MAX3238E/MAX3248E
+3.0V to +5.5V, 10nA, 250kbps RS-232 Transceivers
with ±15kV ESD-Protected I/O and Logic Pins
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_______________Detailed Description
Dual Charge-Pump Voltage Converter
The MAX3238E/MAX3248Es’ internal power supply
consists of a regulated dual charge pump that provides
output voltages of +5.5V (doubling charge pump) and
-5.5V (inverting charge pump), regardless of the input
voltage (V
CC
) over the 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.
RS-232 Transmitters
The MAX3248E transmitters are inverting level translators
that convert a logic low of 0.6V and logic high of 1.2V to
5.0V EIA/TIA-232 levels. The MAX3238E transmitters are
inverting level translators that convert CMOS-logic levels
to 5.0V EIA/TIA-232 levels. The MAX3238E/MAX3248E
transmitters both guarantee a 250kbps data rate with
worst-case loads of 3kΩ in parallel with 1000pF, provid-
ing compatibility with PC-to-PC communication software
(such as LapLink™). Transmitters can be paralleled to
drive multiple receivers. Figure 1 shows a complete sys-
tem connection.
LapLink is a trademark of Traveling Software.
When FORCEOFF is driven to ground, the transmitters
and receivers are disabled and the outputs go high
impedance, except for R1OUTB. When the Auto-
Shutdown Plus circuitry senses that all receiver and
transmitter inputs are inactive for more than 30s, the
transmitters are disabled and the outputs go into a
high-impedance state, but the receivers remain active.
When the power is off, the MAX3238E/MAX3248E per-
mit the outputs to be driven up to ±12V.
The transmitter inputs, FORCEON and FORCEOFF,
have a 400kΩ active positive-feedback resistor. Once
driven to a valid logic level, they will retain this level if
the driving signal is removed or goes high-impedance.
Unused transmitter inputs may be left unconnected.
RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic
output levels. All receivers have inverting three-state
outputs and are inactive in shutdown (FORCEOFF)
(Table 1). The MAX3238E/MAX3248E also feature an
extra, always-active noninverting output, R1OUTB. This
extra output monitors receiver activity while the other
receivers are high impedance, allowing Ring Indicator
to be monitored without forward biasing other devices
connected to the receiver outputs. This is ideal for sys-
tems where V
CC
is set to 0 in shutdown to accommo-
date peripherals, such as UARTs (Figure 2).
Table 1. Output Control Truth Table
FORCEON
FORCEOFF
AutoShutdown
Plus
OPERATION
STATUS
T_OUT R_OUT R1OUTB
X 0 X
Shutdown
(Forced Off)
High-Z High-Z Active
1 1 X
Normal Operation
(Forced On)
Active Active Active
0 1 <30s*
Normal
Operation
(AutoShutdown
Plus)
Active Active Active
0 1 >30s*
Shutdown
(AutoShutdown
Plus)
High-Z Active Active
X = Don’t care
*Time since last receiver or transmitter input transition.
