Datasheet

MAX218
1.8V to 4.25V-Powered,
True RS-232 Dual Transceiver
6 _______________________________________________________________________________________
RS-232 Drivers
The two drivers are identical, and deliver EIA/TIA-232E
and EIA/TIA-562 output voltage levels when V
DD
is
between 1.8V and 4.25V. The transmitters drive up to
3kΩ in parallel with 1000pF at up to 120kbps. Connect
unused driver inputs to either GND or V
CC
. Disable the
drivers by taking S
—
H
—
D
—
N
–
low. The transmitter outputs are
forced into a high-impedance state when S
—
H
—
D
—
N
–
is low.
RS-232 Receivers
The two receivers are identical, and accept both
EIA/TIA-232E and EIA/TIA-562 input signals. The
CMOS receiver outputs swing rail-to-rail. When EN is
high, the receivers are active regardless of the state of
S
—
H
—
D
—
N
–
. When EN is low, the receiver outputs are put
into a high-impedance state. This allows two RS-232
ports (or two ports of different types) to be wired-ORed
at the UART.
Operating Modes
S
—
H
—
D
—
N
–
and EN determine the MAX218’s mode of opera-
tion, as shown in Table 2.
Table 2. Operating Modes
Shutdown
When S
—
H
—
D
—
N
–
is low, the power supplies are disabled and
the transmitters are put into a high-impedance state.
Receiver operation is not affected by taking S
—
H
—
D
—
N
–
low.
Power consumption is dramatically reduced in shutdown
mode. Supply current is minimized when the receiver
inputs are static in any of three states: floating (ground),
GND, or V
CC
.
__________Applications Information
Operation from Regulated/Unregulated
Dual System Power Supplies
The MAX218 is intended for use with three different
power-supply sources: it can be powered directly from
a battery, from a 3.0V or 3.3V power supply, or simulta-
neously from both. Figure 1 shows the single-supply
configuration. Figure 2 shows the circuit for operation
from both a 3V supply and a raw battery supply—an
ideal configuration where a regulated 3V supply is
being derived from two cells. In this application, the
MAX218’s logic levels remain appropriate for interface
with 3V logic, yet most of the power for the MAX218 is
drawn directly from the battery, without suffering the
efficiency losses of the DC-DC converter. This pro-
longs battery life.
Bypass the input supplies with 0.1µF at V
CC
(C4) and at
least 1µF at the inductor (C5). Increase C5 to 4.7µF if
the power supply has no other bypass capacitor con-
nected to it.
MANUFACTURER PART NUMBER PHONE FAX
Murata-Erie LQH4N150K-TA
USA [phone]
Japan [phone]
USA [phone]
Japan [phone]
Sumida CD43150
USA [phone]
Japan [phone]
USA [phone]
Japan [phone]
TDK NLC453232T-150K
USA [phone]
Japan [phone]
USA [phone]
Japan [phone]
Central Semiconductor CMPSH-3, Schottky USA [phone] USA [phone]
Motorola MMBD6050LT1, Silicon USA [phone] USA [phone]
Philips PMBD6050, Silicon USA [phone] USA [phone]
Motorola
1N6050, Silicon
1N5817, Schottky
USA [phone] USA [phone]
Table 1. Suggested Component Suppliers
–
S
—
H
—
D
—
N
–
EN
RECEIVER
OUTPUT
DRIVER
OUTPUT
DC-DC
CONVERTER
L L High-Z High-Z OFF
L H Enabled High-Z OFF
H L High-Z Enabled ON
H H Enabled Enabled ON
SUPPLY
CURRENT
Minimum
Minimum
Normal
Normal
Inductors—Surface Mount
Diodes—Surface Mount
Diodes—Through-Hole