1µA Supply-Current, True +3V to +5.5V
RS-232 Transceivers with AutoShutdown™
FORCEOFF
FORCEON
INVALID
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.
POWER DOWN
Table 2.
INVALID
Truth Table
RS-232 SIGNAL PRESENT
AT ANY RECEIVER INPUT
Yes
No
INVALID
OUTPUT
High
Low
MAX3221E/MAX3223E/MAX3243E
Figure 3c. MAX32_ _E AutoShutdown Logic
TRANSMITTERS ENABLED, INVALID HIGH
0.1µF
1M
RECEIVER INPUT LEVELS
POWER-
MANAGEMENT
UNIT
MASTER SHDN LINE
+2.7V
INDETERMINATE
+0.3V
0
-0.3V
INDETERMINATE
-2.7V
TRANSMITTERS ENABLED, INVALID HIGH
AUTOSHUTDOWN, TRANSMITTERS DISABLED,
1µA SUPPLY CURRENT, INVALID LOW
FORCEOFF FORCEON
MAX3221E
MAX3223E
MAX3243E
Figure 4. AutoShutdown with Initial Turn-On to Wake Up a
Mouse or Another System
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+
is pulled to V
CC
, V- is pulled to ground, and the trans-
mitter outputs are high impedance. The time required to
exit shutdown is typically 100µs (Figure 5b).
a)
RECEIVER
INPUT
VOLTAGE
(V)
INVALID
REGION
V
CC
INVALID
OUTPUT
(V)
0
t
INVL
t
INVH
t
WU
V+
V
CC
0
V-
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.
b)
Figure 5. AutoShutdown Trip Levels
_______________________________________________________________________________________
9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
