Table 3. Required Minimum Capacitance
Values
M
__________Applic ations Information
Capac itor S elec tion
The capacitor type used for C1–C4 is not critical for
proper operation; polarized or nonpolarized capacitors
can be used. The charge pump requires 0.1μF capaci-
tors for 3.3V operation. For other supply voltages, see
Table 3 for required capacitor values. Do not use val-
ues 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, C4,
and C
BYPASS
, 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 capaci-
tors with a larger nominal value. The capacitor’s equiv-
alent series resistance (ESR), which usually rises at low
temperatures, influences the amount of ripple on V+
and V-.
Power-S upply Dec oupling
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
charge-pump capacitor C1. Connect bypass capaci-
tors as close to the IC as possible.
T ransmitter Outputs
when Exiting S hutdown
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 lev-
els (one transmitter input is high; the other is low). Each
transmitter is loaded with 3k
in parallel with 1000pF.
The transmitter outputs display no ringing or undesir-
able transients as they come out of shutdown. Note that
the transmitters are enabled only when the magnitude
of V- exceeds approximately -3V.
High Data Rates
The MAX3224/MAX3226/MAX3244 maintain the RS-232
±5.0V minimum transmitter output voltage even at high
data rates. Figure 7 shows a transmitter loopback test
circuit. Figure 8 shows a loopback test result at
120kbps, and Figure 9 shows the same test at
250kbps. For Figure 8, all transmitters were driven
1μA S upply Current, 1Mbps, 3.0V to 5.5V,
RS -232 Transc eivers with AutoS hutdown Plus
12
______________________________________________________________________________________
V
CC
(V)
C2, C3, C4
(μF)
0.1
0.33
1
3.0 to 3.6
4.5 to 5.5
3.0 to 5.5
C1, C
BYPASS
(μF)
0.1
0.047
0.22
5
μ
s/div
T1OUT
FORCEON =
FORCEOFF
T2OUT
READY
5V/div
0
2V/div
0
5V/div
0
V
CC
= 3.3V
C1–C4 = 0.1
μ
F
Figure 6. Transmitter Outputs when Exiting Shutdown or
Powering Up
MAX3224
MAX3225
MAX3226
MAX3227
MAX3244
MAX3245
5k
R_ IN
R_ OUT
FORCEON
C2-
C2+
C1-
C1+
V-
V+
V
CC
C4
C3*
C1
C2
C
BYPASS
V
CC
FORCEOFF
*C3 CAN BE RETURNED TOV
CC
OR GND.
T_ OUT
T_ IN
GND
V
CC
1000pF
Figure 7. Loopback Test Circuit