LTC2855IGNTRPBF datasheet(10/16 Pages) LINER

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LTC2854/LTC2855

285455fa

Driver

The driver provides full RS485/RS422 compatibility. When

enabled, if DI is high, Y-Z is positive for the full-duplex

device (LTC2855) and A-B is positive for the half-duplex

device (LTC2854).

When the driver is disabled, both outputs are high-

impedance. For the full-duplex LTC2855, the leakage on

the driver output pins is guaranteed to be less than 10μA

over the entire common mode range of –7V to +12V. On

the half-duplex LTC2854, the impedance is dominated by

the receiver input resistance, RIN.

Driver Overvoltage and Overcurrent Protection

The driver outputs are protected from short-circuits to

any voltage within the Absolute Maximum range of (VCC

–15V) to +15V. The typical peak current in this condition

does not exceed 180mA.

If a high driver output is shorted to a voltage just above

VCC, a reverse current will ﬂow into the supply. When this

voltage exceeds VCC by about 1.4V, the reverse current

turns off. Preventing the driver from turning off with outputs

shorted to output voltages just above VCC keeps the driver

active even for receiver loads that have a positive common

mode with respect to the driver — a valid condition.

The worst-case peak reverse short-circuit current can be as

high as 300mA in extreme cold conditions. If this current

cannot be absorbed by the supply, a 3.6V Zener diode can

be added in parallel with the supply to sink this current.

All devices also feature thermal shutdown protection that

disables the driver and receiver in case of excessive power

dissipation (see Note 4).

Receiver and Failsafe

With the receiver enabled, when the absolute value of the

differential voltage between the A and B pins is greater than

200mV, the state of RO will reﬂect the polarity of (A-B).

The LTC2854/LTC2855 have a failsafe feature that guaran-

tees the receiver output to be in a logic-high state when

the inputs are either shorted, left open, or terminated

(externally or internally), but not driven. This failsafe fea-

ture is guaranteed to work for inputs spanning the entire

common mode range of –7V to +12V.

The receiver output is internally driven high (to VCC) or

low (to ground) with no external pull-up needed. When

the receiver is disabled the RO pin becomes high-Z with

leakage of less than ±1μA for voltages within the supply

range.

Receiver Input Resistance

The receiver input resistance from A or B to ground is guar-

anteed to be greater than 96k (C, I-Grade) when the termi-

nation is disabled. This is 8X higher than the requirements

for the RS485 standard and thus this receiver represents a

one-eighth unit load. This, in turn, means that 8X the

standard number of receivers, or 256 total, can be con-

nected to a line without loading it beyond what is called

out in the RS485 standard. The receiver input resistance

from A or B to ground on high temperature H-Grade parts

is greater than 48k providing a one-quarter unit load. The

input resistance of the receivers is unaffected by enabling/

disabling the receiver and by powering/unpowering the

part. The equivalent input resistance looking into A and B

is shown in Figure 9. The termination resistor cannot be

enabled by TE if the device is unpowered or in thermal

shutdown mode.

285455 F09

>96k

Figure 9. Equivalent Input Resistance into A and B

(on the LTC2854, Valid if Driver is Disabled)

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