RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic
output levels. The MAX3222/MAX3237/MAX3241 receiv-
ers have inverting three-state outputs. In shutdown, the
receivers can be active or inactive (Table 1).
The complementary outputs on the MAX3237 (R1OUTB)
and the MAX3241 (R1OUTB, R2OUTB) are always
active, regardless of the state of EN or SHDN. This allows
for Ring Indicator applications without forward biasing
other devices connected to the receiver outputs. This is
ideal for systems where VCC is set to 0V in shutdown to
accommodate peripherals, such as UARTs (Figure 2).
MAX3222/MAX3237/MAX3241
Shutdown Mode
Supply current falls to less than 1μA in shutdown mode
(SHDN = low). When shut down, the device’s charge
pumps are turned off, V+ is pulled down to VCC, V- is
pulled to ground, and the transmitter outputs are disabled
(high impedance). The time required to exit shutdown is
typically 100μs, as shown in Figure 3. Connect SHDN
to VCC if the shutdown mode is not used. SHDN has no
effect on R_OUT or R_OUTB.
MAX3222/MAX3237/MAX3241
Enable Control
The inverting receiver outputs (R_OUT) are put into a
high-impedance state when EN is high. The complemen-
tary outputs R1OUTB and R2OUTB are always active,
regardless of the state of EN and SHDN (Table 1). EN has
no effect on T_OUT.
Applications Information
Capacitor Selection
The capacitor type used for C1–C4 is not critical for
proper operation; polarized or nonpolarized capacitors
can be used. The charge pump requires 0.1μF capaci-
tors for 3.3V operation. For other supply voltages, refer to
Table 2 for required capacitor values. Do not use values
lower than those listed in Table 2. Increasing the capacitor
values (e.g., by a factor of 2) reduces ripple on the trans-
mitter 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 increas-
ing 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
higher nominal value. The capacitor’s equivalent series
resistance (ESR), which usually rises at low tempera-
tures, influences the amount of ripple on V+ and V-.
Figure 2. Detection of RS-232 Activity when the UART and
Interface are Shut Down; Comparison of MAX3237/MAX3241
(b) with Previous Transceivers (a).
MAX3237
MAX3241
T1OUT
R1OUTB
Tx
5kΩ
UART
VCC
T1IN
LOGIC
TRANSITION
DETECTOR
R1IN
R1OUT
EN = VCC
SHDN = GND
VCC
TO
µP
Rx
PREVIOUS
RS-232
Tx
UART
PROTECTION
DIODE
PROTECTION
DIODE
SHDN = GND
VCC
VCC
GND
Rx
5kΩ
a) OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM
ACTIVE RECEIVER OUTPUT IN SHUTDOWN.
b) NEW MAX3237/MAX3241: EN SHUTS DOWN RECEIVER OUTPUTS (EXCEPT FOR
B OUTPUTS), SO NO CURRENT FLOWS TO UART IN SHUTDOWN. B OUTPUTS
INDICATE RECEIVER ACTIVITY DURING SHUTDOWN WITH EN HIGH.
GND
MAX3222/MAX3232/
MAX3237/MAX3241
3.0V to 5.5V, Low-Power, Up to 1Mbps,
True RS-232 Transceivers
www.maximintegrated.com Maxim Integrated
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