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

SP3238E_100_020111

7

DESCRIPTION

The SP3238E device meets the EIA/TIA-232

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

and can be implemented in battery-powered,

portable, or hand-held applications such as

notebook or palmtop computers. The SP3238E

devices feature Exar's proprietary and patented

(U.S.-- 5,306,954) on-board charge pump cir-

cuitrythatgenerates±5.5VRS-232voltagelevels

from a single +3.0V to +5.5V power supply. The

SP3238E devices can guarantee a data rate of

250kbps fully loaded.

The SP3238E is a 5-driver/3-receiver device,

ideal for portable or hand-held applications.

The SP3238E includes one complementary

always-active receiver that can monitor an

external device (such as a modem) in shutdown.

This aids in protecting the UART or serial

controller IC by preventing forward biasing

of the protection diodes where V

disconnected.

The SP3238E device is an ideal choice for power

sensitive designs.The SP3238E device features

power supply drain to a 1µA supply current.

In many portable or hand-held applications, an

RS-232cablecanbedisconnectedoraconnected

peripheral can be turned off. Under these condi-

tions,theinternalchargepumpandthedriverswill

be shut down. Otherwise, the system automati-

cally comes online. This feature allows design

engineers to address power saving concerns

without major design changes.

THEORY OF OPERATION

The SP3238E device is made up of four basic

circuit blocks:

1. Drivers

2. Receivers

3. The Exar proprietary charge pump, and

4.

Drivers

The drivers are inverting level transmitters that

convert 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-232-F and all previous

RS-232 versions.

The drivers can guarantee a data rate of 250kbps

fully loaded with 3kΩ in parallel with 1000pF,

ensuring compatibility with PC-to-PC communi-

cation software. All unused drivers inputs should

be connected to GND or V

The slew rate of the driver output is internally

limited to a maximum of 30V/µs in order to meet

theEIAstandards(EIARS-232D2.1.7,Paragraph

5). The transition of the loaded output from HIGH

toLOWalsomeetsthemonotonicityrequirements

of the standard.

Figure 7 shows a loopback test circuit used to

test the RS-232 drivers. Figure 8 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 9 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 ca-

pacitor. A solid RS-232 data transmission rate

of 120kbps provides compatibility with many

designs in personal computer peripherals and

LAN applications.

Figure 6. Interface Circuitry Controlled by

Microprocessor Supervisory Circuit

SP3238E

28

25

3

1

27

4

26

GND

C1+

C1-

C2+

C2-

V+

V-

0.1µF

+

C2

C5

C1

+

+

C3

C4

+

+

13

14

15

2

ONLINE

SHUTDOWN

STATUS

µP

Supervisor

IC

VIN

RESET

5kΩ

5kΩ

5kΩ

24

23

22

16

21

20

18

5

6

7

8

9

11

RS-232

OUTPUTS

RS-232

INPUTS

UART

or

Serial µC

TxD

RTS

DTR

RxD

CTS

DSR

DCD

RI

19

17

10

12

0.1µF

0.1µF

0.1µF

0.1µF