MAX3228/MAX3229 feature a separate logic supply
input (V
L
) that sets V
OH
for the receiver and
INVALID
outputs. The transmitter inputs (T_IN), FORCEON and
FORCEOFF
, are also referred to V
L
. This feature allows
maximum flexibility in interfacing to different systems
and logic levels. Connect V
L
to the system
’
s logic sup-
ply voltage (+1.65V to +5.5V), and bypass it with a
0.1μF capacitor to GND. If the logic supply is the same
as V
CC
, connect V
L
to V
CC
. Always enable V
CC
before
enabling the V
L
supply. V
CC
must be greater than or
equal to the V
L
supply.
Software-Controlled Shutdown
If direct software control is desired, connect
FORCEOFF
and FORCEON together to disable AutoShutdown. The
microcontroller then drives
FORCEOFF
and FORCEON
like a
SHDN
input,
INVALID
can be used to alert the
microcontroller to indicate serial data activity.
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. However, ceramic chip
capacitors with an X7R or X5R dielectric work best. The
charge pump requires 0.1μF capacitors for 3.3V opera-
tion. For other supply voltages, refer to Table 4 for
required capacitor values. Do not use values smaller
than those listed in Table 4. Increasing the capacitor
values (e.g., by a factor of 2) reduces ripple on the
transmitter outputs and slightly reduces power con-
sumption. 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 capac-
itors).
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) usually 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-
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.
M
+2.5V to +5.5V RS-232 Transceivers
in UCSP
_______________________________________________________________________________________
9
Table 3. Output Control Truth Table
TRANSCEIVER STATUS
Shutdown (AutoShutdown)
Shutdown (Forced Off)
Normal Operation (Forced On)
Normal Operation (AutoShutdown)
FORCEON
Low
X
High
Low
FORCEOFF
High
Low
High
High
RECEIVER STATUS
High-Z
High-Z
Active
Active
INVALID
L
H
X = Don’t care.
= I
NVALID
output state is determined by R_IN input levels.
V
CC
(V)
2.5 to 3.0
3.0 to 3.6
4.5 to 5.5
3.0 to 5.5
C1, C
BYPASS
(
μ
F)
0.22
0.1
0.047
0.22
C2, C3, C4 (
μ
F)
0.22
0.1
0.33
1
Table 4. Required Capacitor Values
Figure 5. Transmitter Outputs Exiting Shutdown or Powering Up
5V/div
FORCEON =
FORCEOFF
T
OUT
0
2V/div
4
μ
s/div
0