LTC1487 LINER [Linear Technology], LTC1487 Datasheet - Page 8

• Manufacturer Part Number: LTC1487
• Description: Ultra-Low Power RS485 with Low EMI, Shutdown and High Input Impedance
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
PACKAGE
LTC1487
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 A 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 A. With the
receiver active and the driver outputs disabled, the LTC1487
will typically draw 80 A 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 A. Typical quiescent current rise with the driver
enabled is about 40 A.
The quiescent current rises significantly if the driver is
enabled when it is externally terminated. With 1/2
termination load (120 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 line attached, the current will
rise to greater than 25mA with the driver enabled,
completely overshadowing the extra 40 A 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.
For package descriptions consult the 1994 Linear Databook Volume III .
8
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
[phone]
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In shutdown the LTC1487 typically draws only 1 A 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 s.
If the driver is active immediately prior to shutdown, the
supply current will not drop to 1 A until the driver
outputs have reached a steady state; this can take as long
as 2.6 s under worst case conditions. If the driver is
disabled prior to shutdown the supply current will drop
to 1 A 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.
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:
Skew = 250ns Typ, V
t
r
, t
t
PLH
TTL IN
f
Figure 11. Receiver Propagation Delay Test Circuit
< 6ns
– t
600ns Max, V
PHL
D
= 13ns Typ, V
CC
CC
R
100
= 5V
100pF
100pF
= 5V, T
LINEAR TECHNOLOGY CORPORATION 1995
CC
LT/GP 0395 10K • PRINTED IN THE USA
= 5V
LTC1487 • F11
A
BR
= – 40 C to 85 C
R
RECEIVER
OUT