3V, ±15kV ESD-Protected, AutoShutdown Plus
RS-232 Transceiver for PDAs and Cell Phones
Detailed Description
Dual Charge-Pump
Voltage Converter
The MAX3387E’s internal power supply consists of a
regulated dual charge pump that provides output volt-
ages of +5.5V (doubling charge pump) and -5.5V
(inverting charge pump), regardless of the input volt-
age (V
CC
) over a +3.0V to +5.5V range. The charge
pumps operate in a discontinuous mode: if the output
voltages are less than 5.5V, the charge pumps are
enabled; if the output voltages exceed 5.5V, the charge
pumps are disabled. Each charge pump requires a fly-
ing capacitor (C1, C2) and a reservoir capacitor (C3,
C4) to generate the V+ and V- supplies.
These RS-232 output stages are turned off (high imped-
ance) when the device is in shutdown mode. When the
power is off, the MAX3387E permits the outputs to be
driven up to ±12V.
The transmitter inputs do not have pull-up resistors.
Connect unused inputs to GND or V
L
.
MAX3387E
RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic
output levels. The MAX3387E’s receivers are always
active, even when the device is in shutdown.
The MAX3387E features an
INVALID
output that indi-
cates when no signal is present on any RS-232 receiver
inputs.
INVALID
is independent of other control logic
functions; it indicates the receiver input conditions only
(Figures 2 and 3).
RS-232 Transmitters
The transmitters are inverting level translators that con-
vert CMOS-logic levels to 5.0V EIA/TIA-232 levels.
The MAX3387E transmitters guarantee a 250kbps data
rate with worst-case loads of 3kΩ in parallel with
1000pF, providing compatibility with PC-to-PC commu-
nication software (such as Laplink
®
). Transmitters can
be paralleled to drive multiple receivers or mice. Figure 1
shows a complete system connection.
FORCEON
FORCEOFF
INVALID
+0.3V
R_IN
-0.3V
POWER-
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
I/O CHIP
POWER SUPPLY
30μs
COUNTER
R
INVALID
TRANSMITTERS ARE DISABLED, REDUCING SUPPLY CURRENT TO 1μA IF
ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR
AT LEAST
30μs.
V
L
Figure 2a.
INVALID
Function Diagram,
INVALID
= Low
MAX3387E
+2.7V
V
CC
I/O
CHIP
WITH
UART
CPU
R_IN
RS-232
-2.7V
30μs
COUNTER
R
INVALID
TRANSMITTERS ARE ENABLED IF:
ANY RECEIVER INPUT IS GREATER THAN +2.7V OR LESS THAN -2.7V;
ANY RECEIVER INPUT HAS BEEN BETWEEN +0.3V AND -0.3V FOR
LESS THAN
30μs.
Figure 2b.
INVALID
Function Diagram,
INVALID
= High
Figure 1. Interface Under Control of PMU
Laplink is a registered trademark of Laplink Software, Inc.
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