±15kV ESD-Protected, Down to 10nA, 3.0V to 5.5V,
Up to 1Mbps, True RS-232 Transceivers
MAX3222E/MAX3232E/MAX3237E/MAX3241E
_______________Detailed Description
Dual Charge-Pump Voltage Converter
The MAX3222E/MAX3232E/MAX3237E/MAX3241E’s
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), over the 3.0V to 5.5V V
CC
range. The charge
pump operates in discontinuous mode; if the output
voltages are less than 5.5V, the charge pump is
enabled, and if the output voltages exceed 5.5V, the
charge pump is disabled. Each charge pump requires
a flying capacitor (C1, C2) and a reservoir capacitor
(C3, C4) to generate the V+ and V- supplies (Figure
1).
state. Receivers can be either active or inactive in shut-
down (Table 1).
The complementary outputs on the MAX3237E/MAX3241E
(R_OUTB) are always active, regardless of the state of
EN
or
SHDN.
This allows the device to be used for ring indica-
tor applications without forward biasing other devices con-
nected to the receiver outputs. This is ideal for systems
where V
CC
drops to 0 in shutdown to accommodate
peripherals such as UARTs (Figure 2).
MAX3222E/MAX3237E/MAX3241E
Shutdown Mode
Supply current falls to less than 1µA in shutdown mode
(SHDN = low). The MAX3237E’s supply current falls
to10nA (typ) when all receiver inputs are in the invalid
range (-0.3V < R_IN < +0.3). When shut down, the
device’s charge pumps are shut off, V+ is pulled down
to V
CC
, V- is pulled to ground, and the transmitter
outputs are disabled (high impedance). The time
required to recover from shutdown is typically 100µs,
as shown in Figure 3. Connect
SHDN
to V
CC
if the shut-
down mode is not used.
SHDN
has no effect on R_OUT
or R_OUTB (MAX3237E/MAX3241E).
RS-232 Transmitters
The transmitters are inverting level translators that con-
vert TTL/CMOS-logic levels to ±5.0V EIA/TIA-232 com-
pliant levels.
The MAX3222E/MAX3232E/MAX3237E/MAX3241E
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 paralleled to
drive multiple receivers or mice.
The MAX3222E/MAX3237E/MAX3241E’s transmitters are
disabled and the outputs are forced into a high-imped-
ance state when the device is in shutdown mode (SHDN =
GND). The MAX3222E/MAX3232E/MAX3237E/MAX3241E
permit the outputs to be driven up to ±12V in shutdown.
The MAX3222E/MAX3232E/MAX3241E transmitter
inputs do not have pullup resistors. Connect unused
inputs to GND or V
CC
. The MAX3237E’s transmitter
inputs have a 400kΩ active positive feedback resistor,
allowing unused inputs to be left unconnected.
±15kV ESD Protection
As with all Maxim devices, ESD-protection structures
are incorporated to protect against electrostatic dis-
charges encountered during handling and assembly.
The driver outputs and receiver inputs of the
MAX3222E/MAX3232E/MAX3237E/MAX3241E have
extra protection against static electricity. Maxim’s engi-
neers have developed state-of-the-art structures to pro-
tect these pins against ESD of ±15kV without damage.
The ESD structures withstand high ESD in all states:
normal operation, shutdown, and powered down. After
an ESD event, Maxim’s E versions keep working without
latchup, whereas competing RS-232 products can latch
and must be powered down to remove latchup.
Furthermore, the MAX3237E logic I/O pins also have
±15kV ESD protection. Protecting the logic I/O pins to
±15kV makes the MAX3237E ideal for data cable appli-
cations.
MAX3237E MegaBaud Operation
For higher-speed serial communications, the MAX3237E
features MegaBaud operation. In MegaBaud operating
mode (MBAUD = V
CC
), the MAX3237E transmitters
guarantee a 1Mbps data rate with worst-case loads of
3kΩ in parallel with 250pF for 3.0V < V
CC
< 4.5V. For 5V
±10% operation, the MAX3237E transmitters guarantee a
1Mbps data rate into worst-case loads of 3kΩ in parallel
with 1000pF.
Table 1. MAX3222E/MAX3237E/MAX3241E
Shutdown and Enable Control Truth Table
SHDN
0
0
1
1
EN
0
1
0
1
T_OUT
High-Z
High-Z
Active
Active
R_OUT
Active
High-Z
Active
High-Z
R_OUTB
(MAX3237E/
MAX3241E)
Active
Active
Active
Active
RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic
output levels. The MAX3222E/MAX3237E/MAX3241E
receivers have inverting three-state outputs. Drive
EN
high to place the receiver(s) into a high-impedance
LapLink is a trademark of Traveling Software.
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