SP332 Datasheet, PDF (6/11 Pages) Sipex Corporation – RS-232/RS-485 Multi-Mode Serial Transceiver

The SP332 is 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 elements, single-ended drivers and
receivers, differential 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 propa-
gation delay from driver input to output is
60ns. The driver outputs are current limited
to less than 250mA, and can tolerate shorts
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 com-
ply with the RS-485 and RS-422 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...
Theory of Operation
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 transmission 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 to 7kΩ over
a +/-15V range. The maximum operating
voltage range for the receiver is +/-30V,
under these conditions the input current to
the receiver must be limited to less than
100mA. Due to the on-chip ESD protec-
tion 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 Exar–patented design
(U.S. 5,306,954) and uses a unique approach
compared 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 7(a) shows the
waveform found on the positive side of capci-
tor C2, and Figure 7(b) shows the negative
side of capcitor C2. There is a free–running
oscillator that controls the four phases of
the voltage shifting. A description of each
phase follows.
RS-232 (V.28) Drivers...
The single-ended drivers and receivers
comply with the RS-232 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 driv-
ers rely on the V+ and V- voltages generated
by the on-chip charge pump to maintain
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.
Phase 1
— V_SS charge storage —During this phase of
the clock cycle, the positive side of capacitors
C₁ and C₂ are initially charged to +5V. C_l+ is
then switched
transferred to
to ground and
C_2–. Since C₂₊
charge on C_1– is
is connected to
+5V, the voltage potential across capacitor
C₂ is now 10V.
Phase 2
— V_SS transfer — Phase two of the clock con-
nects the negative terminal of C₂ to the V_SS
storage capacitor and the positive terminal
of C₂ to ground, and transfers the generated
–l0V to C₃. Simultaneously, the positive side
of capacitor C ₁ is switched to +5V and the
negative side is connected to ground.
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com
SP332_100_012610