8

LTC1487

APPLICATIO S I FOR ATIO

effects of noise on the line signals. If the line is terminated

or the receiver inputs are shorted together, the receiver

output will retain the last valid line signal due to the 45mV

of hysteresis incorporated in the receiver circuit. If the

LTC1487 transceiver inputs are left 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 LTC1487 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

receiver active and the driver outputs disabled, the LTC1487

will typically draw 80

µA quiescent current. With the driver

outputs enabled but unterminated, quiescent current will

rise slightly as one of the two outputs sources current into

the internal receiver input resistance. With the minimum

receiver input resistance of 70k and the maximum output

swing of 5V, the quiescent current will rise by a maximum

of 72

µA. Typical quiescent current rise with the driver

enabled is about 40

µA.

The quiescent current rises significantly if the driver is

enabled when it is externally terminated. With 1/2

termination 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 40

µA drawn by the

internal receiver 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 LTC1487 will

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

In shutdown the LTC1487 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. Toggling

either RE or DE will wake the LTC1487 back up within

3.5

µs.

If the driver is active immediately prior to shutdown, the

supply current will not drop to 1

µA until the driver

outputs have reached a steady state; this can take as long

as 2.6

µs under worst case conditions. If the driver is

disabled prior to shutdown the supply current will drop

to 1

µA immediately.

Slew Rate and 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 LTC1487

propagation delay.

Figure 11. Receiver Propagation Delay Test Circuit

D

BR

R

RECEIVER

OUT

R

100

Ω

100pF

100pF

TTL IN

LTC1487 • F11

The receiver delay times are:

The LTC1487 drivers feature controlled slew rate to reduce

system EMI and improve signal fidelity by reducing reflec-

tions due to misterminated cables.

The driver’s skew times are:

For package descriptions consult the

1994 Linear Databook Volume III.

© LINEAR TECHNOLOGY CORPORATION 1995

LT/GP 0395 10K • PRINTED IN THE USA

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7487