MAX3218 shuts down after sensing invalid RS-232 levels

for greater than 30µs, ensuring that the AutoShutdown

mode is not enabled for slow-moving signals (>1V/µs).

Another system with AutoShutdown may need a period

of time to wake up. Figure 5 shows a circuit that forces

the transmitters on for 100ms after start-up, allowing

enough time for the other system to realize that the

MAX3218 system is awake. If the other system outputs

valid RS-232 signals within that time, the RS-232 ports on

both systems remain enabled.

__________Applications Information

Operation from Regulated/Unregulated

Dual System Power Supplies

The MAX3218 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 6 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

MAX3218’s logic levels remain appropriate for interface

with 3V logic, yet most of the power for the MAX3218 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.

Low-Power Operation

The following suggestions will help you get maximum

life out of your batteries.

Transmit at the highest practical data rate. Although

this raises the supply current while transmission is in

progress, the transmission will be over sooner. If the

MAX3218 is shut down (using FORCEOFF) as soon as

each transmission 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 6. This circuit draws most of the MAX3218’s

power straight from the battery, but still provides logic-

level compatibility with the 3V logic.

1µA Supply Current, 1.8V to 4.25V-Powered

RS-232 Transceiver with AutoShutdown™

8

_______________________________________________________________________________________

INVALID IS AN INTERNALLY GENERATED SIGNAL

THAT IS USED BY THE AUTOSHUTDOWN LOGIC

AND APPEARS AS AN OUTPUT OF THE DEVICE.

POWER DOWN IS ONLY AN INTERNAL SIGNAL.

IT CONTROLS THE OPERATIONAL STATUS OF

THE TRANSMITTERS AND THE POWER SUPPLIES.

FORCEOFF

0

0

0

0

1

1

1

1

FORCEON

0

0

1

1

0

0

1

1

INVALID

0

1

0

1

0

1

0

1

POWER DOWN

0

0

0

0

0

1

1

1

POWER DOWN

FORCEOFF

FORCEON

INVALID

+

-

+

-

Rx

R_IN

FORCEON

FORCEOFF

R_OUT

30µs

DELAY

•

•

•

•

•

•

+0.3V to +2.8V

-0.3V to -2.8V

Figure 3. AutoShutdown Logic