1
dc1794afa
DEMO MANUAL DC1794A
Description
LTM2885
Isolated RS485/RS422
µModule Trans Power
Demonstration circuit 1794A is an isolated RS485/RS422
μModule
®
trans power featuring the LTM
®
2885. The
demo circuit is a 6500V
RMS
galvanically isolated RS485/
RS422 transceiver interface. The demo circuit features
an EMI optimized circuit configuration and printed circuit
board layout. The demo circuit operates from a single
external supply on V
CC
. The part generates the output
L
, LT, LTC, LTM, µModule, Linear Technology and the Linear logo are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
perForMAnce sUMMArY
operAting principles
voltage V
CC2
and communicates all necessary signaling
across the isolation barrier using LTC’s isolator µModule
technology.
Design files for this circuit board are available at
http://www.linear.com/demo
(T
A
= 25°C)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
CC
Input Supply Range
LTM2885
4.5
5
5.5
V
V
CC2
Output Voltage
I
LOAD
= 0mA to 100mA, DE = 0V
4.75
5
V
f
MAX
Maximum Data rate
SLO
= V
CC2
20
Mbps
V
IORM
Maximum Working Insulation Voltage
GND to GND2
890
600
V
DC
V
RMS
Common Mode Transient Immunity
50
kV/µs
The LTM2885 contains an isolated DC/DC converter, de-
livering power to V
CC2
at 5V from the input supply, V
CC
.
Isolation is maintained by the separation of GND and GND2,
which significant operating voltages and transients can
exist without affecting the operation of the LTM2885. The
logic side ON pin enables or shuts down the LTM2885.
RS485/RS422 signaling is controlled by the logic inputs
DE, DI, TE and
RE
. Connection to the transceiver pins (A,
B, Y and Z) allows full- or half-duplex operation on the
isolated side of the demo circuit. A full-/half-duplex jumper
is included on the demo circuit to ease setting the system
configuration. The
SLO
pin configures the slew rate of the
driver output pins Y and Z.
Data is transmitted out the driver pins Y and Z from the
input DI with DE set on. Data is received through the dif-
ference in A and B to the output RO with
RE
set on.
The LTM2885 µModule and demo circuit has been designed
and optimized for low RF emissions. To this end some
features of the LTM2885 are not available for evaluation
on the demo circuit, primarily to minimize the size of the
demo board and to limit the number of I/O connections.
The logic supply voltage, V
L
, is tied to V
CC
on the demo
circuit. All control signals are selectable by jumper pro-
gramming only, including ON,
RE
, DE, TE and
SLO
. The
spare logic channel D
IN
to D
OUT
is not available.
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