SP3485 Datasheet, PDF (7/9 Pages) Exar Corporation – +3.3V Low Power Half-Duplex RS-485 Transceiver

The SP3485 is a member in the family of
+3.3V low power half-duplex transceivers
that meet the electrical specifications of the
RS-485 and RS-422 serial protocols. This
device is pin-to-pin compatible with the Exar
SP481, SP483 and SP485 devices as well
as popular industry standards. The SP3485
feature Exar's BiCMOS process allowing
low power operation without sacrificing
performance.
Driver
The driver outputs of the SP3485 are dif-
ferential outputs meeting the RS-485 and
RS-422 standards. The typical voltage
output swing with no load will be 0 volts to
+3.3 Volts. With a load of 54Ω across the
differential outputs, the drivers can maintain
greater than 1.5V voltage levels.
The driver of the SP3485 has a driver enable
control line which is active HIGH. A logic
HIGH on DE (pin 3) will enable the differential
driver outputs. A logic LOW on the DE (pin
3) will tri-state the driver outputs.
The driver of the SP3485 operates up to
10Mbps. The 250mA I_SC maximum limit
on the driver output allows the SP3485 to
withstand an infinite short circuit over the
-7.0V to +12V common mode range without
catastrophic damage to the IC.
Receiver
The SP3485 receiver has differential inputs
with an input sensitivity of ±200mV. Input
impedance of the receiver is 12kΩ minimum.
A wide common mode range of -7V to +12V
allows for large ground potential differences
between systems. The receiver is equipped
with a fail-safe feature that guarantees the
receiver output will be in a HIGH state when
the input is left unconnected. The receiver of
the SP3485 operates up to 10Mbps.
DESCRIPTION
The receiver of the SP3485 has an enable
control line which is active LOW. A logic
LOW on RE (pin 2) will enable the differential
receiver. A logic HIGH on RE (pin 2) of the
SP3485 will disable the receiver.
Low Power Shutdown Mode
Low-power shutdown mode is initiated
by bringing both RE high and DE low. In
shutdown, the devices typically draw only
50nA of supply current. RE and DE can be
driven simultaneously; the part is guaranteed
not to enter shutdown if RE is high and DE
is low for less than 50ns. If the inputs are in
this state for at least 600ns, the parts are
guaranteed to enter shutdown.
Enable times t_PRZH, t_PZH, t_PRZL and t_PZL assume
the part was not in a low-power shutdown
state. Enable times t_PRSH, t_PSH, t_PRSL and t_PSL
assume the parts were shut down. It takes
drivers and receivers longer to become
enabled from low-power shutdown mode
(t_PRSH, t_PSH, t_PRSL, t_PSL) than from driver/receiver-
disable mode (t_PRZH, t_PZH, t_PRZL, t_PZL).
INPUTS
OUTPUTS
RE
DE
DI
B
A
X
1
1
0
1
X
1
0
1
0
0
0
X
High-Z
1
0
X
Shutdown
Table 1. Transmit Function Truth Table
INPUTS
RE
DE
0
X
V_A - V_B
-50mV
0
X
-200mV
X
X
Open/Shorted
1
1
X
1
0
X
Table 2. Receive Function Truth Table
OUTPUTS
RO
1
0
1
High-Z
Shutdown
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7000 • www.exar.com
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SP3485_200_051916