1.8V to 4.25V-Powered,

True RS-232 Dual Transceiver

6

_______________________________________________________________________________________

RS-232 Drivers

The two drivers are identical, and deliver EIA/TIA-232E

between 1.8V and 4.25V. The transmitters drive up to

3k

Ω in parallel with 1000pF at up to 120kbps. Connect

drivers by taking S

—

H

—

D

—

N

–

low. The transmitter outputs are

forced into a high-impedance state when S

—

H

—

D

—

N

–

is low.

RS-232 Receivers

The two receivers are identical, and accept both

EIA/TIA-232E and EIA/TIA-562 input signals. The

CMOS receiver outputs swing rail-to-rail. When EN is

high, the receivers are active regardless of the state of

S

—

H

—

D

—

N

–

. When EN is low, the receiver outputs are put

into a high-impedance state. This allows two RS-232

ports (or two ports of different types) to be wired-ORed

at the UART.

Operating Modes

S

—

H

—

D

—

N

–

and EN determine the MAX218’s mode of opera-

tion, as shown in Table 2.

Table 2. Operating Modes

Shutdown

When S

—

H

—

D

—

N

–

is low, the power supplies are disabled and

the transmitters are put into a high-impedance state.

Receiver operation is not affected by taking S

—

H

—

D

—

N

–

low.

Power consumption is dramatically reduced in shutdown

mode. Supply current is minimized when the receiver

inputs are static in any of three states: floating (ground),

__________Applications Information

Operation from Regulated/Unregulated

Dual System Power Supplies

The MAX218 is intended for use with three different

power-supply sources: it can be powered directly from

a battery, from a 3.0V or 3.3V power supply, or simulta-

neously from both. Figure 1 shows the single-supply

configuration. Figure 2 shows the circuit for operation

from both a 3V supply and a raw battery supply—an

ideal configuration where a regulated 3V supply is

being derived from two cells. In this application, the

MAX218’s logic levels remain appropriate for interface

with 3V logic, yet most of the power for the MAX218 is

drawn directly from the battery, without suffering the

efficiency losses of the DC-DC converter. This pro-

longs battery life.

least 1µF at the inductor (C5). Increase C5 to 4.7µF if

the power supply has no other bypass capacitor con-

nected to it.

MANUFACTURER

PART NUMBER

PHONE

FAX

Murata-Erie

LQH4N150K-TA

USA (404) 436-1300

Japan (075) 951-9111

USA (404) 436-3030

Japan (075) 955-6526

Sumida

CD43150

USA (708) 956-0666

Japan (03) 3607-5111

USA (708) 956-0702

Japan (03) 3607-5428

TDK

NLC453232T-150K

USA (708) 803-6100

Japan (03) 3278-5111

USA (708) 803-6296

Japan (03) 3278-5358

Central Semiconductor

CMPSH-3, Schottky

USA (516) 435-1110

USA (516) 435-1824

Motorola

MMBD6050LT1, Silicon

USA (408) 749-0510

USA (408) 991-7420

Philips

PMBD6050, Silicon

USA (401) 762-3800

USA (401) 767-4493

Motorola

1N6050, Silicon

1N5817, Schottky

USA (408) 749-0510

USA (408) 991-7420

Table 1. Suggested Component Suppliers

–

S

—

H

—

D

—

N

–

EN

RECEIVER

OUTPUT

DRIVER

OUTPUT

DC-DC

CONVERTER

L

L

High-Z

High-Z

OFF

L

H

Enabled

High-Z

OFF

H

L

High-Z

Enabled

ON

H

H

Enabled

Enabled

ON

SUPPLY

CURRENT

Minimum

Minimum

Normal

Normal

Inductors—Surface Mount

Diodes—Surface Mount

Diodes—Through-Hole