Datasheet

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 disconnected or the con-
nected 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 with-
out 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 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.
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 MAX3221/MAX3223/
*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.
Table 1a. MAX3221/MAX3223 FORCEOFF
and Enable Control Truth Table
Table 1b. MAX3243 FORCEOFF Control
Truth Table
Figure 3a. MAX32_ _ Entering 1μA Supply Mode Through
AutoShutdown
Figure 3b. MAX32_ _ with Transmitters Enabled Using
AutoShutdown
Figure 3c. AutoShutdown Logic
Figure 4. AutoShutdown with Initial Turn-On to Wake Up a
Mouse or Another System
FORCEOFF EN T_OUT R_OUT
0 0 High-Z Active
0 1 High-Z High-Z
1 0 Active* Active
1 1 Active* High-Z
FORCEOFF T_OUT R_OUT R2OUTB
0 High-Z High-Z Active
1 Active* Active* Active
+0.3V
-0.3V
INVALID
TO MAX32_ _
POWER SUPPLY
AND TRANSMITTERS
R_IN
* TRANSMITTERS ARE DISABLED, REDUCING SUPPLY CURRENT TO 1
µ A IF
ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR AT LEAST 30µs.
30µs
COUNTER
R
+2.7V
-2.7V
INVALID
TO MAX32_ _
POWER SUPPLY
R_IN
* 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 30
µ s.
30µs
COUNTER
R
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.
FORCEON
MASTER SHDN LINE
0.1µF 1MΩ
FORCEOFF
MAX3221
MAX3223
MAX3243
POWER
MANAGEMENT
UNIT
MAX3221/MAX3223/
MAX3243
+3V to +5.5V RS-232
Transceivers with AutoShutdown
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