3V, ±15kV ESD-Protected, AutoShutdown Plus
RS-232 Transceiver for PDAs and Cell Phones
MAX3387E
T_IN
EDGE
DETECT
TRANSMITTERS ENABLED, INVALID HIGH
+2.7V
RECEIVER INPUT LEVELS
INDETERMINATE
+0.3V
0V
-0.3V
INDETERMINATE
-2.7V
TRANSMITTERS ENABLED, INVALID HIGH
AutoShutdown, TRANSMITTERS DISABLED,
1μA SUPPLY CURRENT, INVALID LOW
AUTOSHDN
FORCEOFF
S
30s
TIMER
R
R_IN
EDGE
DETECT
FORCEON
Figure 2c. AutoShutdown Plus Logic
Figure 3a. Receiver Thresholds for
INVALID
FORCEOFF
POWERDOWN
FORCEON
AUTOSHDN
POWERDOWN IS ONLY AN INTERNAL SIGNAL. IT CONTROLS THE
OPERATIONAL STATUS OF THE TRANSMITTERS AND THE POWER SUPPLIES.
Figure 2d. Power-Down Logic
AutoShutdown Plus Mode
The MAX3387E achieves a1µA supply current with
Maxim’s AutoShutdown Plus feature, which operates
when
FORCEOFF
is high and a FORCEON is low. When
these devices do not sense a valid signal transition on
any receiver and transmitter input for 30sec, the on-
board charge pumps are shut down, reducing supply
current to 1µA. This occurs if the RS-232 cable is dis-
connected or if the connected peripheral transmitters
are turned off, and if the UART driving the transmitter
inputs is inactive. The system turns on again when a
valid transition is applied to any RS-232 receiver or
transmitter input. As a result, the system saves power
without changes to the existing BIOS or operating sys-
tem.
Figures 2a and 2b show valid and invalid RS-232
receiver voltage levels.
INVALID
indicates the receiver
input’s condition, and is independent of FORCEON and
FORCEOFF
states. Figure 2 and Table 1 summarize the
MAX3387E’s operating modes. FORCEON and
FORCE-
OFF
override AutoShutdown Plus circuitry. When nei-
ther control is asserted, the IC selects between these
states automatically based on the last receiver or trans-
mitter input edge received.
By connecting FORCEON to
INVALID,
the MAX3387E
shuts down when no valid receiver level and no receiver or
transmitter edge is detected for 30sec, and wakes up
when a valid receiver level or receiver or transmitter
edge is detected.
By connecting FORCEON and
FORCEOFF
to
INVALID,
the MAX3387E shuts down when no valid receiver level
is detected.
A mouse or other system with AutoShutdown Plus may
need time to wake up. Figure 4 shows a circuit that
forces the transmitters on for 100ms, allowing enough
time for the other system to realize that the MAX3387E
is awake. If the other system outputs valid RS-232 sig-
nal transitions within that time, the RS-232 ports on both
systems remain enabled.
V
L
Logic Supply Input
Unlike other RS-232 interface devices where the receiv-
er outputs swing between 0V and V
CC
, the MAX3387E
features a separate logic supply input (V
L
) that sets
V
OH
for the receiver outputs and sets thresholds for the
receiver inputs. This feature allows a great deal of flexi-
bility in interfacing to many different types of systems
with different logic levels. Connect this input to the host
logic supply (1.8V
≤
V
L
≤
V
CC
). Also, see the
Typical
PDA/Cell-Phone Application
section.
8
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
