LTC2854 LINER [Linear Technology], LTC2854 Datasheet - Page 12

• Manufacturer Part Number: LTC2854
• Description: 3.3V 20Mbps RS485/RS422 Transceivers with Integrated Switchable Termination
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC2854/LTC2855
Supply Current
The unloaded static supply currents in the LTC2854/
LTC2855 are very low —typically under 500μA for all
modes of operation. In applications with resistively ter-
minated cables, the supply current is dominated by the
driver load. For example, when using two 120Ω termina-
tors with a differential driver output voltage of 2V, the DC
current is 33mA, which is sourced by the positive voltage
supply. This is true whether the terminators are external
or internal such as in the LTC2854/LTC2855. Power sup-
ply current increases with toggling rate due to capacitive
loading and this term can increase signifi cantly at high
data rates. Figure 13 shows supply current vs data rate for
two different capacitive loads for the circuit confi guration
of Figure 4.
High Speed Considerations
A ground plane layout is recommended for the LTC2854/
LTC2855. A 0.1μF bypass capacitor less than one-quarter
inch away from the V
board traces connected to signals A/B and Z/Y (LTC2855)
should be symmetrical and as short as possible to maintain
good differential signal integrity. To minimize capacitive
effects, the differential signals should be separated by more
than the width of a trace and should not be routed on top
of each other if they are on different signal planes.
Care should be taken to route outputs away from any
sensitive inputs to reduce feedback effects that might
cause noise, jitter, or even oscillations. For example, in
12
Figure 13. Supply Current vs Data Rate
80
70
60
50
40
30
20
0.1
R
DIFF
= 54Ω
CC
DATA RATE (Mbps)
pin is also recommended. The PC
1
C
L
C
= 1000pF
L
= 100pF
10
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100
the full duplex LTC2855, DI and A/B should not be routed
near the driver or receiver outputs.
The logic inputs of the LTC2854/LTC2855 have 150mV of
hysteresis to provide noise immunity. Fast edges on the
outputs can cause glitches in the ground and power supplies
which are exacerbated by capacitive loading. If a logic input
is held near its threshold (typically 1.5V), a noise glitch
from a driver transition may exceed the hysteresis levels
on the logic and data input pins causing an unintended
state change. This can be avoided by maintaining normal
logic levels on the pins and by slewing inputs through
their thresholds by faster than 1V/μs when transitioning.
Good supply decoupling and proper line termination also
reduces glitches caused by driver transitions.
Cable Length vs Data Rate
For a given data rate, the maximum transmission distance
is bounded by the cable properties. A typical curve of
cable length vs data rate compliant with the RS485/RS422
standards is shown in Figure 14. Three regions of this
curve refl ect different performance limiting factors in data
transmission. In the fl at region of the curve, maximum
distance is determined by resistive losses in the cable. The
downward sloping region represents limits in distance and
data rate due to AC losses in the cable. The solid vertical
line represents the specifi ed maximum data rate in the
RS485/RS422 standards. The dashed lines at 20Mbps show
the maximum data rates of the LTC2854/LTC2855.
Figure 14. Cable Length vs Data Rate (RS485/
RS422 Standards Shown in Vertical Solid Line)
100
10k
10
1k
10k
100k
MAX DATA RATE
DATA RATE (bps)
RS485/RS422
1M
LTC2854/LTC2855
MAX DATA RATE
10M
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100M
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