7

Date: 02/24/05

SP3282EB Intelligent +2.35V to +5.5V RS-232 Transceivers

© Copyright 2005 Sipex Corporation

Since receiver input is usually from a transmission

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 5k

Ωpulldownresistorto

ground will commit the output of the receiver to a

HIGH state.

Charge Pump

The charge pump is a Sipex–patented design

(U.S. #5,306,954) and uses a unique approach

compared to older less–efficient designs. The

charge pump uses a four–phase voltage

shifting technique to attain symmetrical

±5.5V

power supplies. The internal power supply con-

sists of a regulated dual charge pump that provides

output voltages

±5.5V regardless of the input

voltage (V

This is important to maintain compliant RS-232

levels

regardless

of

power

supply

fluctuations. The charge pump will provide out-

put voltage levels of

±4.0V when the input voltage

(V

The charge pump operates in a discontinuous

mode using an internal oscillator. If the output

voltages are less than a magnitude of 5.5V ( V

CC

> 3.3V ) and 4.0V (V

is enabled. If the output voltages exceed a

magnitude of 5.5V (V

3.1V), the charge pump is disabled. This oscilla-

tor controls the four phases of the voltage shifting

(Figure 10).

A description of each phase follows.

Phase 1 (Figure 11)

Figure 6. Loopback Test Circuit for RS-232 Driver Data

Transmission Rates

Figure 10. Charge Pump Waveforms

2

1

T

T

[]

T

2

+6V

-6V

0V

0V

Figure 7. Loopback Test Circuit Result at 120kbps

(All Drivers Fully Loaded)

Figure 8. Loopback Test Circuit result at 250kbps

(All Drivers Fully Loaded)

SP3282EB

TxIN

TxOUT

C1+

C1-

C2+

C2-

V+

V-

0.1µF

0.1µF

0.1µF

+

C2

C5

C1

+

+

C3

C4

+

+

0.1µF

0.1µF

LOGIC

INPUTS

5kΩ

RxIN

RxOUT

LOGIC

OUTPUTS

SHUTDOWN

GND

ONLINE

1000pF