±15kV ESD-Protected, 1µA, 3.0V to 5.5V, 250kbps,
RS-232 Transceivers with AutoShutdown
MAX3221E/MAX3223E/MAX3243E
POWER-
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
FORCEOFF
FORCEON
INVALID
PROTECTION
DIODE
V
CC
MAX3243E
V
CC
Rx
UART
Tx
I
PREVIOUS
RS-232
5k
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
LOGIC
µP
TRANSITION
DETECTOR
Figure 1. Interface Under Control of PMU
I
PROTECTION
DIODE
V
CC
Rx
UART
R2OUT
THREE-STATED
MAX3243E
RS-232 Receivers
The MAX3221E/MAX3223E/MAX3243E’s receivers con-
vert RS-232 signals to CMOS-logic output levels. All
receivers have inverting three-state outputs and can be
active or inactive. In shutdown (FORCEOFF = low) or in
AutoShutdown, the MAX3221E/MAX3223E’s receivers
are active (Table 1). Drive
EN
high to place the receiv-
er(s) in a high-impedance state. The MAX3243E’s
receivers are high-impedance when the part is in shut-
down (FORCEOFF = low).
The MAX3243E features an extra, always-active com-
plementary output (R2OUTB). R2OUTB monitors
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
drops to 0 in shutdown to accommodate peripherals
such as UARTs (Figure 2).
R2OUTB
R2IN
5k
Tx
GND
T1IN
FORCEOFF = GND
T1OUT
b)
NEW MAX3243E: 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 MAX3243E detects RS-232 activity when the
UART and interface are shut down.
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7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
