1.8V to 4.25V-Powered,

True RS-232 Dual Transceiver

_______________________________________________________________________________________

7

Low-Power Operation

The following suggestions will help you get maximum

life out of your batteries.

Shut the MAX218 down when it is not being used for

transmission. The receivers can remain active when

the MAX218 is shut down, to alert your system to exter-

nal activity.

Transmit at the highest practical data rate. Although

this raises the supply current while transmission is in

progress, the transmission will be over sooner. As long

as the MAX218 is shut down as soon as each transmis-

sion ends, this practice will save energy.

Operate your whole system from the raw battery volt-

age rather than suffer the losses of a regulator or DC-

DC converter. If this is not possible, but your system is

powered from two cells and employs a 3V DC-DC con-

verter to generate the main logic supply, use the circuit

of Figure 2. This circuit draws most of the MAX218’s

power straight from the battery, but still provides logic-

level compatibility with the 3V logic.

Keep communications cables short to minimize capaci-

tive loading. Lowering the capacitive loading on the

transmitter outputs reduces the MAX218’s power con-

sumption. Using short, low-capacitance cable also

helps transmission at the highest data rates.

Keep the S

—

H

—

D

—

N

–

pin low while power is being applied to

the MAX218, and take S

—

H

—

D

—

N

–

risen above about 1.5V. This avoids active operation at

very low voltages, where currents of up to 150mA can be

drawn. This is especially important with systems pow-

ered from rechargeable cells; if S

—

H

—

D

—

N

–

is high while the

cells are being trickle charged from a deep discharge,

the MAX218 could draw a significant amount of the

charging current until the battery voltage rises above

1.5V.

Pin Configuration Change

The

Pin Configuration shows pin 2 as N.C. (no con-

nect). Early samples had a bypass capacitor for the

internal reference connected to pin 2, which was

labeled REF. This bypass capacitor proved to be

unnecessary and the connection has been omitted. Pin

2 may now be connected to ground, left open, or

bypassed to GND with a capacitor.

EIA/TIA-232E and

_____________EIA/TIA-562 Standards

RS-232 circuits consume much of their power because

the EIA/TIA-232E standard demands that the transmit-

ters deliver at least 5V to receivers with impedances

that can be as low as 3k

Ω. For applications where

power consumption is critical, the EIA/TIA-562 standard

provides an alternative.

EIA/TIA-562 transmitter output voltage levels need only

reach ±3.7V, and because they have to drive the same

3k

Ω receiver loads, the total power consumption is con-

siderably reduced. Since the EIA/TIA-232E and

EIA/TIA-562 receiver input voltage thresholds are the

same, interoperability between EIA/TIA-232E and

EIA/TIA-562 devices is guaranteed. Maxim’s MAX560

and MAX561 are EIA/TIA-562 transceivers that operate

on a single supply from 3.0V to 3.6V, and the MAX562

transceiver operates from 2.7V to 5.25V while produc-

ing EIA/TIA-562 levels.

R1

R2

T1

T2

T1OUT

T2OUT

T1IN

T2IN

R1OUT

R2OUT

R1IN

R2IN

GND

EN

SHDN

LX

C1+

C1-

V-

6

3

7

8

9

10

11

12

13

14

16

18

15

4

5, 17, 20

MAX218

ENABLE

ON/OFF

1

V+

19

0.1

µF

C4

1

µF

C5

1

µF

C2

1

µF

C3

0.47

µF

C1

15

µH

D1

1N6050

3V

DC-DC

CONVERTER

MAX878

OR

MAX756

OR

MAX856

Figure 2. Operating from Unregulated and Regulated Supplies