sp301ct Exar Corporation, sp301ct Datasheet - Page 5

• Manufacturer Part Number: sp301ct
• Description: Rs-232/rs-422 Line Drivers/receivers
• Manufacturer: Exar Corporation

Figure 1. Isolating SP301/302 From Ground;
a) Single SP301/302; b) Multiple SP301/302
SOIC packages for operation over the commercial
temperature range.
USING THE SP301/302
POWER SUPPLIES
The SP301/SP302 require 12V and +5V for
full RS-232 and RS-422 operation. The 12V
supplies set up the RS-232 9V voltage swings,
and the +5V is used for the internal logic that
formats the communications mode and controls
the loopback function. The supply voltages can
be decreased to as low as 7V for V
and 4.0V for V
tions, derated performance can be expected.
POWER SUPPLY SEQUENCING
There are two requirements for power supply
sequencing for the SP301/302. The first is that
V
that when the part is powered up, V
applied 20ms before V
GENERAL USAGE
RS232 Operation
The SP301 and SP302 are fully compliant
RS-232 devices. Their outputs are fully pro-
tected against shorts to 20V with no external
SIGNAL PROCESSING EXCELLENCE
a)
b)
DD
Corporation
is always greater than V
3.3k Ω
3.3kΩ
Q
3
2N2222
Q
3
CC
3.3k Ω
2N2222
3.3kΩ
Q
. Under these supply condi-
V
4
DD
Q
V
4
DD
V
GND
V
GND
SP301/302
SP301/302
CC
DD
DD
V
GND
SP301/302
DD
.
V
V
V
V
CC
EE
CC
EE
CC
V
V
CC
EE
. The second is
V
V
DD
CC
EE
V
V
DD
CC
EE
and V
must be
EE
,
circuitry. If the potential exists for momentary
shorts to voltages greater than 20V, it is recom-
mended that a 220
with each driver output. This will limit any
damage from the higher short-circuit current
from these higher voltage potentials. Voltage
clamps such as back-to-back Zener diodes can
be used to clamp the driver outputs to “safe”
levels. Short circuit current to ground is inter-
nally limited, and can therefore be sustained
infinitely. Under normal operating conditions,
the drivers can typically source 7mA at 5V
output, which exceeds the minimum RS-232
standards requirement.
If an SP301/302 transmitter output occupies a
data transmission line with other RS-232 de-
vices which are not powered by the same power
supplies, it is possible that a device that is not
powered will have a low impedance path to
ground at its driver output. The RS-232 stan-
dards require that with no power applied to the
device, the impedance from a transmitter output
to ground must be greater than 300 . This can
be easily achieved as shown in Figure 1a, where
an external transistor is used as a switch to
Figure 2. Typical Circuit
3.3k Ω
Q
3
2N2222
3.3k Ω
ROB
ROB
ROA
Q
TIB
TIB
TIA
V
4
DD
1
2
1
1
2
1
V
GND
resistor be wired in series
DD
SP302
V
V
CC
EE
TOB
TOB
RIB
TOA
TOA
RIB
RIA
RIA
V
V
1
2
1
2
SELECT A = 1
SELECT B = 0
LOOPBACK = 1
1
2
1
2
CC
EE
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