Datasheet

ADM2485
Rev. A | Page 14 of 20
CIRCUIT DESCRIPTION
ELECTRICAL ISOLATION
In the ADM2485, electrical isolation is implemented on the
logic side of the interface. Therefore, the part has two main
sections: a digital isolation section and a transceiver section
(see
Figure 30). Driver input and data enable, applied to the
TxD and RTS pins, respectively, and referenced to logic ground
(GND
1
), are coupled across an isolation barrier to appear at the
transceiver section referenced to isolated ground (GND
2
).
Similarly, the receiver output, referenced to isolated ground in
the transceiver section, is coupled across the isolation barrier to
appear at the RxD pin referenced to logic ground.
iCoupler Technology
The digital signals are transmitted across the isolation barrier
using iCoupler technology. This technique uses chip-scale
transformer windings to couple the digital signals magnetically
from one side of the barrier to the other. Digital inputs are
encoded into waveforms that are capable of exciting the
primary transformer winding. At the secondary winding, the
induced waveforms are then decoded into the binary value that
was originally transmitted.
ISOLATION
BARRIER
V
DD2
V
DD1
A
B
DE OUT
GND
2
GND
1
TxD
RTS
RxD
RE
ENCODE
DECODE
DECODE
DECODE
ENCODE
ENCODE
D
R
TRANSCEIVER
DIGITAL ISOLATION
D1 D2
OSC
06021-026
Figure 30. ADM2485 Digital Isolation and Transceiver Sections
TRUTH TABLES
Table 1 0 and Table 11 use the abbreviations found in Table 9.
Table 9. Truth Table Abbreviations
Letter Description
H High level
I Indeterminate
L Low level
X Irrelevant
Z High impedance (off)
NC Disconnected
Table 10. Transmitting
Supply Status Inputs Outputs
V
DD1
V
DD2
RTS TxD A B DE OUT
On On H H H L H
On On H L L H H
On On L X Z Z L
On Off X X Z Z L
Off On X X Z Z L
Off Off X X Z Z L
Table 11. Receiving
Supply Status Input Outputs
V
DD1
V
DD2
A − B
RE
RxD
On On >+0.2 V L or NC H
On On <–0.2 V L or NC L
On On −0.2 V < A − B < +0.2 V L or NC I
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
THERMAL SHUTDOWN
The ADM2485 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.
RECEIVER FAIL-SAFE INPUTS
The receiver input includes a fail-safe feature that guarantees a
Logic high RxD output when the A and B inputs are floating or
open-circuited.