LINEAR TECHNOLOGY LTC2854 Technical data
L DESIGN FEATURES
DE
RO
DI
A
(15kV)
TE
V
CC
B
(15kV)
Z
(15kV)
Y
(15kV)
SLEEP/SHUTDOWN
LOGIC AND DELAY
RECEIVER
DRIVER
RE
120Ω
R
TERM
120Ω
R
TERM
DE
RO
DI
A
(25kV)
TE
B
(25kV)
SLEEP/SHUTDOWN
LOGIC AND DELAY
RECEIVER
DRIVER
RE
V
CC
GND
125k
R
IN
125k
R
IN
125k
R
IN
125k
R
IN
GND
LTC2854 LTC2855
Rugged 3.3V RS485/RS422
Transceivers with Integrated
Switchable Termination
by Steven Tanghe and Ray Schuler
Introduction
Medium and high speed RS485 networks must be terminated to avoid
data-corrupting reflections. This
means a termination resistor is placed
at each end of the bus. Of course, if
the network is expanded or reconfigured, the termination resistors must
also move. The 3.3V LTC2854 and
LTC2855 transceivers eliminate the
cumbersome task of shuffling termination resistors. These devices have
an integrated termination resistor
connected across the receiver inputs
that can be enabled or disabled with
simple logical control of an input
pin, making network configuration
and reconfiguration a snap. These
devices come in tiny packages and are
extremely robust, withstanding ESD
strikes of up to ±25kV HBM (LTC2854)
on the line I/O pins—the industry’s
highest protection level for an RS485
transceiver.
Other features of the LTC2854
and LTC2855 include a receiver with
balanced thresholds for excellent
duty cycle performance, high input
Figure 1. Photograph of the (left to right)
LTC2854 3mm × 3mm DFN, LTC2855
4mm × 3mm DFN, and the LTC2855 SSOP
resistance allowing as many as 256
devices to be connected to one bus,
and a full failsafe output. The driver
offers low power operation, which in
conjunction with the receiver and
integrated termination resistor, provide a single die impedance-matched
network solution. Parts are available
in half- and full-duplex configurations
in tiny packages including 10- and
12-pin DFN as well as 16-lead SSOP
(see Table 1 and photo in Figure 1).
Block diagrams for the LTC2854 and
LTC2855 are shown in Figure 2.
Switchable Termination
Differential signals propagating down
a twisted pair transmission line are
partially reflected when an impedance mismatch is encountered. The
reflected signal causes constructive
and/or destructive interference on the
line that can corrupt data. To prevent
this condition and optimize system
performance, transmission lines
should be terminated at each end with
a resistor matching the characteristic
impedance of the cable.
The LTC2854 and LTC2855 transceivers integrate this termination
resistor so that it can be selectively
included or excluded simply by controlling the Termination Enable pin
(TE). The resistor is effectively connected across the receiver input pins
by setting TE high and disconnected
when TE is low or the device is unpowered. This arrangement is nearly
ideal from a system management
14
Figure 2. Block diagrams of the LTC2854 and LTC2855
Linear Technology Magazine • March 2007
DESIGN FEATURES L
RO RE TE DE DI
120Ω
LTC2854
R
D
RO RE TE DE DI
120Ω
LTC2854
200 FEET
CAT 5 CABLE
100 FEET
CAT 5 CABLE
R
D
RO RE TE DE DI
120Ω
LTC2854
NODE 1 - Tx NODE 2 - Rx NODE 3 - Rx
R
D
NODES 1 AND 2 PRESENT;
TE ON AT NODES 1 AND 2
NODE 2
NODE 2
NODES 1, 2 AND 3 PRESENT;
TE ON AT NODES 1 AND 2
NODE 3
NODES 1, 2 AND 3 PRESENT;
TE ON AT NODES 1 AND 3
NODE 2
NODE 3
Figure 3. Effects of termination placement with network expansion
standpoint, especially under conditions where a network configuration
changes and the termination resistor
needs to be moved to the new end of
the bus. In this case, manual removal
and placement of a discrete resistor
is not necessary; rather the change
is controlled digitally with the appropriate selection of TE pins on the
LTC2854 or LTC2855.
To illustrate the importance of
termination placement, consider the
configuration shown in Figure 3 where
the effects of network expansion are
presented. The initial configuration
consists of nodes 1 and 2, made up of
LTC2854 transceivers connected with
200 feet of Cat 5 cable. The waveforms
in the lower left of the figure show
the signal received at node 2, driven
Table 1. Product selection
PART NUMBER DUPLEX PACKAGE
from node 1. Both ends of the cable
are terminated by setting the TE
pins high on both transceivers. The
received signal looks clean because
the bus is properly terminated. A
small impedance mismatch between
the cable characteristic impedance of
100Ω and the termination resistor of
120Ω, results in a slight bump in the
waveform. This effect is minor and the
figure serves to illustrate that the termination resistor in the LTC2854 and
LTC2855 is compatible with popular
low cost 100Ω cables.
The second set of waveforms on the
bottom of Figure 3 show the results of
introducing a third node to the system through 100 feet of added cable
but without moving the termination
resistor to the new end location. The
ESD on Line I/O
(HBM)
LTC2854 HALF DFN-10 ±25kV
LTC2855 FULL SSOP-16, DFN-12 ±15kV
waveforms at node 3 and node 2 are
both severely distorted from reflections
caused by the improper termination.
In the third set of waveforms, the
termination placement has been corrected by setting TE high at nodes 1
and 3 only, thereby cleaning up the
signals received at nodes 2 and 3. The
logic-selectable termination resistors
in the LTC2854 permit this correction with no physical intervention
required.
The termination resistance is well
maintained over temperature, common mode voltage and frequency (as
illustrated in Figure 4). Furthermore,
the termination network adds only
insignificant capacitive loading to the
receiver pins. The input capacitance
on the LTC2855’s A and B pins is approximately 9pF measured to ground
and 3.5pF differentially.
Balanced Threshold Receiver
with Full Failsafe
The LTC2854 and LTC2855 feature
a low power receiver that draws
only 450µA. The single-ended input
resistance to ground on each of the
Linear Technology Magazine • March 2007
15
Loading...