Datasheet

Data Sheet ADuM2250/ADuM2251
Rev. A | Page 11 of 16
STARTUP
Both the V
DD1
and V
DD2
supplies have an under voltage lockout
feature that prevents the signal channels from operating unless
certain criteria is met. This feature is to avoid the possibility of
input logic low signals from pulling down the I
2
C bus inadver-
tently during power-up/power-down.
Criteria that must be met for the signal channels to be enabled
are as follows:
 Both supplies must be at least 2.5 V.
 At least 40 μs must elapse after both supplies exceed the
internal start-up threshold of 2.0 V.
Until both of these criteria are met for both supplies, the
ADuM2250/ADuM2251 outputs are pulled high thereby
ensuring a startup that avoids any disturbances on the bus.
Figure 8 and Figure 9 illustrate the supply conditions for fast
and slow input supply slew rates.
SUPPLY VALID
MINIMUM RECOMMENDED
OPERATING SUPPLY, 3.0V
MINIMUM VALID SUPPLY, 2.5V
INTERNAL STARTUP
THRESHOLD, 2.0V
40µs
06670-008
Figure 8. Start-Up Condition, Supply Slew Rate < 12.5 V/ms
SUPPLY VALID
MINIMUM RECOMMENDED
OPERATING SUPPLY, 3.0V
MINIMUM VALID SUPPLY, 2.5V
INTERNAL STARTUP
THRESHOLD, 2.0V
40µs
06670-009
Figure 9. Start-Up Condition, Supply Slew Rate > 12.5 V/ms
MAGNETIC FIELD IMMUNITY
The ADuM2250/ADuM2251 are extremely immune to external
magnetic fields. The limitation on the magnetic field immunity
of the ADuM2250/ADuM2251 is set by the condition in which
induced voltage in the receiving coil of the transformer is suffi-
ciently 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 ADuM2250/ADuM2251 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

 NnrdtdV
n
...,2,1;)/β(
2
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.
Given the geometry of the receiving coil in the ADuM2250/
ADuM2251 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 10.
MAGNETIC FIELD FREQUENCY (Hz)
100
0.001
1M
10
0.01
1k 10k 10M
0.1
1
100M100k
06670-010
MAXIMUM ALLOWABLE MAGNETIC FLUX
DENSITY (kgauss)
Figure 10. Maximum Allowable External Magnetic Flux Density
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 were to occur during a transmitted
pulse (and had the worst-case polarity), it would reduce the
received pulse from >1.0 V to 0.75 V (still well above the 0.5 V
sensing threshold of the decoder).