Datasheet

MAX3238
+3.0V to +5.5V, 1µA, up to 250kbps, True
RS-232 Transceiver with AutoShutdown Plus
_______________________________________________________________________________________ 7
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).
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.
__________Applications Information
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).
Table 2.
IINNVVAALLIIDD
Truth Table
+0.3V
-0.3V
INVALID
R_IN
INVALID ASSERTED IF ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR
AT LEAST 30µs.
30µs
TIMER
R
+2.7V
-2.7V
INVALID
R_IN
INVALID DEASSERTED IF ANY RECEIVER INPUT HAS BEEN BETWEEN +2.7V AND -2.7V
FOR LESS THAN 30µs.
30µs
TIMER
R
AUTOSHDN
R_IN
T_IN
R
S
30sec
TIMER
EDGE
DETECT
EDGE
DETECT
FORCEOFF
FORCEON
Figure 3a. INVALID Functional Diagram, INVALID Low
Figure 3b. INVALID Functional Diagram, INVALID High
Figure 3c. AutoShutdown Plus Logic
POWERDOWN*
AUTOSHDN
FORCEOFF
FORCEON
* POWERDOWN IS ONLY AN INTERNAL SIGNAL.
IT CONTROLS THE OPERATIONAL STATUS OF
THE TRANSMITTERS AND THE POWER SUPPLIES.
Figure 3d. Power-Down Logic
L
No
HYes
IINNVVAALLIIDD
OUTPUT
RS-232 SIGNAL PRESENT AT
ANY RECEIVER INPUT