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Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com

SP332_00_0260

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...

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

accepteitherTTLorCMOSinputsandoutput

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.

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 +/-5V levels. The RS-232

receivers can operate up to 120kbps.

Charge–Pump

Thechargepumpisa

Exar–patenteddesign

(U.S.5,306,954)andusesauniqueapproach

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

waveformfoundonthepositivesideofcapci-

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.

Phase 1

— V

theclockcycle,thepositivesideofcapacitors

C

+

is

then switched to ground and charge on C



–

is

transferred to C

2

–

. Since C

2

+

is connected to

+5V, the voltage potential across capacitor

C

Phase 2

— V

nects the negative terminal of C

storage capacitor and the positive terminal

of C

–l0V to C

of capacitor C

negative side is connected to ground.

THEORY OF OPERATION