Exar Corporation 48720 Kato Road, Fremont CA, 94538 • (50)668-7000 • Fax (50)668-707 • www.exar.com SP206, 207, 208, 2_0_060

6

DESCRIPTION

RECEIVERS

The receivers convert RS-232 input signals to

inverted TTL signals. Since the input is usu-

ally from a transmission line where long cable

length and system interference can degrade

the signal, the inputs have a typical hysteresis

margin of 500mV. This ensures that the re-

ceiver is virtually immune to noisy transmission

lines. Should an input be left unconnected, an

internal 5kohm pull-down resistor to ground will

commit the output of the receiver to a HIGH

state.

SHUTDOWN MODE

The SP206 and SP2 feature a control input

(SD) which will disable the device and reduce

the power supply current to less than 0µA,

making the parts ideal for battery-powered sys-

tems. In the "shutdown" mode the receivers

and transmitters will both be tri-stated. The V+

output of the charge pump will discharge to Vcc

and the V- output will discharge to ground.

ENAblE (EN)

The SP206 and SP2 feature an enable input

which allows the receiver outputs to be either

tri-stated or enabled. This can be especially

useful when the receiver is tied directly to a

microprocessor data bus. The enable pin is

active high.

Figure . Charge Pump Waveforms

+0V

-0V

GND

GND

a) C2+

b) C2+

–5V

–5V

+5V

+

+

+

+

–

–

–

–

Figure 2, Charge pump - Phase 

TRANSMITTER/DRIVER

The drivers are inverting transmitters, which ac-

cept either TTL or CMOS inputs and output the

RS-232 output signals with an inverted sense

relative to the input logic levels. Typically, the

RS-232 output voltage swing is +/-9V with no

load, and +/-5V minimum with full load. The

transmitter outputs are protected against infi-

nite short-circuits to ground without degrada-

tion in reliability. The drivers of the SP206 and

SP2 can be tri-stated by using the SHUT-

DOWN function.

In the "power off" state, the output impedance

will remain greater than 300 Ohms, again satis-

fying the RS-232 specifications. Should the input

of the driver be left open, an internal 400kOhm

pull-up resistor to Vcc forces the input high, thus

committing 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 he EIA

standards (EIA RS-232D 2,,7, Paragraph 5).

The transition of the loaded output from high to

low also meets the monotonicity requirements

of the standard.