ADuM1250/ADuM1251 Data Sheet
Rev. H | Page 10 of 16
APPLICATIONS INFORMATION
FUNCTIONAL DESCRIPTION
The ADuM1250/ADuM1251 interface on each side to a
bidirectional I
2
C signal. Internally, the I
2
C interface is split
into two unidirectional channels communicating in opposing
directions via a dedicated iCoupler isolation channel for each.
One channel (the bottom channel of each channel pair shown
in Figure 6) senses the voltage state of the Side 1 I
2
C pin and
transmits its state to its respective Side 2 I
2
C pin.
Both the Side 1 and the Side 2 I
2
C pins are designed to interface
to an I
2
C bus operating in the 3.0 V to 5.5 V range. A logic low
on either pin causes the opposite pin to be pulled low enough to
comply with the logic low threshold requirements of other I
2
C
devices on the bus. Avoidance of I
2
C bus contention is ensured
by an input low threshold at SDA
1
or SCL
1
guaranteed to be at
least 50 mV less than the output low signal at the same pin. This
prevents an output logic low at Side 1 being transmitted back to
Side 2 and pulling down the I
2
C bus.
Because the Side 2 logic levels/thresholds are standard I
2
C values,
multiple ADuM1250/ADuM1251 devices connected to a bus by
their Side 2 pins can communicate with each other and with other
I
2
C compatible devices. A distinction is made between I
2
C compat-
ibility and I
2
C compliance. I
2
C compatibility refers to situations in
which the logic levels of a component do not necessarily meet the
requirements of the I
2
C specification but still allow the component
to communicate with an I
2
C compliant device. I
2
C compliance
refers to situations in which the logic levels of a component meet
the requirements of the I
2
C specification.
However, because the Side 1 pin has a modified output level/
input threshold, this side of the ADuM1250/ADuM1251 can
communicate only with devices that conform to the I
2
C stan-
dard. In other words, Side 2 of the ADuM1250/ADuM1251 is
I
2
C compliant, whereas Side 1 is only I
2
C compatible.
The output logic low levels are independent of the V
DD1
and
V
DD2
voltages. The input logic low threshold at Side 1 is also
independent of V
DD1
. However, the input logic low threshold at
Side 2 is designed to be at 0.3 V
DD2
, consistent with I
2
C require-
ments. The Side 1 and Side 2 pins have open-collector outputs
whose high levels are set via pull-up resistors to their respective
supply voltages.
ENCODE DECODE
DECODE ENCODE
ENCODE DECODE
DECODE ENCODEV
DD1
SDA
1
SCL
1
V
DD2
SDA
2
SCL
2
C
L
GND
2
1
2
3
8
7
6
5
GND
1
4
C
L
R2 R2
6113-006
Figure 6. ADuM1250 Block Diagram
STARTUP
Both the V
DD1
and V
DD2
supplies have an undervoltage lockout
feature to prevent the signal channels from operating unless
certain criteria are met. This feature prevents input logic low
signals from pulling down the I
2
C bus inadvertently during
power-up/power-down.
For the signal channels to be enabled, the following two criteria
must be met:
Both supplies must be at least 2.5 V.
At least 40 μs must elapse after both supplies exceed the
internal startup threshold of 2.0 V.
Until both criteria are met for both supplies, the ADuM1250/
ADuM1251 outputs are pulled high, ensuring a startup that
avoids any disturbances on the bus. Figure 7 and Figure 8 illustrate
the supply conditions for fast and slow input supply slew rates.
MINIMUM RECOMMENDED
OPERATING SUPPLY, 3.0V
MINIMUM VALID SUPPLY, 2.5V
INTERNAL START-UP
THRESHOLD, 2.0V
40µs
SUPPLY VALID
06113-007
Figure 7. Start-Up Condition, Supply Slew Rate > 12.5 V/ms
40µs
SUPPLY VALID
MINIMUM RECOMMENDED
OPERATING SUPPLY, 3.0V
MINIMUM VALID SUPPLY, 2.5V
INTERNAL START-UP
THRESHOLD, 2.0V
06113-008
Figure 8. Start-Up Condition, Supply Slew Rate < 12.5 V/ms
