+3.0V to +5.5V, 1µA, up to 250kbps, True
RS-232 Transceiver with AutoShutdown Plus™
Table 2.
INVALID
Truth Table
RS-232 SIGNAL PRESENT AT
ANY RECEIVER INPUT
Yes
No
MAX3238
+0.3V
INVALID
OUTPUT
H
L
R_IN
-0.3V
30µs
TIMER
R
INVALID
INVALID ASSERTED IF ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR
AT LEAST
30µs.
Figure 3a.
INVALID
Functional Diagram,
INVALID
Low
Tables 1 and 2 and Figure 3 summarize the MAX3238’s
operating modes. FORCEON and
FORCEOFF
override
the automatic circuitry and force the transceiver into its
normal operating state or into its low-power standby
state. When neither control is asserted, the IC enters
AutoShutdown Plus mode and selects between these
states automatically, based on the last receiver or
transmitter input edge received.
When shut down, the device’s charge pumps turn off,
V+ decays to V
CC,
V- decays to ground, and the trans-
mitter outputs are disabled (high impedance). The time
required to exit shutdown is typically 25µs (Figure 4b).
+2.7V
R_IN
-2.7V
30µs
TIMER
R
INVALID
INVALID DEASSERTED IF ANY RECEIVER INPUT HAS BEEN BETWEEN +2.7V AND -2.7V
FOR
LESS THAN
30µs.
Figure 3b.
INVALID
Functional Diagram,
INVALID
High
Software-Controlled Shutdown
If direct software control is desired, use
INVALID
to
indicate DTR or Ring Indicator signal. Tie
FORCEOFF
and FORCEON together to bypass the AutoShutdown
feature so the line acts like a
SHDN
input.
T_IN
EDGE
DETECT
FORCEOFF
S
30sec
TIMER
R
__________Applications Information
R_IN
AUTOSHDN
Capacitor Selection
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).
EDGE
DETECT
FORCEON
Figure 3c. AutoShutdown Plus Logic
FORCEOFF
POWERDOWN*
FORCEON
AUTOSHDN
* POWERDOWN IS ONLY AN INTERNAL SIGNAL.
IT CONTROLS THE OPERATIONAL STATUS OF
THE TRANSMITTERS AND THE POWER SUPPLIES.
Figure 3d. Power-Down Logic
_______________________________________________________________________________________
7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
