max221e Maxim Integrated Products, Inc., max221e Datasheet - Page 9

• Manufacturer Part Number: max221e
• Description: Max221e ?15kv Esd-protected, +5v, 1?a, Single Rs-232 Transceiver With Autoshutdown
• Manufacturer: Maxim Integrated Products, Inc.

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.
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
Figure 7. Transmitter Output when Exiting Shutdown or
Powering Up
__________Applications Information
±15kV ESD-Protected, +5V, 1µA, Single RS-232
5V/div
5V/div
_______________________________________________________________________________________
50 s/div
Capacitor Selection
TIN = GND
TIN = V
Using I I N N V V A A L L I I D D
CC
Transceiver with AutoShutdown
FORCEON =
FORCEOFF
TOUT
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-.
In most circumstances, a 0.1µF V
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.
Figure 7 shows the transmitter output when exiting
shutdown mode. The transmitter is loaded with 3k
parallel with 1000pF. The transmitter output displays no
ringing or undesirable transients as the MAX221E
comes out of shutdown.
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.
Figure 8. Loopback Test Circuit
V
C1
C2
0V
CC
+5V
0.1 F
FORCEOFF
TIN
ROUT
EN
FORCEON
C1+
C1-
C2+
C2-
Power-Supply Decoupling
Transmitter Output when
MAX221E
GND
V
CC
Exiting Shutdown
CC
INVALID
High Data Rates
5k
TOUT
RIN
V+
V-
bypass capacitor
C3
C4
1000pF
in
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