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Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP3238E_100_020111

Receivers

The receivers convert +5.0V EIA/TIA-232

levels to TTL or CMOS logic output levels. Re-

circuitry is enabled or when in shutdown.

The SP3238E includes an additional non-in-

verting receiver with an output R

is an extra output that remains active and

monitors activity while the other receiver

outputs are forced into high impedance.

This allows a Ring Indicator (RI) signal from a

peripheral to be monitored without forward

biasing the TTL/CMOS inputs of the other

devices connected to the receiver outputs.

Since receiver input is usually from a transmis-

sion line where long cable lengths and system

interference can degrade the signal, the inputs

have a typical hysteresis margin of 300mV. This

ensures that the receiver is virtually immune to

noisy transmission lines. Should an input be left

unconnected, an internal 5kΩ pulldown resistor

to ground will commit the output of the receiver

to a HIGH state.

Charge Pump

The charge pump is an 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

Figure 7. Loopback Test Circuit for RS-232 Driver

Data Transmission Rates

capacitors, but uses a four–phase voltage

shifting technique to attain symmetrical 5.5V

power supplies. The internal power supply

consists of a regulated dual charge pump that

provides output voltages 5.5V regardless of

the input voltage (V

range. This is important to maintain compli-

ant RS-232 levels regardless of power supply

fluctuations.

Figure 9. Loopback Test results at 250Kbps

(All Drivers Fully Loaded)

Figure 8. Loopback Test results at 120kbps

(All Drivers Fully Loaded)

SP3238E

TxIN

TxOUT

C1+

C1-

C2+

C2-

V+

V-

0.1µF

+

C2

C5

C1

+

+

C3

C4

+

+

LOGIC

INPUTS

5kΩ

RxIN

RxOUT

LOGIC

OUTPUTS

SHUTDOWN

GND

ONLINE

1000pF

0.1µF

0.1µF

0.1µF

0.1µF