DESCRiPTion
DETAiLED DESCRiPTion
The SP232A transceiver is a two driver and two
receiver device that meets the EIA/TIA- 232 and
V.28 serial communication protocol. The device
is pin-to-pin compatible with popular industry
standard pinouts. The SP232A offers 20kbps
data rate, 0V/
μ
s slew rate and a regulated
charge pump that operates from a single 5V
supply. The proprietary on-board charge pump
generates a regulated output of +/-6.5V for RS-
232 compliant voltage levels.
Theory of operation
The SP232A is made up of three basic circuit
blocks: . Driver, 2. Receiver, and 3. charge
pump.
Drivers
The drivers are inverting level transmitters that
convert TTL or CMOS logic levels to EIA/TIA-
232 levels with an inverted sense relative to the
input logic levels. Typically, the RS-232 output
voltage swing is +/-6.5V with no load and +/-
5.0V minimum when fully loaded. The driver out-
puts are protected against infinite short-circuits
to ground without degradation in reliability.
The drivers can guarantee output data rates of
20Kbps under worst case loading of 3k ohms
and 2500pF.
The driver output Slew Rate is internally limited
to 30V/μs in order to meet the EIA standards
(EIA-232F). Additionally, the driver output LOW
to HIGH transition meet the monotonicity output
requirements of the standard.
Receivers
The receivers convert EIA/TIA-232 signal levels
to TTL or CMOS logic output levels. Since the
input is usually from a transmission line, where
long cable length and system interference can
degrade the signal, the receiver inputs have a
typical hysteresis margin of 500mV. This en-
sures that the receiver is virtually immune to
noisy transmission lines. Should a input be left
unconnected, an internal 5Kohm pull-down re-
sistor to ground will commit the output of the re-
ceiver to a logic HIGH state. The input voltage
range for the SP232A Receiver is +/-5V.
Charge pump
The charge pump is a patented design and
uses a unique approach compared to older less
efficient designs. The charge pump requires 4
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 +/-6.5V. This
is important to maintain compliant RS-232 lev-
els regardless of power supply fluctuations.
The charge pump operates in a discontinous
mode using an internal oscillator. If the voltages
are less than a magnitude of 6.5V, the charge
pump is enabled. If the output voltage exceed
a magnitude of 6.5V then the charge pump is
disabled. The internal oscillator controls the four
phases of the voltage shifting. A description of
each phase follows:
Phase 1
Vss charge store and double: The positive ter-
minals of capacitors C and C2 are charged
from Vcc with their negative terminals initially
connected to ground. C+ is then connected
to ground and the stored charge from C- is
superimposed onto C2-. Since C2+ is still con-
nected to Vcc the voltage potential across C2 is
now 2 x Vcc.
Phase 2
Vss transfer and invert: Phase two connects
the negative terminal of C2 to the Vss storage
capacitor and the positive terminal of C2 to
ground. This transfers the doubled and inverted
(V-) voltage onto C4. Meanwhile, capacitor C
is charged from Vcc in order to prepare it for its
next phase.
Phase 3
Vdd charge store and double: Phase three is
identical to the first phase. The positive termi-
nals of C and C2 are charged from Vcc with
their negative terminals initially connected to
ground. C+ is then connected to ground and
the stored charge from C- is superimposed
onto C2-. Since C2+ is still connected to Vcc
the voltage potential across capacitor C2 is now
2 x Vcc.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (50)668-7000 • Fax (50)668-707 • www.exar.com
SP232A_00_05308
4
EXAR [ EXAR CORPORATION ]
