MAX3221 Просмотр технического описания (PDF) - Maxim Integrated
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MAX3221 Datasheet PDF : 16 Pages
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1µA Supply-Current, True +3V to +5.5V
RS-232 Transceivers with AutoShutdown
Table 1a. MAX3221/MAX3223 FORCEOFF and
Enable Control Truth Table
FORCEOFF
0
0
1
1
EN
T_OUT
R_OUT
0
High-Z
Active
1
High-Z
High-Z
0
Active*
Active
1
Active*
High-Z
Table 1b. MAX3243 FORCEOFF Control
Truth Table
FORCEOFF
T_OUT
R_OUT R2OUTB
0
High-Z
High-Z
Active
1
Active*
Active*
Active
*Note: If the part is in AutoShutdown mode (FORCEOFF = VCC,
FORCEON = GND), it is shut down if no valid RS-232
levels are present on all receiver inputs.
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
+0.3V
R_IN
-0.3V
TO MAX32_ _
POWER SUPPLY
30µs AND TRANSMITTERS
COUNTER INVALID
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
+2.7V
TO MAX32_ _
R_IN
30µs
POWER SUPPLY
COUNTER INVALID
-2.7V
R
* 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.
Figure 3b. MAX32_ _ with Transmitters Enabled Using
AutoShutdown
FORCEOFF
FORCEON
INVALID
POWER DOWN
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. 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
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