Applications Information
Using
INVALID
INVALID indicates when an RS-232 signal is present at
the receiver input, and therefore when the port is in use.
INVALID can be used in alternative shutdown control
schemes where it relieves the processor from constantly
polling the port for activity.
Capacitor Selection
The capacitor type used for C1–C4 is not critical for
proper operation; either polarized or nonpolarized
capacitors are acceptable. If polarized capacitors are
used, connect polarity as shown in the
Typical Operating
Circuit
. The charge pump requires 0.1µF capacitors.
Increasing the capacitor values (e.g., by a factor of 2)
reduces ripple on the transmitter output and slightly
reduces power consumption. C2, C3, and C4 can be
increased without changing C1’s value. However, do not
increase C1’s value without also increasing the values of
C2, C3, and C4 to maintain the proper ratios (C1 to the
other capacitors).
When using the minimum 0.1µF capacitors, make sure
the capacitance does not degrade excessively with
temperature. If in doubt, use capacitors with a larger
nominal value. The capacitor’s equivalent series resis-
tance (ESR) usually rises at low temperatures and influ-
ences the amount of ripple on V+ and V-.
Power-Supply Decoupling
In most circumstances, a 0.1µF V
CC
bypass capacitor
is adequate. In applications that are sensitive to power-
supply noise, use a capacitor of the same value as the
charge-pump capacitor C1. Connect bypass capaci-
tors as close to the IC as possible.
Transmitter Output when
Exiting Shutdown
Figure 5 shows the transmitter output when exiting shut-
down mode. The transmitter is loaded with 3kΩ in paral-
lel with 2500pF. The transmitter output displays no
ringing or undesirable transients as the MAX221 comes
out of shutdown. Note that the transmitter is enabled only
when the magnitude of V- exceeds approximately 3V.
High Data Rates
The MAX221 maintains the RS-232 ±5.0V minimum
transmitter output voltage even at high data rates.
Figure 6 shows a transmitter loopback test circuit.
Figure 7 shows the loopback test result at 120kbps,
and Figure 8 shows the same test at 250kbps.
MAX221
+5V, 1µA, Single RS-232 Transceiver
with AutoShutdown
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