1µA Supply-Current, True +3V to +5.5V
RS-232 Transceivers with AutoShutdown
MAX3221/MAX3223/MAX3243
Table 1a. MAX3221/MAX3223
FORCEOFF
and
Enable Control Truth Table
FORCEOFF
0
0
1
1
EN
0
1
0
1
T_OUT
High-Z
High-Z
Active*
Active*
R_OUT
Active
High-Z
Active
High-Z
-0.3V
+0.3V
TO MAX32_ _
POWER SUPPLY
AND TRANSMITTERS
INVALID
R_IN
30µs
COUNTER
R
* TRANSMITTERS ARE DISABLED, REDUCING SUPPLY CURRENT TO 1µA IF
ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR
AT LEAST
30µs.
Figure 3a. MAX32_ _ Entering 1µA Supply Mode via
AutoShutdown
Table 1b. MAX3243
FORCEOFF
Control
Truth Table
FORCEOFF
0
1
T_OUT
High-Z
Active*
R_OUT
High-Z
Active*
R2OUTB
Active
Active
+2.7V
TO MAX32_ _
POWER SUPPLY
INVALID
R_IN
-2.7V
30µs
COUNTER
R
*Note: If the part is in AutoShutdown mode (FORCEOFF = V
CC
,
FORCEON = GND), it is shut down if no valid RS-232
levels are present on all receiver inputs.
* TRANSMITTERS ARE ENABLED IF:
ANY RECEIVER INPUT IS GREATER THAN +2.7V OR LESS THAN -2.7V.
ANY RECEIVER INPUT HAS BEEN BETWEEN +0.3V AND -0.3V FOR
LESS THAN
30µs.
AutoShutdown
A 1µA supply current is achieved with Maxim’s new
AutoShutdown feature, which operates when
FORCEON is low and
FORCEOFF
is high. When the
MAX3221/MAX3223/MAX3243 sense no valid signal
levels on all receiver inputs for 30µs, the on-board
power supply and drivers are shut off, reducing supply
current to 1µA. This occurs if the RS-232 cable is dis-
connected or the connected peripheral transmitters are
turned off. The system turns on again when a valid level
is applied to any RS-232 receiver input. As a result, the
system saves power without changes to the existing
BIOS or operating system. When using AutoShutdown,
the
INVALID
output is high when the device is on and
low when the device is shut down. Because INVALID
indicates the receiver inputs’ condition, it can be used
in any mode (Figure 3).
Table 2 and Figure 3c summarize the MAX3221/
MAX3223/MAX3243 operating modes. FORCEON and
FORCEOFF
override the automatic circuitry and force
the transceiver into its normal operating state or into its
low-power standby state. When neither control is assert-
ed, 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.
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
Figure 3b. MAX32_ _ with Transmitters Enabled Using
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
Figure 3c. AutoShutdown Logic
POWER
MANAGEMENT
UNIT
MASTER SHDN LINE
0.1µF
1MΩ
FORCEOFF FORCEON
MAX3221
MAX3223
MAX3243
Figure 4. AutoShutdown with Initial Turn-On to Wake Up a
Mouse or Another System
_______________________________________________________________________________________
7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
