±15kV ESD-Protected, +5V, 1µA, Single RS-232
Transceiver with AutoShutdown
__________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 part for activity.
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-.
MAX221E
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.
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
Transmitter Output when
Exiting Shutdown
Figure 7 shows the transmitter output when exiting
shutdown mode. The transmitter is loaded with 3kΩ in
parallel with 1000pF. The transmitter output displays no
ringing or undesirable transients as the MAX221E
comes out of shutdown.
High Data Rates
The MAX221E maintains 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 the loopback test result at 120kbps,
and Figure 10 shows the same test at 250kbps.
+5V
5V/div
TIN = GND
FORCEON =
FORCEOFF
0.1µF
C3
V
CC
C1+
C1
C1-
5V/div
TOUT
C2
C2-
TIN = V
CC
TIN
ROUT
50µs/div
0V
EN
FORCEOFF
FORCEON
C2+
V+
MAX221E
V-
C4
TOUT
RIN
5k
1000pF
Figure 7. Transmitter Output when Exiting Shutdown or
Powering Up
V
CC
GND
INVALID
Figure 8. Loopback Test Circuit
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
