Datasheet

Data Sheet ADM2483
Rev. F | Page 15 of 18
TRUTH TABLES
The following truth tables use these abbreviations:
Letter Description
H
High level
L Low level
X Irrelevant
Z High impedance (off)
NC Disconnected
Table 10. Transmitting
Supply Status Inputs Outputs
V
DD1
V
DD2
DE TxD A B
On On H H H L
On
On
H
L
L
H
On On L X Z Z
On Off X X Z Z
Off On X X Z Z
Off Off X X Z Z
Table 11. Receiving
Supply Status Inputs Outputs
V
DD1
V
DD2
A − B (V)
RE
RxD
On On >−0.03 L or NC H
On On <−0.2 L or NC L
On On
−0.2 < A − B <
−0.03
L or NC Indeterminate
On On Inputs open L or NC H
On On X H Z
On Off X L or NC H
Off On X L or NC H
Off Off X L or NC L
POWER-UP/POWER-DOWN CHARACTERISTICS
The power-up/power-down characteristics of the ADM2483 are
in accordance with the supply thresholds shown in Table 12.
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
DD1
power-down threshold
is crossed, the ADM2483 output signals reach their unpowered
states within 4 µs.
Table 12. Power-Up/Power-Down Thresholds
Supply Transition Threshold (V)
V
DD1
Power-up 2.0
V
DD1
Power-down 1.0
V
DD2
Power-up 3.3
V
DD2
Power-down 2.4
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 3 V
operating condition of the ADM2483 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
;π
2
n
r
dt
d
V ∑






β−
=
N
n ,...,2
,1=
where if the pulses at the transformer output are greater than
1.0 V in amplitude:
β
= magnetic flux density (gauss)
N
= number of turns in receiving coil
r
n
= 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.
Given the geometry of the receiving coil and an imposed
requirement that the induced voltage is, at most, 50% of the
0.5 V margin at the decoder, a maximum allowable magnetic
field is calculated, as shown in Figure 27.
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.