1ꢀ5k EꢁDꢂ-rotected, Down to 10nA, 3.0k to ꢀ.ꢀk,
Up to 1Mbps, True Rꢁꢂ232 Transceivers
6
Table 2. Required Minimum Capacitor
Values
5
V
4
3
OUT+
V
= 3.0V
CC
V
(V)
C1
(µF)
C2, C3, C4
(µF)
CC
2
V
V
1
OUT+
OUT-
MAX3222E/MAX3232E/MAX3241E
0
3.0 to 3.6
4.5 to 5.5
3.0 to 5.5
MAX3237E
3.0 to 3.6
3.15 to 3.6
4.5 to 5.5
3.0 to 5.5
0.1
0.047
0.1
0.1
-1
-2
-3
-4
-5
-6
0.33
0.47
V
CC
1
V
OUT-
9
0.22
0.1
0.22
0.1
0
2
3
4
5
6
7
8
10
LOAD CURRENT PER TRANSMITTER (mA)
0.047
0.22
0.33
1.0
Figure 6a. MAX3241E Transmitter Output Voltage vs. Load
Current per Transmitter
power supplies. It has been tested with leading mouse
brands from manufacturers such as Microsoft and
Logitech. The MAX3241E successfully drove all serial
mice tested and met their respective current and volt-
age requirements. Figure 6a shows the transmitter out-
put voltages under increasing load current at 3.0V.
Figure 6b shows a typical mouse connection using the
MAX3241E.
When using the minimum required capacitor values,
make sure the capacitor value does not degrade
excessively with temperature. If in doubt, use capaci-
tors with a larger nominal value. The capacitor’s equiv-
alent series resistance (ESR), which usually rises at low
temperatures, influences the amount of ripple on V+
and V-.
-owerꢂꢁupply Decoupling
High Data Rates
The MAX3222E/MAX3232E/MAX3237E/MAX3241E
maintain the RS-232 5.0V minimum transmitter output
voltage even at high data rates. Figure 7 shows a trans-
mitter loopback test circuit. Figure 8 shows a loopback
test result at 120kbps, and Figure 9 shows the same test
at 250kbps. For Figure 8, all transmitters were driven
simultaneously at 120kbps into RS-232 loads in parallel
with 1000pF. For Figure 9, a single transmitter was driv-
en at 250kbps, and all transmitters were loaded with an
RS-232 receiver in parallel with 1000pF.
In most circumstances, a 0.1µF V
bypass capacitor
CC
is adequate. In applications that are sensitive to power-
supply noise, use a capacitor of the same value as
charge-pump capacitor C1. Connect bypass capaci-
tors as close to the IC as possible.
Operation Down to 2.7k
Transmitter outputs will meet EIA/TIA-562 levels of
3.7V with supply voltages as low as 2.7V.
Transmitter Outputs when
Recovering from ꢁhutdown
The MAX3237 maintains the RS-232 5.0V minimum
transmitter output voltage at data rates up to 1Mbps.
Figure 10 shows a loopback test result at 1Mbps with
MBAUD = V . For Figure 10, all transmitters were
CC
loaded with an RS-232 receiver in parallel with 250pF.
Figure 3 shows two transmitter outputs when recover-
ing from shutdown mode. As they become active, the
two transmitter outputs are shown going to opposite
RS-232 levels (one transmitter input is high, the other is
low). Each transmitter is loaded with 3k in parallel with
2500pF. The transmitter outputs display no ringing or
undesirable transients as they come out of shutdown.
Note that the transmitters are enabled only when the
magnitude of V- exceeds approximately -3V.
Interconnection with 3k and ꢀk Logic
The MAX3222E/MAX3232E/MAX3237E/MAX3241E can
directly interface with various 5V logic families, includ-
ing ACT and HCT CMOS. See Table 3 for more infor-
mation on possible combinations of interconnections.
Mouse Driveability
The MAX3241E has been specifically designed to
power serial mice while operating from low-voltage
______________________________________________________________________________________ 11
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
