1µA Supply Current, 1Mbps, 3.0V to 5.5V,
RS-232 Transceivers with AutoShutdown Plus
MAX3224–MAX3227/MAX3244/MAX3245
†
Table 1. Output Control Truth Table
OPERATION
STATUS
VALID
RECEIVER
LEVEL
RECEIVER OR
TRANSMITTER
EDGE WITHIN
30sec
X
R_OUT
(MAX3224/
MAX3225/
MAX3226/
MAX3227)
Active
R_OUT
(MAX3244/
MAX3245)
R2OUTB
(MAX3244/
MAX3245)
FORCEON
FORCEOFF
T_OUT
Shutdown
(Forced Off)
Normal
Operation
(Forced On)
Normal
Operation
(AutoShutdown
Plus)
Shutdown (Auto-
Shutdown Plus)
Normal
Operation
Normal
Operation
Shutdown
Normal
Operation
(AutoShutdown)
Shutdown
(AutoShutdown)
X
0
X
High-Z
High-Z
Active
1
1
X
X
Active
Active
Active
Active
0
1
X
Yes
Active
Active
Active
Active
0
INVALID*
INVALID*
INVALID*
INVALID*
1
1
1
1
INVALID**
X
Yes
X
No
Yes
No
X
Yes
No
X
High-Z
Active
Active
High-Z
Active
Active
Active
Active
Active
Active
Active
Active
Active
Active
Active
Active
Active
Active
Active
Active
INVALID*
INVALID**
No
X
High-Z
Active
High-Z
Active
X = Don’t care
*
INVALID
connected to FORCEON
**
INVALID
connected to FORCEON and
FORCEOFF
RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic
output levels. The MAX3224–MAX3227 feature inverting
outputs that always remain active (Table 1). The
MAX3244/MAX3245 have inverting three-state outputs
that are high impedance when shut down (FORCEOFF
= GND) (Table 1).
The MAX3244/MAX3245 feature an extra, always
active, noninverting output, R2OUTB. R2OUTB output
monitors receiver activity while the other receivers are
high impedance, allowing Ring Indicator applications to
be monitored without forward biasing other devices
connected to the receiver outputs. This is ideal for sys-
tems where V
CC
is set to ground in shutdown to
accommodate peripherals such as UARTs (Figure 2).
The MAX3224–MAX3227/MAX3244/MAX3245 feature
an
INVALID
output that is enabled low when no valid
RS-232 voltage levels have been detected on all receiv-
er inputs. Because
INVALID
indicates the receiver
input’s condition, it is independent of FORCEON and
FORCEOFF
states (Figures 3 and 4).
AutoShutdown Plus Mode
The MAX3224–MAX3227/MAX3244/MAX3245 achieve a
1µA supply current with Maxim’s AutoShutdown
Plusfeature, which operates when
FORCEOFF
is high
and a FORCEON is low. When these devices do not
sense a valid signal transition on any receiver and trans-
mitter input for 30sec, the on-board charge pumps are
shut down, reducing supply current to 1µA. This occurs
if the RS-232 cable is disconnected or if the connected
9
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
