a logic signal to propagate through a component. The propagation
delay to a logic low output may differ from the propagation
delay to a logic high.
INPUT (
V
IX
)
OUTPUT (V
OX
)
t
PLH
t
PHL
50%
50%
03786-018
Figure 18. 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’s timing is preserved.
Channel-to-channel matching refers to the maximum amount
the propagation delay differs between channels within a single
ADuM140x component.
Propagation delay skew refers to the maximum amount the
propagation delay differs between multiple ADuM140x
components operating under the same conditions.
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 2 μs, a
periodic set 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 about 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.
The limitation on the ADuM140x’s magnetic field immunity is
set by the condition in which induced voltage in the transformer’s
receiving coil is sufficiently large to either falsely set or reset the
decoder. The following analysis defines the conditions under
which this may occur. The 3 V operating condition of the
ADuM140x 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, thus
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)
∑∏r
n
2
; n = 1, 2, … , N
where:
β is magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
is the radius of the n
th
turn in the receiving coil (cm).
Given the geometry of the receiving coil in the ADuM140x 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.
MAGNETIC FIELD FREQUENCY (Hz)
100
MAXIMUM ALLOWABLE MAGNETIC FLUX
DENSITY (kgauss)
0.001
1M
10
0.01
1k 10k 10M
0.1
1
100M100k
03786-019
Figure 19. Maximum Allowable External Magnetic Flux Density