MAX3387E
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 a1A 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 1A. 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.
Figure 3a shows valid and invalid RS-232 receiver volt-
age levels. INVALID indicates the receiver input’s con-
dition, 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.
VL Logic Supply Input
Unlike other RS-232 interface devices where the receiv-
er outputs swing between 0V and VCC, the MAX3387E
features a separate logic supply input (VL) that sets
VOH 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
≤ VL ≤ VCC). 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
RECEIVER
INPUT
LEVELS
-2.7V
-0.3V
+2.7V
+0.3V
0V
INDETERMINATE
TRANSMITTERS ENABLED, INVALID HIGH
AutoShutdown, TRANSMITTERS DISABLED,
1
μA SUPPLY CURRENT, INVALID LOW
TRANSMITTERS ENABLED, INVALID HIGH
INDETERMINATE
Figure 3a. Receiver Thresholds for INVALID
AUTOSHDN
R_IN
T_IN
R
S
30s
TIMER
EDGE
DETECT
EDGE
DETECT
FORCEOFF
FORCEON
Figure 2d. Power-Down Logic