M
3V, ±15kV ESD-Protected, AutoShutdown Plus
RS-232 Transceiver for PDAs and Cell Phones
8
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AutoShutdown Plus Mode
The MAX3387E achieves a1μAsupply 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 0 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.
POWERDOWN
AUTOSHDN
FORCEOFF
FORCEON
POWERDOWN IS ONLY AN INTERNAL SIGNAL. IT CONTROLS THE
OPERATIONAL STATUS OF THE TRANSMITTERS AND THE POWER SUPPLIES.
Figure 2c. AutoShutdown Plus Logic
R
-2.7V
-0.3V
+2.7V
+0.3V
0V
INDETERMINATE
TRANSMITTERS ENABLED, INVALID HIGH
AutoShutdown, TRANSMITTERS DISABLED,
1
μ
A SUPPLY CURRENT, INVALID LOW
TRANSMITTERS ENABLED, INVALID HIGH
INDETERMINATE
Figure 3a. Receiver Thresholds for
INVALID
AUTOSHDN
R_IN
T_IN
R
S
30sec
TIMER
EDGE
DETECT
EDGE
DETECT
FORCEOFF
FORCEON
Figure 2d. Power-Down Logic