Datasheet

MAX3243 is awake. If the other system outputs 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 turned
off, V+ decays to V
CC
, V- decays to ground, and
the transmitter outputs are disabled (high impedance).
The time required to exit shutdown is typically 100µs
(Figure 5b).
Software-Controlled Shutdown
If direct software control is desired, INVALID can be used
to indicate DTR or Ring Indicator signal. Tie FORCEOFF
and FORCEON together to bypass AutoShutdown so the
line acts like a SHDN input.
Applications Information
Capacitor Selection
The capacitor type used for C1–C4 is not critical for
proper operation; either polarized or nonpolarized capaci-
tors may be used. The charge pump requires 0.1µF
capacitors for 3.3V operation. For other supply voltages,
refer to Table 3 for required capacitor values. Do not use
values smaller than those listed in Table 3. Increasing
the capacitor values (e.g., by a factor of 2) reduces ripple
on the transmitter outputs and slightly reduces power
consumption. C2, C3, and C4 can be increased without
changing C1’s value. However, do not increase C1
without also increasing the values of C2, C3, and
C4 to maintain the proper ratios (C1 to the other
capacitors).
When using the minimum required capacitor values,
make sure the capacitor value does not degrade exces-
sively with temperature. If in doubt, use capacitors with
a larger nominal value. The capacitor’s equivalent series
resistance (ESR) usually rises at low temperatures and
influences the amount of ripple on V+ and V-.
Table 2. AutoShutdown Logic
Figure 5. AutoShutdown Trip Levels
RS-232 SIGNAL
PRESENT AT
RECEIVER INPUT
FORCEOFF
INPUT
FORCEON
INPUT
INVALID
INPUT
TRANSCEIVER
STATUS
Yes H X H Normal Operation
No H H L Normal Operation (Forced On)
No H L L Shutdown (AutoShutdown)
Yes L X H Shutdown (Forced Off)
No L X L Shutdown (Forced Off)
TRANSMITTERS ENABLED, INVALID HIGH
RECEIVER INPUT LEVELS
AUTOSHUTDOWN, TRANSMITTERS DISABLED,
1µA SUPPLY CURRENT, INVALID LOW
TRANSMITTERS ENABLED, INVALID HIGH
(a)
(b)
-2.7V
-0.3V
+2.7V
+0.3V
0V
INDETERMINATE
INDETERMINATE
V-
V
CC
0
V+
0
V
CC
t
INVL
t
WU
INVALID
REGION
RECEIVER
INPUT
VOLTAGE
(V)
INVALID
OUTPUT
(V)
t
INVH
MAX3221/MAX3223/
MAX3243
+3V to +5.5V RS-232
Transceivers with AutoShutdown
www.maximintegrated.com
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