ad51-069-0reel Analog Devices, Inc., ad51-069-0reel Datasheet - Page 15
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ad51-069-0reel
Manufacturer Part Number
ad51-069-0reel
Description
Half-duplex Icoupler-r Isolated Rs-485 Transceiver
Manufacturer
Analog Devices, Inc.
Datasheet
1.AD51-069-0REEL.pdf
(20 pages)
TRUTH TABLES
The following truth tables use these abbreviations:
Letter
H
L
X
Z
NC
Table 9. Transmitting
V
On
On
On
On
Off
Off
Table 10. Receiving
Supply Status
V
On
On
On
On
On
On
Off
Off
POWER-UP/POWER-DOWN CHARACTERISTICS
The power-up/power-down characteristics of the ADM2483 are
in accordance with the supply thresholds shown in Table 11.
Upon power-up, the ADM2483 output signals (A, B, and RxD)
reach their correct state once both supplies exceed their
thresholds. Upon power-down, the ADM2483 output signals
retain their correct state until at least one of the supplies drops
below its power-down threshold. When the V
threshold is crossed, the ADM2483 output signals reach their
unpowered states within 4 µs.
Table 11. Power-Up/Power-Down Thresholds
Supply
V
V
V
V
DD1
DD1
DD2
DD2
DD1
DD1
Supply Status
V
On
On
On
On
On
Off
On
Off
DD2
V
On
On
On
Off
On
Off
DD2
Transition
Power-up
Power-down
Power-up
Power-down
Description
High level
Low level
Irrelevant
High impedance (off)
Disconnected
A − B (V)
>−0.03
<−0.2
−0.2 < A − B <
−0.03
Inputs open
X
X
X
X
DE
H
H
L
X
X
X
Inputs
Inputs
L or NC
L or NC
L or NC
L or NC
H
L or NC
L or NC
L or NC
RE
TxD
H
L
X
X
X
X
Threshold (V)
2.0
1.0
3.3
2.4
DD1
RxD
H
L
Indeterminate
H
Z
H
H
L
A
H
L
Z
Z
Z
Z
power-down
Outputs
Outputs
B
L
H
Z
Z
Z
Z
Rev. B | Page 15 of 20
THERMAL SHUTDOWN
The ADM2483 contains thermal shutdown circuitry that
protects the part from excessive power dissipation during fault
conditions. Shorting the driver outputs to a low impedance
source can result in high driver currents. The thermal sensing
circuitry detects the increase in die temperature under this
condition and disables the driver outputs. This circuitry is
designed to disable the driver outputs when a die temperature
of 150°C is reached. As the device cools, the drivers are re-enabled
at a temperature of 140°C.
TRUE FAIL-SAFE RECEIVER INPUTS
The receiver inputs have a true fail-safe feature, which ensures
that the receiver output is high when the inputs are open or
shorted. During line-idle conditions, when no driver on the bus
is enabled, the voltage across a terminating resistance at the
receiver input decays to 0 V. With traditional transceivers,
receiver input thresholds specified between −200 mV and
+200 mV mean that external bias resistors are required on the
A and B pins to ensure that the receiver outputs are in a known
state. The true fail-safe receiver input feature eliminates the
need for bias resistors by specifying the receiver input threshold
between −30 mV and −200 mV. The guaranteed negative
threshold means that when the voltage between A and B decays
to 0 V, the receiver output is guaranteed to be high.
MAGNETIC FIELD IMMUNITY
Because iCouplers use a coreless technology, no magnetic
components are present, and the problem of magnetic
saturation of the core material does not exist. Therefore,
iCouplers have essentially infinite dc field immunity. The
analysis that follows defines the conditions under which this
might occur. The ADM2483’s 3 V operating condition is
examined because it represents the most susceptible mode of
operation.
The limitation on the iCoupler’s ac magnetic field immunity is
set by the condition in which the induced error voltage in the
receiving coil (the bottom coil in this case) is made sufficiently
large, either to falsely set or reset the decoder. The voltage
induced across the bottom coil is given by
where if the pulses at the transformer output are greater than
1.0 V in amplitude:
r
The decoder has a sensing threshold of about 0.5 V; therefore,
there is a 0.5 V margin in which induced voltages can be
tolerated.
β
N= number of turns in receiving coil
n
= radius of nth turn in receiving coil (cm)
= magnetic flux density (gauss)
V
=
⎛
⎜
⎝
−
dt
d
β
⎞
⎟
⎠
∑
π
r
n
2
;
n
=
, 1
, 2
...
,
N
ADM2483