7

LTC1480

APPLICATIO S I FOR ATIO

receiver active and the driver outputs disabled, the LTC1480

will typically draw 300

µA quiescent current. With the

driver outputs enabled but unterminated, quiescent cur-

rent will rise as one of the two outputs sources current into

the internal receiver input resistance. With the minimum

receiver input resistance of 12k and the maximum output

swing of 3.3V, the quiescent current will rise by a maxi-

mum of 275

µA. Typical quiescent current rise with the

driver enabled is about 100

µA.

The quiescent current rises significantly if the driver is

enabled when it is externally terminated. With 1/2 termina-

tion load (120

Ω between the driver outputs) the quiescent

current will jump to at least 13mA as the drivers force a

minimum of 1.5V across the termination resistance. With

a fully terminated 60

Ω line attached, the current will rise

to greater than 25mA with the driver enabled, completely

overshadowing the extra 100

µAdrawnbyinternalreceiver

inputs.

Shutdown Mode

Both the receiver output (RO) and the driver outputs (A, B)

can be placed in three-state mode by bringing RE HIGH

and DE LOW respectively. In addition, the LTC1480 will

enter shutdown mode when RE is HIGH and DE is LOW.

In shutdown the LTC1480 typically draws only 1

µA of

supply current. In order to guarantee that the part goes

into shutdown, RE must be high and DE must be LOW for

at least 600ns simultaneously. If this time duration is less

than 50ns the part will not enter shutdown mode.

Propagation Delay

Many digital encoding schemes are dependent upon the

difference in the propagation delay times of the driver and

receiver. Figure 11 shows the test circuit for the LTC1480

propagation delay.

The receiver delay times are:

The driver’s skew times are:

voltage exceeds the supply rails. Latch-up at the output

drivers is virtually eliminated and the driver is prevented

from loading the line under RS485 specified fault condi-

tions. A proprietary output protection structure protects

the transceiver line terminals against ESD strikes of up to

±10kV.

When two or more drivers are connected to the same

transmission line, a potential condition exists whereby

more than two drivers are simultaneously active. If one or

more drivers is sourcing current while another driver is

sinking current, excessive power dissipation may occur

within either the sourcing or sinking element. This condi-

tion is defined as driver contention, since multiple drivers

are competing for one transmission line. The LTC1480

provides a current limiting scheme to prevent driver

contention failure. When driver contention occurs, the

current drawn is limited to about 70mA preventing exces-

sive power dissipation within the drivers.

The LTC1480 has a thermal shutdown feature which

protects the part from excessive power dissipation. Under

extreme fault conditions, up to 250mA can flow through

the part causing rapid internal temperature rise. The

thermal shutdown circuit will disable the driver outputs

when the internal temperature reaches 150

°C and turns

them back on when the temperature cools to 130

°C. This

cycle will repeat as necessary until the fault condition is

removed.

Receiver Inputs

The LTC1480 features an input common-mode range

covering the entire RS485 specified range of –7V to 12V.

Differential signals of greater than

±200mV within the

specified input common-mode range will be converted to

a TTL compatible signal at the receiver output. A small

amount of input hysteresis is included to minimize the

effects of noise on the line signals. If the receiver inputs are

floating (unterminated) an internal pull-up of 10

µA at the

A input will force the receiver output to a known high state.

Low Power Operation

The LTC1480 draws very little supply current whenever

the driver outputs are disabled. In shutdown mode the

quiescent current is typically less than 1

µA. With the

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of circuits as described herein will not infringe on existing patent rights.