ADM2481 Datasheet, PDF (14/20 Pages) Analog Devices – Half-Duplex, iCoupler Isolated RS-485 Transceiver

ADM2481
TRUTH TABLES
The following truth tables use the abbreviations shown in Table 9.
Table 9.
Letter
H
L
X
Z
NC
Description
High level
Low level
Don’t care
High impedance (off )
Disconnected
Table 10. Transmitting
Supply Status
VDD1
VDD2
DE
On
On
H
On
On
H
On
On
L
On
Off
X
Off
On
L
Off
Off
X
Inputs
TxD
H
L
X
X
L
X
Outputs
A
B
H
L
L
H
Z
Z
Z
Z
Z
Z
Z
Z
Table 11. Receiving
Supply Status
Inputs
VDD1
VDD2
A − B (V)
RE
On
On
>−0.03
L or NC
On
On
<−0.2
L or NC
On
On
−0.2 < A − B < −0.03 L or NC
On
On
Inputs open
L or NC
On
On
X
H
On
Off
X
L or NC
Off
Off
X
L or NC
Outputs
RxD
H
L
Indeterminate
H
Z
H
L
THERMAL SHUTDOWN
The ADM2481 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 that 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 thresh-
old 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 satura-
tion 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 3 V operating condition of the ADM2481 is examined
because it represents the most susceptible mode of operation.
The limitation on the ac magnetic field immunity of the
iCoupler 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
V
=
⎜⎛
⎝
− dβ
dt
⎟⎞
⎠
∑
πr_n²
;
n = 1, 2, ... , N
where, if the pulses at the transformer output are greater than
1.0 V in amplitude:
β is the magnetic flux density (gauss).
N is the number of turns in receiving coil.
rn is the radius of nth turn in receiving coil (cm).
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.
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