SP332 Datasheet, PDF (5/10 Pages) Sipex Corporation – RS-232/RS-485 Multi-Mode Serial Transceiver

THEORY OF OPERATION...
The SP332 is a single chip device that can be
configured via software for either RS-232, RS-
485 or both interface modes at any time. The
SP332 is made up of three basic circuit ele-
ments, single-ended drivers and receivers, dif-
ferential drivers and receivers and charge pump.
DIFFERENTIAL DRIVER/RECEIVER...
RS-485, RS-422 Drivers...
The differential drivers and receivers comply
with the RS-485 and RS-422 standards. The
driver circuits are able to drive a minimum of
1.5V when terminated with a 54Ω resistor
across the two outputs. The typical propagation
delay from the driver input to output is 60ns. The
driver outputs are current limited to less than
250mA, and can tolerate short circuits to ground,
or to any voltage within a +10V to -7V range
with no damage.
RS-485, RS-422 Receivers...
The differential receivers of the SP332 comply
with the RS-485, RS-422 and V.11 standards.
The input to the receiver is equipped with a
common mode range of +12V to -7V. The input
threshold over this range is a minimum of
±200mV. The differential receivers can receive
data up to 10Mbps. The typical propagation
delay from the receiver input to output is 90ns.
SINGLE ENDED DRIVER/RECEIVER...
RS-232 (V.28) Drivers...
The single-ended drivers and receivers comply
the with the RS-232E and V.28 standards. The
drivers are inverting transmitters which accept
either TTL or CMOS inputs and output the RS-
232 signals with an inverted sense relative to the
input logic levels. Typically, the RS-232 driver
output voltage swing is ±9V with no load and is
guaranteed to be greater than ±5V under full
load. The drivers rely on the V+ and V- voltages
generated by the on-chip charge pump to main-
tain proper RS-232 output levels. With worst
case load conditions of 3kΩ and 2500pF, the
four RS-232 drivers can still maintain ±5V
output levels. The drivers can operate up to
120kbps; the propagation delay from input to
output is typically 2µs.
RS-232 (V.28) Receivers...
The RS-232 receivers convert RS-232 input
signals to inverted TTL signals. Each of the four
receivers features 500mV of hysteresis margin
to minimize the affects of noisy tranmission
lines. The inputs also have a 5kΩ resistor to
ground, in an open circuit situation the input of
the receiver will be forced low, committing the
output to a logic high state. The input resistance
will maintain 3kΩ-7kΩ over a ±15V range. The
maximum operating voltage range for the re-
ceiver is ±30V, under these conditions the input
current to the receiver must be limited to less
than 100mA. Due to the on-chip ESD protection
circuitry, the receiver inputs will be clamped to
±15V levels. The RS-232 receivers can operate
up to 120kbps.
CHARGE PUMP...
The charge pump is a Sipex-patented design
(5,306,954) and uses a unique approach com-
pared to older less-efficient designs. The charge
pump still requires four external capacitors, but
uses a four-phase voltage shifting technique to
attain symmetrical 10V power supplies. Figure
8a shows the waveform found on the positive
side of capacitor C2, and Figure 8b shows the
negative side of capacitor C2. There is a free-
running oscillator that controls the four phases
of the voltage shifting. A description of each
phase follows.
Phase 1
-Vss charge storage- During this phase of the
clock cycle, the positive side of capacitors C1
and C2 are initially charged to +5V. C1+ is then
switched to ground and charge in C1- is trans-
ferred to C2-. Since C2+ is connected to +5V,
the voltage potential across capacitor C2 is
now 10V.
Phase 2
-Vss transfer- Phase two of the clock connects
the negative terminal of C2 to the Vss storage
capacitor and the positive terminal of C2 to
ground, and transfers the generated -10V to C3.
Simultaneously, the positive side of capacitor
C1 is switched to +5V and the negative side is
connected to ground.
TGoddard/SP332/9617R0
SP332 RS-232/RS-485 Multi-Mode Serial Transceiver
5
© Copyright 2000 Sipex Corporation