3.0V to 5.5V, Low-Power, up to 1Mbps, True RS-232
Transceivers Using Four 0.1µF External Capacitors
MAX3222/MAX3232/MAX3237/MAX3241
RS-232 Receivers
V
CC
V
CC
Rx
PROTECTION
DIODE
UART
Tx
GND
SHDN = GND
PREVIOUS
RS-232
5kΩ
The receivers convert RS-232 signals to CMOS-logic out-
put levels. The MAX3222/MAX3237/MAX3241 receivers
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 V
CC
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 V
CC
, V- is
pulled to ground, and the transmitter outputs are dis-
abled (high impedance). The time required to exit shut-
down is typically 100µs, as shown in Figure 3. Connect
SHDN
to V
CC
if the shutdown mode is not used.
SHDN
has no effect on R_OUT or R_OUTB.
a)
OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM
ACTIVE RECEIVER OUTPUT IN SHUTDOWN.
V
CC
TO
µP
LOGIC
TRANSITION
DETECTOR
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.
MAX3237
MAX3241
R1OUTB
V
CC
Rx
PROTECTION
DIODE
UART
Tx
GND
T1IN
SHDN = GND
R1OUT
EN = V
CC
5kΩ
T1OUT
R1IN
__________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 capaci-
tor values (e.g., by a factor of 2) reduces ripple on the
transmitter outputs and slightly reduces power consump-
tion. 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
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-.
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.
Figure 2. Detection of RS-232 Activity when the UART and
Interface are Shut Down; Comparison of MAX3237/MAX3241
(b) with Previous Transceivers (a).
8
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