+3V to +5.5V RS-232 Transceivers
with AutoShutdown
MAX3221/MAX3223/MAX3243
5
V
+
4
3
OUT
V
CC
= 3.0V
5V/div
2V/div
2
T2
T
T
T
1
2
3
1
V
+
-
OUT
0
-1
-2
-3
-4
-5
-6
V
V
CC
1
OUT
V
-
OUT
6
T1
0
2
3
4
5
7
LOAD CURRENT PER TRANSMITTER (mA)
50μs/div
V
CC
= +3.3V
C1–C4 = 0.1μF
Figure 7a. MAX3243 Transmitter Output Voltage vs. Load
Current per Transmitter
Figure 6. Transmitter Outputs when Exiting Shutdown or
Powering Up
mice and met their respective current and voltage
requirements. Figure 7a shows the transmitter outputs
under increasing load current. The MAX3243’s switching
regulator ensures the transmitters will supply at least 5V
during worst-case conditions. The AutoShutdown feature
does not work with a mouse, so FORCEOFF and
Power-Supply Decoupling
In most circumstances a 0.1µF bypass capacitor is
adequate. In applications that are sensitive to power-
supply noise, decouple V
to ground with a capacitor
CC
of the same value as the charge-pump capacitor C1.
Connect bypass capacitors as close to the IC as possible.
FORCEON should be connected to V
.
CC
Transmitter Outputs
High Data Rates
when Exiting Shutdown
The MAX3221/MAX3223/MAX3243 maintain the RS-232
5.0V minimum transmitter output voltage even at high
data rates. Figure 8 shows a transmitter loopback test
circuit. Figure 9 shows a loopback test result at
120kbps, and Figure 10 shows the same test at
235kbps. For Figure 9, all three transmitters were driv-
en simultaneously at 120kbps into RS-232 loads in par-
allel with 1000pF. For Figure 10, a single transmitter
was driven at 235kbps, but all three transmitters were
loaded with an RS-232 receiver in parallel with 1000pF.
Figure 6 shows two transmitter outputs when exiting
shutdown mode. As they become active, the two trans-
mitter outputs are shown going to opposite RS-232
levels. Each transmitter is loaded with 3kΩ in parallel
with 2500pF. The transmitter outputs display no ringing
or undesirable transients as they come out of shut-
down. Note that the transmitters are enabled only when
the magnitude of V- exceeds approximately 3V. Figure
7a shows the MAX3243’s transmitter output voltage
vs. current per transmitter. Figure 7b is a mouse driver
test circuit.
Interconnection with 3V and 5V Logic
The MAX3221/MAX3223/MAX3243 can directly inter-
face with various 5V logic families, including ACT and
HCT CMOS. See Table 4 for more information on possi-
ble combinations of interconnections.
Mouse Driveability (MAX3243)
The MAX3243 has been specifically designed to power
serial mice while operating from low-voltage power sup-
plies. It has been tested with samples of ten major mouse
models from six manufacturers including the leading
three: Logitech (5 models), Mouse Systems, and
Microsoft. The MAX3243 successfully drove all serial
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