3.0V to 5.5V, Low -Pow e r, up to 1Mbps , True RS-232
Tra ns c e ive rs Us ing Four 0.1µF Exte rna l Ca pa c itors
RS -2 3 2 Re c e ive rs
The receivers convert RS-232 signals to CMOS-logic out-
V
CC
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
PREVIOUS
RS-232
V
CC
Rx
for s ys te ms whe re V
is s e t to 0V in s hutd own to
CC
PROTECTION
DIODE
accommodate peripherals, such as UARTs (Figure 2).
5k
MAX3 2 2 2 /MAX3 2 3 7 /MAX3 2 4 1
S h u t d o w n Mo d e
UART
Tx
Supply current falls to less than 1µA in shutdown mode
(SHDN = low). When shut down, the device’s charge
GND
SHDN = GND
pumps are turned off, V+ is pulled down to V , V- is
CC
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
a) OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM
ACTIVE RECEIVER OUTPUT IN SHUTDOWN.
SHDN to V if the shutdown mode is not used. SHDN
CC
has no effect on R_OUT or R_OUTB.
MAX3 2 2 2 /MAX3 2 3 7 /MAX3 2 4 1
En a b le Co n t ro l
V
CC
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.
TO
µP
LOGIC
TRANSITION
DETECTOR
MAX3237
MAX3241
__________Ap p lic a t io n s In fo rm a t io n
R1OUTB
R1OUT
Ca p a c it o r S e le c t io n
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).
V
CC
R1IN
Rx
PROTECTION
DIODE
7/MAX3241
EN = V
CC
5k
UART
T1OUT
T1IN
Tx
GND
SHDN = GND
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.
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).
8
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