Rev. 6/19/03

SP3281EB Intelligent +2.35V to +5.5V RS-232 Transceivers

© Copyright 2003 Sipex Corporation

6

DESCRIPTION

The SP3281EB device meets the EIA/TIA-232

and ITU-T V.28/V.24 communication protocols

and can be implemented in battery-powered, por-

table, or hand-held applications such as notebook

or palmtop computers. The SP3281EB device

features Sipex's proprietary and patented (U.S.

#5,306,954) on-board charge pump circuitry that

generates

±5.5V RS-232 voltage levels from a

single +3.3V to +5.5V power supply. The

SP3281EB will adhere to EIA/TIA-562 voltage

levels with V

The SP3281EB device is an ideal choice for power

sensitive designs. The SP3281EB device features

power supply drain to a 1

µA supply current. In

many portable or hand-held applications, an RS-

232 cable can be disconnected or a connected

peripheral can be turned off. Under these condi-

tions, the internal charge pump and the drivers will

be shut down. Otherwise, the system automati-

cally comes online. This feature allows design

engineers to address power saving concerns with-

out major design changes.

THEORY OF OPERATION

The SP3281EB device is made up of four basic

circuit blocks:

1.

Drivers,

2.

Receivers,

3.

The Sipex proprietary charge pump, and

Drivers

The drivers are inverting level transmitters that ,

when V

TTL or CMOS logic levels to 5.0V EIA/TIA-232

levels with an inverted sense relative to the input

logic levels . Typically, the RS-232 output voltage

swing is +5.4V with no load and +5V minimum

fully loaded. The driver outputs are protected

against infinite short-circuits to ground without

degradation in reliability. These drivers comply

with the EIA-TIA-232F and all previous RS-232

versions. The driver outputs will adhere to EIA/

TIA-562 when V

The SP3281EB drivers can guarantee a data rate of

250 kbps fully loaded with 3k

Ω in parallel with

1000pF, ensuring compatibility with PC-to-PC

communication software. All unused driver inputs

must be connected to V

Figure 6 shows a loopback test circuit used to test

the SP3281EB RS-232 drivers. Figure 7 shows the

test results of the loopback circuit with all five

drivers active at 120kbps with typical RS-232

loads in parallel with 1000pF capacitors. Figure 8

shows the test results where one driver was active

at 250kbps and all five drivers loaded with an RS-

232 receiver in parallel with a 1000pF capacitor. A

solid RS-232 data transmission rate of 120kbps

provides compatibility with many designs in per-

sonal computer peripherals and LAN applications.

Receivers

The receivers convert

±5.0V EIA/TIA-232

levels to TTL or CMOS logic output levels.

Receivers are not active when in shutdown. If

there is no activity present at the receivers for a

period longer than 100

µs during AUTO ON-

the device goes into a standby mode where the

circuit draws 1

µA. The truth table logic of the

driver and receiver outputs can be found in

Table 3.

Figure 5. Interface Circuitry Being Controlled by

Microprocessor Supervisory Circuit

SP3281EB

28

25

3

1

27

4

26

GND

C1+

C1-

C2+

C2-

V+

V-

0.1µF

0.1µF

0.1µF

+

C2

C5

C1

+

+

C3

C4

+

+

0.1µF

0.1µF

13

14

15

2

ONLINE

SHUTDOWN

STATUS

µP

Supervisor

IC

VIN

RESET

5kΩ

5kΩ

5kΩ

24

23

22

21

20

18

5

6

7

8

9

11

RS-232

OUTPUTS

RS-232

INPUTS

T

R

R

T

T

T

T

T

R

R

R

R

UART

or

Serial µC

TxD

RTS

DTR

RxD

CTS

DSR

DCD

RI

19

17

10

12

T

T

T

T

16