3.0V, ±15kV ESD-Protected RS-232
Transceiver for PDAs and Cell Phones
MAX3386E
Detailed Description
Dual Charge-Pump
Voltage Converter
The MAX3386E’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 voltage
(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 flying 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
impedance) when the device is in shutdown mode.
When the power is off, the MAX3386E 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
.
RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic
output levels. The MAX3386E’s receivers have inverting
three-state outputs, which depend on the shutdown
state of the device.
Shutdown Mode
Supply current falls to less than 1µA when the MAX3386E
is placed in shutdown mode (SHDN logic low). When
shut down, the device’s charge pumps are turned off, V+
decays to V
CC
, V- is pulled to ground, and the transmitter
outputs are disabled (high impedance). The time
required to exit shutdown is typically 100µs, as shown in
Figure 2. Connect
SHDN
to V
CC
if the shutdown mode
is not used. In shutdown mode, the receiver outputs are
high impedance (Table 1).
RS-232 Transmitters
The transmitters are inverting level translators that convert
CMOS-logic levels to 5.0V EIA/TIA-232 levels.
The MAX3386E’s transmitters guarantee a 250kbps data
rate with worst-case loads of 3kΩ in parallel with 1000pF,
providing compatibility with PC-to-PC communication
software (such as LapLink™). Transmitters can be paral-
leled to drive multiple receivers or mice. Figure 1 shows a
complete system connection.
Table 1. Shutdown Logic Truth Table
SHDN
TRANSMITTER
OUTPUTS
High-Z
Active
RECEIVER
OUTPUTS
High-Z
Active
CHARGE
PUMP
Inactive
Active
POWER-
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
I/O CHIP
POWER SUPPLY
SHDN
L
H
V
L
V
CC
MAX3386E
5V/div
T2
I/O
CHIP
WITH
UART
CPU
RS-232
2V/div
V
CC
= 3.3V
C1–C4 = 0.1µF
50µs/div
T1
Figure 1. Interface Under Control of PMU
LapLink is a trademark of Traveling Software.
6
Figure 2. Transmitter Outputs when Exiting Shutdown
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