SP238A Datasheet, PDF (4/12 Pages) Exar Corporation – +5V Powered Multi-Channel RS-232 Drivers/Receivers

Features…
The SP238A multi-channel RS-232 line trans-
ceiver provides 4 drivers and 4 receivers to fit
most communication needs, especially those
applications where ±12V is not available. The
device features low power CMOS operation,
which is particularly beneficial in battery-powered
systems.
The device includes two charge pump voltage
converters which allows it to operate from a
single +5V supply. These converters convert the
+5V input power to the minimum loaded voltage
of ±5.0V for compliant RS-232 output levels. All
drivers and receivers meet all EIA RS-232D and
CCITT V.28 specifications.
The device is available for use over the com-
mercial and industrial temperature ranges. It is
packaged in a WSOIC package.
THEORY OF OPERATION
The SP238A is made up of three basic circuit
blocks - 1) transmitter, 2) receiver and 3) charge
pump.
Driver/Transmitter
The drivers are inverting transmitters, which ac-
cept TTL or CMOS inputs and output the RS-232
signals with an inverted sense relative to the input
logic levels. The transmitter outputs are protected
against infinite short‑circuits to ground without
degradation in reliability. Should the input of the
driver be left open, an internal 400kΩ pull-up
resistor to Vcc forces the input HIGH, thus com-
mitting the output to a low state.
The slew rate of the transmitter output is internally
limited to a maximum of 30V/µs in order to meet
the standards [EIA RS-232-D 2.1.7, Paragraph
(5)]. The transition of the loaded output from V_OL to
V_OH clearly meets the monotonicity requirements
of the standard [EIA RS-232-D 2.1.7, Paragraphs
(1) & (2)].
DESCRIPTION
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
protected against voltages up to ±30V. 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 re-
ceiver input at the printer. When the printer power
is turned on, the receiver will operate normally.
All series devices are fully protected.
Charge Pump
The charge pump is a patented design and
uses a unique approach compared to older less
efficient designs. The charge pump requires4
external capacitors and uses a four phase volt-
age shifting technique. The internal power sup-
ply consists of a regulated dual charge pump
that provides output voltages of +/-5.5V. This
is important to maintain compliant RS-232 lev-
els regardless of power supply fluctuations.
The charge pump operates in a discontinuous
mode using an internal oscillator. If the voltages
are less than a magnitude of 5.5V, the charge
pump is enabled. If the output voltage exceed
a magnitude of 5.5V then the charge pump is
disabled. The internal oscillator controls the four
phases of the voltage shifting. A description of
each phase follows:
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • [phone] • www.exar.com

SP238A_100_071410