Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP385E_100_030811

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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.

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

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.

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 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.