5

SP385ADS/05

SP385A Line Driver/Receiver

© Copyright 2000 Sipex Corporation

FEATURES…

The Sipex SP385A is a +3V to +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 0.1

µF to 10µF, either polar-

izedornon–polarized,in+3Vsupplies. TheSP385A

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 SP385A also includes a shutdown feature that

tri-states the drivers and the receivers.

The SP385A includes a charge pump voltage con-

verter which allows it to operate from a single +3.3V

or +5V supply. These converters double the V

CC

voltageinputinordertogeneratetheEIA-232orEIA-

562 output levels. For +5V operation, the SP385A

driver outputs adhere to all EIA-232D and CCITT

V.28 specifications. While at +3.3V operation, the

outputs adhere to EIA-562 specifications. Due to

Sipex's efficient charge pump design, the charge

pump levels and the driver outputs are less noisy

than other 3V EIA-232 transceivers.

The SP385A has a single control line which simul-

taneously shuts down the internal DC/DC con-

verter and puts all transmitter and receiver outputs

into a high impedance state.

The SP385A 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 SP385A is ideal for +3.3V battery applica-

tions 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 SP385A 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 3k

Ω and 2500pF, the output is guar-

anteed to be

±5V for a 5V supply and ±3.7V for

a 3.3V supply which adheres to EIA-232 and EIA-

562 specifications, respecitively. The transmitter

outputs are protected against infinite short-circuits

to ground without degradation in reliability.

The instantaneous slew rate of the transmitter

output is internally limited to a maximum of 30V/

µsinordertomeetthestandards[EIA232-D2.1.7,

Paragraph (5)]. However, the transition region

slew rate of these enhanced products is typically

10V/

µs.Thesmoothtransitionoftheloadedoutput

from V

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 hysteresis margin of 500mV.

This ensures that the receiver is virtually immune

to noisy transmission 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 pro-

tected against voltages up to

±15V. Should an

input be left unconnected, a 5k

Ω pull-down resis-

tor to ground will commit the output of the receiver

to a high state.

In actual system applications, it is quite possible

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.