SP385ECT-L Datasheet, PDF (7/15 Pages) Exar Corporation – Enhanced +3V or +5V RS-232 Line Driver/Receiver

FEATURES
The Exar SP385E is a +3V or +5V EIA-232/
EIA-562 line transceiver. It is a pin-for-pin
alternative for the SP310A and will operate in
the same socket with capacitors ranging from
0.1µF to 10µF, either polarized or non–polar-
ized, in +3V supplies. The SP385E offers the
same features such as 120kbps guaranteed
transmission rate, increased drive current for
longer and more flexible cable configurations,
low power dissipation and overall ruggedized
construction for commercial and industrial
environments. The SP385E also includes a
shutdown feature that tri-states the drivers and
the receivers.
Driver/Transmitter
The drivers are inverting transmitters, which
accept TTL or CMOS inputs and output the
RS-232 signals with an inverted sense relative
to the input logic levels. Typically the RS-232
output voltage swing is ±9V for 5V supply and
±4.2V for 3.3V supply. Even under worst case
loading conditions of 3kΩ and 2500pF, the
output is guaranteed to be ±5V for a 5V supply
and ±3.7V for a 3.3V supply which adheres to
EIA-232 and EIA-562 specifications, respec-
tively. The transmitter outputs are protected
against infinite short-circuits to ground without
degradation in reliability.
The SP385E includes a charge pump voltage
converter which allows it to operate from a
single +3.3V or +5V supply. These convert-
ers double the V_CC voltage input in order to
generate the EIA-232 or EIA-562 output levels.
For +5V operation, the SP385E driver out-
puts adhere to all EIA-232D and CCITT V.28
specifications. While at +3.3V operation, the
outputs adhere to EIA-562 specifications. Due
to Exar's efficient charge pump design, the
charge pump levels and the driver outputs are
less noisy than other 3V EIA-232 transceivers.
The SP385E has a single control line which
simultaneously shuts down the internal DC/DC
converter and puts all transmitter and receiver
outputs into a high impedance state.
The SP385E is available in 18-pin plastic
SOIC and 20-pin plastic SSOP packages
for operation over commercial and industrial
temperature ranges. Please consult the factory
for surface-mount packaged parts supplied on
tape-on-reel as well as parts screened to MIL-
M-38510.
The SP385E is ideal for +3.3V battery ap-
plications requiring low power operation. The
charge pump strength allows the drivers to
provide ±4.0V signals, plenty for typical EIA-
232 applications since the EIA-232 receivers
have input sensitivity levels of less than ±3V.
THEORY OF OPERATION
The SP385E device is made up of three basic
circuit blocks — 1) a driver/transmitter, 2) a
receiver and 3) a charge pump.
The instantaneous slew rate of the transmit-
ter output is internally limited to a maximum
of 30V/µs in order to meet the standards [EIA
232-D 2.1.7, Paragraph (5)]. However, the
transition region slew rate of these enhanced
products is typically 10V/µs. The smooth tran-
sition of the loaded output from VOL to VOH
clearly meets the monotonicity requirements of
the standard [EIA 232-D 2.1.7, Paragraphs (1)
& (2)].
Receivers
The receivers convert RS-232 input signals to
inverted TTL signals. Since the input is usually
from a transmission line, where long cable
lengths and system interference can degrade
the signal, the inputs have a typical hyster-
esis margin of 500mV. This ensures that the
receiver is virtually immune to noisy transmis-
sion lines.
The input thresholds are 0.8V minimum and
2.4V maximum, again well within the ±3V RS-
232 requirements. The receiver inputs are also
protected against voltages up to ±15V. Should
an input be left unconnected, a 5kΩ pull-down
resistor to ground will commit the output of the
receiver to a high state.
In actual system applications, it is quite pos-
sible for signals to be applied to the receiver
inputs before power is applied to the receiver
circuitry. This occurs for example when a PC
user attempts to print only to realize the printer
wasn’t turned on. In this case an RS-232 signal
from the PC will appear on the receiver input
at the printer. When the printer power is turned
on, the receiver will operate normally. All of
these enhanced devices are fully protected.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP385E_100_030811