LTC2850/LTC2851/LTC2852

10

285012fc

BLOCK DIAGRAM

SLEEP/SHUTDOWN

LOGIC AND DELAY

RECEIVER

DRIVER

RO

RE

DE

DI

A

(15kV)

GND

B

(15kV)

285012 BDa

RECEIVER

DRIVER

RO

DI

285012 BDb

A

(15kV)

Z

(15kV)

Y

(15kV)

B

(15kV)

GND

SLEEP/SHUTDOWN

LOGIC AND DELAY

RECEIVER

DRIVER

RO

RE

DE

DI

285012 BDc

A

(15kV)

Z

(15kV)

Y

(15kV)

B

(15kV)

GND

LTC2850

LTC2851

LTC2852

APPLICATIONS INFORMATION

Driver

The driver provides full RS485/RS422 compatibility. When

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

devices (LTC2851, LTC2852) and A-B is positive for the

half-duplex device (LTC2850).

When the driver is disabled, both outputs are high-im-

pedance. For the full-duplex devices, 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 LTC2850, the impedance is dominated by

Driver Overvoltage and Overcurrent Protection

The driver outputs are protected from short-circuits to

– 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

off. Preventing the driver from turning off with outputs

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

can not 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).

These parts have a failsafe feature that guarantees the

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

are either shorted, left open, or terminated but not driven.

This failsafe feature is guaranteed to work for inputs span-

ning the entire common mode range of –7V to 12V.

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.