+3V to +5.5V RS-232 Transceivers
with AutoShutdown
MAX3221/MAX3223/MAX3243
V
POWER
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
CC
FORCEOFF
FORCEON
INVALID
PROTECTION
DIODE
PREVIOUS
MAX3243
RS-232
V
CC
I
Rx
Tx
5kΩ
UART
I/O
GND
SHDN = GND
CHIP
WITH
UART
CPU
RS-232
(a) OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM ACTIVE
RECEIVER OUTPUT IN SHUTDOWN.
V
CC
TO
μP
LOGIC
TRANSITION
DETECTOR
Figure 1. Interface Under Control of PMU
MAX3243
I
PROTECTION
DIODE
R2OUTB
RS-232 Receivers
V
CC
The MAX3221/MAX3223/MAX3243’s receivers convert
RS-232 signals to CMOS-logic output levels. All
receivers have one inverting three-state output. In shut-
down (FORCEOFF = low) or in AutoShutdown, the
MAX3221/MAX3223’s receivers are active (Table 1).
Driving EN high places the receiver(s) in a high-imped-
ance state. The MAX3243’s receivers are high imped-
ance when the part is shut down.
R2IN
Rx
R2OUT
THREE-STATED
UART
5kΩ
T1OUT
T1IN
Tx
GND
FORCEOFF = GND
The MAX3243 has an always-active complementary
output (R2OUTB). R2OUTB is an extra output that mon-
itors receiver activity while the other receivers are high
impedance. This allows Ring Indicator to be monitored
without forward biasing other devices connected to the
(b) NEW MAX3243: IN SHUTDOWN, R2OUTB IS USED TO MONITOR EXTERNAL
DEVICES AND R2OUT IS THREE STATED, ELIMINATING A CURRENT PATH
THROUGH THE UART'S PROTECTION DIODE.
receiver outputs. This is ideal for systems where V
is
CC
set to 0V in shutdown to accommodate peripherals,
such as UARTs (Figure 2).
Figure 2. The MAX3243 detects RS-232 activity when the
UART and interface are shut down.
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