1µA Supply-Current, True +3V to +5.5V
RS-232 Transceivers with AutoShutdown
MAX3221/MAX3223/MAX3243
POWER
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
FORCEOFF
FORCEON
INVALID
PROTECTION
DIODE
V
CC
MAX3243
V
CC
Rx
UART
Tx
I
PREVIOUS
RS-232
5kΩ
CPU
I/O
CHIP
WITH
UART
GND
RS-232
SHDN = GND
(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
PROTECTION
DIODE
I
MAX3243
R2OUTB
RS-232 Receivers
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.
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
receiver outputs. This is ideal for systems where V
CC
is
set to 0V in shutdown to accommodate peripherals,
such as UARTs (Figure 2).
V
CC
Rx
UART
R2OUT
THREE-STATED
5kΩ
T1IN
FORCEOFF = GND
T1OUT
R2IN
Tx
GND
(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.
Figure 2. The MAX3243 detects RS-232 activity when the
UART and interface are shut down.
6
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