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9

1µA Supply-Current, True +3V to +5.5V

RS-232 Transceivers with AutoShutdown™

Table 2 and Figure 3c summarize the MAX3221E/

MAX3223E/MAX3243E operating modes. FORCEON

and FORCEOFF override AutoShutdown. When neither

control is asserted, the IC selects between these states

automatically, based on receiver input levels. Figures

3a, 3b, and 5a depict valid and invalid RS-232 receiver

levels. Figure 5 shows the input levels and timing dia-

gram for AutoShutdown operation.

A mouse or other system with AutoShutdown may need

time to wake up. Figure 4 shows a circuit that forces the

transmitters on for 100ms, allowing enough time for the

other system to realize that the MAX3221E/MAX3223E/

MAX3243E are awake. If the other system transmits

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

on both systems remain enabled.

When shut down, the device’s charge pumps are off, V+

mitter outputs are high impedance. The time required to

exit shutdown is typically 100µs (Figure 5b).

Software-Controlled Shutdown

If direct software control is desired, INVALID can be

used to indicate DTR or Ring Indicator signal. Connect

FORCEOFF and FORCEON together to disable Auto-

Shutdown so the line acts like a SHDN input.

V-

0

V+

0

INVALID

REGION

RECEIVER

INPUT

VOLTAGE

(V)

INVALID

OUTPUT

(V)

TRANSMITTERS ENABLED, INVALID HIGH

AUTOSHUTDOWN, TRANSMITTERS DISABLED,

1

µA SUPPLY CURRENT, INVALID LOW

TRANSMITTERS ENABLED, INVALID HIGH

-2.7V

-0.3V

+2.7V

+0.3V

0

INDETERMINATE

INDETERMINATE

b)

a)

Figure 5. AutoShutdown Trip Levels

FORCEON

MASTER SHDN LINE

0.1µF

1M

FORCEOFF

MAX3221E

MAX3223E

MAX3243E

POWER-

MANAGEMENT

UNIT

Figure 4. AutoShutdown with Initial Turn-On to Wake Up a

Mouse or Another System

FORCEOFF

POWER DOWN

INVALID

FORCEON

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.

Figure 3c. MAX32_ _E AutoShutdown Logic

Table 2. INVALID Truth Table

Low

No

High

Yes

INVALID OUTPUT

RS-232 SIGNAL PRESENT

AT ANY RECEIVER INPUT