LTC1482CS8 datasheet(12/16 Pages) LINER

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LTC1482

When VCC is applied, some time is needed for CD and RO

to become valid. The time needed depends on the capaci-

tance at the CD pin, the VCC rise time and the loads

connected to the A and B pins. For a load capacitance of

15pF and a 1

µs VCC rise time, a wait time of 10µs is

recommended.

Receiver Output and Fail-Safe

If CD is low, the receiver output, RO, responds to the input

differential voltage and is guaranteed (by testing) to go

high if (A – B)

≥VTHRO(MAX) andlowif(A–B)≤VTHCD(MIN).

Some data encoding schemes require that the output of

the receiver maintain a known state (usually logic 1) when

data transmission ends and all drivers on the line are

forced into three-state. The carrier detect mechanism

ensures that RO will be high regardless of whether the line

is open, floating or shorted together, or whether the line is

terminated or not. This removes external components

required with earlier RS485 devices for the case where the

required known state is a logic 1. External components are

needed if the required state is a logic 0.

Fail-safe operates over the – 7V to 12V common mode

range and fast common mode steps do not affect the

receiver output.

Note that the CD output only goes high after all the drivers

are three-stated due to built-in delays (tCDH) in the CD

signal path (see Carrier Detect Operation). During the time

interval (see Figure 11) beginning at driver three-state and

ending at CD going high, the receiver output stays at the

last state just prior to the driver three-stating.

ICC Shutdown Mode

The supply current of the LTC1482 is reduced to 20

µAmax

by taking both the DE and DI/SHDN pins low. In shutdown,

all internal circuits are powered down and the driver and

receiver outputs are three-stated. The CD output is taken

high by the weak internal pull-up.

Logic within the LTC1482 prevents slow DE and DI/SHDN

transitions from generating internal shutdown pulses by

rejecting “shutdown pulses” of less than 50ns (typ) in

duration. Without this logic, the driver outputs will glitch

when three-stated momentarily.

Carrier Detect Operation

The carrier detect or CD pin is an open-drain output with

a weak internal pull-up (30

µA typical). This allows several

LTC1482s to share the same carrier detect line. The

internal pull-up has a series diode, permitting users to tie

the CD output to a voltage higher than VCC (8V max). When

driving low, the CD output can sink up to 4mA while

maintaining the output below a TTL VOL of 0.4V. An

external pull-up resistor is recommended if fast rise times

are important.

The LTC1482 defines the presence of a carrier as

VTHCD(MIN) ≥ (A – B) ≥ VTHCD(MAX). CD pulls low when a

carrier is present. When the carrier is absent, the weak

internal pull-up pulls CD high. For slow moving input

signals (below about 32kHz for signals conforming to

RS485 specifications), the CD output will go high when the

(A – B) signal is within the VTHCD(MIN) to VTHCD(MAX) range.

For faster input signals, the CD output does not glitch high

when the (A – B) signal is traversing the transition region.

This is achieved through internal delays in the CD signal

path. It takes tCDH (≤ 5µs) for CD to go high after the carrier

signal is removed. There are no additional built-in delays

for CD going low so that tCDL is only 300ns max.

When the LTC1482 is not in shutdown mode, CD = 1

always forces the receiver output (RO) high. If the driver

is enabled (DE = 1), CD = 0 as long as VTHCD min ≥ (A – B)

≥ VTHCD max. Shorting the A and B pins together or

excessive loading between these pins will cause this

condition to be violated and the CD pin will pull high. If the

driver is disabled, CD is guaranteed to go high when:

a) A is shorted to B,

b) A and B are open (with or without termination) or

c) VTHCD min ≤ A – B ≤ VTHCD max

The last condition occurs if the external driver is loaded

excessively.

In shutdown mode, RO is three-stated and CD is taken high

by the weak internal pull-up. On exiting shutdown, it takes

longer (tCDL(SHDN) = 5µs max) for CD to pull low when a

carrier is present.

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