LTC2851 LINER [Linear Technology], LTC2851 Datasheet - Page 12

• Manufacturer Part Number: LTC2851
• Description: 20Mbps RS485 Transceivers
• Manufacturer: LINER [Linear Technology]

applicaTions inFormaTion
LTC2859/LTC2861
The integrated termination resistor has a high frequency
response which does not limit performance at the maxi-
mum specified data rate. Figure 12 shows the magnitude
and phase of the termination impedance vs frequency. The
termination resistor cannot be enabled by TE if the device
is unpowered or in thermal shutdown mode.
Supply Current
The unloaded static supply currents in the LTC2859/
LTC2861 are very low —typically under 700µA for all modes
of operation without the internal terminator enabled. In
applications with resistively terminated cables, the supply
terminators are external or internal such as in the LTC2859/
LTC2861. Power supply current increases with toggling
data due to capacitive loading and this term can increase
current vs data rate for two different capacitive loads (for
the circuit configuration of Figure 3).
High Speed Considerations
A ground plane layout is recommended for the LTC2859/
LTC2861. A 0.1µF bypass capacitor less than one quarter
inch away from the V
board traces connected to signals A/B and Z/Y (LTC2861)
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 sen-
sitive inputs to reduce feedback effects that might cause
noise, jitter, or even oscillations. For example, in the full
duplex LTC2861, DI and A/B should not be routed near
the driver or receiver outputs.
The logic inputs of the LTC2859/LTC2861 have 50mV 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
current is dominated by the driver load. For example, when
using two 120Ω terminators 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
significantly at high data rates. Figure 13 shows supply
12
CC
pin is also recommended. The PC
150
140
130
120
110
185
170
155
140
125
110
Figure 13. Supply Current vs Data Rate
75
70
65
60
55
50
45
95
80
–10
10
10
Figure 11. Termination Resistance
vs Common Mode Voltage
Figure 12. Termination Magnitude
and Phase vs Frequency
2
–1
R
DIFF
= 54Ω
–5
COMMON MODE VOLTAGE (V)
10
FREQUENCY (MHz)
DATA RATE (kbps)
C
3
MAGNITUDE
L
0
= 1000pF
10
0
C
5
L
= 100pF
10
4
PHASE
10
285961 F13
285961 F11
10
285455 F12
10
15
1
30
15
0
–15
–30
–45
–60
–75
5
285961fc