+3V to +5.5V RS-232 Transceivers
with AutoShutdown
MAX3221/MAX3223/MAX3243
Table 2. AutoShutdown Logic
RS-232 SIGNAL
PRESENT AT
RECEIVER INPUT
FORCEON
INPUT
FORCEOFF INPUT
INVALID OUTPUT
TRANSCEIVER STATUS
Yes
No
H
H
H
L
X
H
L
H
L
Normal Operation
Normal Operation (Forced On)
Shutdown (AutoShutdown)
Shutdown (Forced Off)
No
L
Yes
No
X
X
H
L
L
Shutdown (Forced Off)
MAX3243 is awake. If the other system outputs valid
RS-232 signals within that time, the RS-232 ports on
both systems remain enabled.
TRANSMITTERS ENABLED, INVALID HIGH
+2.7V
+0.3V
When shut down, the device’s charge pumps are turned
off, V+ decays to V , V- decays to ground, and the
CC
transmitter outputs are disabled (high impedance). The
time required to exit shutdown is typically 100µs (Figure
5b).
INDETERMINATE
AUTOSHUTDOWN, TRANSMITTERS DISABLED,
0V
1μA SUPPLY CURRENT, INVALID LOW
-0.3V
Software-Controlled Shutdown
INDETERMINATE
If direct software control is desired, INVALID can be
used to indicate DTR or Ring Indicator signal. Tie
FORCEOFF and FORCEON together to bypass Auto-
Shutdown so the line acts like a SHDN input.
-2.7V
TRANSMITTERS ENABLED, INVALID HIGH
(a)
Applications Information
Capacitor Selection
The capacitor type used for C1–C4 is not critical for
proper operation; either polarized or nonpolarized
capacitors 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).
RECEIVER
INPUT
VOLTAGE
(V)
INVALID
REGION
V
CC
0
INVALID
OUTPUT
(V)
t
t
INVH
INVL
t
WU
V+
V
CC
0
When using the minimum required capacitor values,
make sure the capacitor value does not degrade
excessively with temperature. If in doubt, use capaci-
tors with a larger nominal value. The capacitor’s equiva-
lent series resistance (ESR) usually rises at
low temperatures and influences the amount of ripple
on V+ and V-.
V-
(b)
Figure 5. AutoShutdown Trip Levels
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