LTC2854/LTC2855

12

285455fc

For more information www.linear.com/LTC2854

285455 F14

DATA RATE (bps)

10k

1M

10M

100k

100M

100

1k

10

10k

RS485/RS422

MAX DATA RATE

LTC2854/LTC2855

MAX DATA RATE

Figure 14. Cable Length vs Data Rate (RS485/

RS422 Standards Shown in Vertical Solid Line)

Figure 13. Supply Current vs Data Rate

DATA RATE (Mbps)

0.1

20

60

70

80

1

10

100

285455 F13

50

40

30

RDIFF = 54

CL = 1000pF

CL = 100pF

APPLICATIONS INFORMATION

Supply Current

The unloaded static supply currents in the LTC2854/

LTC2855areverylow—typicallyunder500µAforallmodes

of operation. In applications with resistively terminated

cables, the supply 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.

Thisistruewhethertheterminatorsareexternalorinternal

such as in the LTC2854/LTC2855. Power supply current

increases with toggling rate due to capacitive loading and

this term can increase significantly at high data rates. Fig-

ure 13 shows supply current vs data rate for two different

capacitive loads for the circuit configuration 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 VCC pin is also recommended. The PC

board traces connected to signals A/B and Z/Y (LTC2855)

shouldbesymmetricalandasshortaspossibletomaintain

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 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

outputscancauseglitchesinthegroundandpowersupplies

whichareexacerbatedbycapacitiveloading.Ifalogicinput

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 reflect different performance limiting factors in data

transmission. In the flat region of the curve, maximum

distance is determined by resistive losses in the cable. The

downwardslopingregionrepresentslimitsindistanceand

data rate due to AC losses in the cable. The solid vertical

line represents the specified maximum data rate in the

RS485/RS422 standards. The dashed lines at 20Mbps

show the maximum data rates of the LTC2854/LTC2855.