Rainbow Electronics MAX13223E - Manual (Page 9)

Applications Information
Capacitor Selection
The capacitor type used for C1–C4 is not critical for
proper operation; either polarized or nonpolarized
capacitors may be used. The charge pump requires
0.1µF capacitors for 3.3V operation. For other supply
voltages, see Table 3 for required capacitor values. Do
not use values smaller than those listed in Table 3.
Increasing the capacitor values (e.g., by a factor of 2)
reduces ripple on the transmitter outputs and slightly
reduces power consumption. C2, C3, and C4 can be
increased without changing C1’s value. However, do
not increase C1 without also increasing the values
of C2, C3, and C4 to maintain the proper ratios (C1
to the other capacitors). When using the minimum
required capacitor values, make sure the capacitor
value does not degrade excessively with temperature.
If in doubt, use capacitors with a larger nominal value.
The capacitor’s equivalent series resistance (ESR) usu-
ally rises at low temperatures and influences 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-sup-
ply noise, decouple V
CC
to ground with a capacitor of the
same value as the charge-pump capacitor C1. Connect
bypass capacitors as close to the IC as possible.
Transmitter Outputs when
Exiting Shutdown
Figure 7 shows two transmitter outputs when exiting
shutdown mode. As they become active, the two trans-
mitter outputs are shown going to opposite RS-232 lev-
els. Each transmitter is loaded with 3k
Ω 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.
MAX13223E
±70V Fault-Protected, 3.0V to 5.5V,
2Tx/2Rx RS-232 Transceiver
_______________________________________________________________________________________ 9
Table 2. AutoShutdown Control
INPUTS OUTPUTS
FORCEOFF FORCEON
VALID RECEIVER INPUT
LEVEL
INVALID
OUTPUT
OPERATION MODE T_OUT
0 X No 0 Shutdown (Forced Off) High-Z
0 X Yes 1 Shutdown (Forced Off) High-Z
1 0 No 0 Shutdown (AutoShutdown) High-Z
1 0 Yes 1 Active (AutoShutdown) Active
1 1 No 0 Active (Forced On) Active
1 1 Yes 1 Active (Forced On) Active
Table 3. Required Capacitor Values
V
CC
(V) C1, C
BYPASS
(µF) C2, C3 (µF) C4 (µF)
3.0 to 3.6 0.1 0.1 1
4.5 to 5.5 0.047 0.33 1
X = Don’t Care.
40 µs/div
5V/div
2V/div
T2OUT
T1OUT
V
CC
= +3.3V
C1–C4 = 0.1 µF
FORCEON =
FORCEOFF
Figure 7. Transmitter Outputs when Exiting Shutdown or
Powering Up