MAX218
1.8V to 4.25V-Powered,
True RS-232 Dual Transceiver
_______________________________________________________________________________________ 7
Low-Power Operation
The following suggestions will help you get maximum
life out of your batteries.
Shut the MAX218 down when it is not being used for
transmission. The receivers can remain active when
the MAX218 is shut down, to alert your system to exter-
nal activity.
Transmit at the highest practical data rate. Although
this raises the supply current while transmission is in
progress, the transmission will be over sooner. As long
as the MAX218 is shut down as soon as each transmis-
sion ends, this practice will save energy.
Operate your whole system from the raw battery volt-
age rather than suffer the losses of a regulator or DC-
DC converter. If this is not possible, but your system is
powered from two cells and employs a 3V DC-DC con-
verter to generate the main logic supply, use the circuit
of Figure 2. This circuit draws most of the MAX218’s
power straight from the battery, but still provides logic-
level compatibility with the 3V logic.
Keep communications cables short to minimize capaci-
tive loading. Lowering the capacitive loading on the
transmitter outputs reduces the MAX218’s power con-
sumption. Using short, low-capacitance cable also
helps transmission at the highest data rates.
Keep the S
—
H
—
D
—
N
–
pin low while power is being applied to
the MAX218, and take S
—
H
—
D
—
N
–
high only after V
CC
has
risen above about 1.5V. This avoids active operation at
very low voltages, where currents of up to 150mA can be
drawn. This is especially important with systems pow-
ered from rechargeable cells; if S
—
H
—
D
—
N
–
is high while the
cells are being trickle charged from a deep discharge,
the MAX218 could draw a significant amount of the
charging current until the battery voltage rises above
1.5V.
Pin Configuration Change
The
Pin Configuration
shows pin 2 as N.C. (no con-
nect). Early samples had a bypass capacitor for the
internal reference connected to pin 2, which was
labeled REF. This bypass capacitor proved to be
unnecessary and the connection has been omitted. Pin
2 may now be connected to ground, left open, or
bypassed to GND with a capacitor.
EIA/TIA-232E and
_____________EIA/TIA-562 Standards
RS-232 circuits consume much of their power because
the EIA/TIA-232E standard demands that the transmit-
ters deliver at least 5V to receivers with impedances
that can be as low as 3kΩ. For applications where
power consumption is critical, the EIA/TIA-562 standard
provides an alternative.
EIA/TIA-562 transmitter output voltage levels need only
reach ±3.7V, and because they have to drive the same
3kΩ receiver loads, the total power consumption is con-
siderably reduced. Since the EIA/TIA-232E and
EIA/TIA-562 receiver input voltage thresholds are the
same, interoperability between EIA/TIA-232E and
EIA/TIA-562 devices is guaranteed. Maxim’s MAX560
and MAX561 are EIA/TIA-562 transceivers that operate
on a single supply from 3.0V to 3.6V, and the MAX562
transceiver operates from 2.7V to 5.25V while produc-
ing EIA/TIA-562 levels.
R1
R2
T1
T2
T1OUT
T2OUT
T1IN
T2IN
R1OUT
R2OUT
R1IN
R2IN
GND
EN
V
CC
SHDN
LX
C1+
C1-
V-
6
3
7
8
9
10
11
12
13
14
16
18
15
4
5, 17, 20
MAX218
ENABLE
ON/OFF
1
V+
19
0.1µF
C4
1µF
C5
1µF
C2
1µF
C3
0.47µF
C1
15µH
D1
1N6050
3V
DC-DC
CONVERTER
MAX878
OR
MAX756
OR
MAX856
Figure 2. Operating from Unregulated and Regulated Supplies
