1µA Supply Current, 1Mbps, 3.0V to 5.5V,
RS-232 Transceivers with AutoShutdown Plus
MAX3224–MAX3227/MAX3244/MAX3245
†
+0.3V
T_IN
EDGE
DETECT
FORCEOFF
S
30sec
TIMER
R
R_IN
-0.3V
30µs
TIMER
R
AUTOSHDN
INVALID
R_IN
EDGE
DETECT
INVALID ASSERTED IF ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR
AT LEAST
30μs.
FORCEON
Figure 3a.
INVALID
Functional Diagram,
INVALID
Low
Figure 3c. AutoShutdown Plus Logic
+2.7V
FORCEOFF
POWERDOWN*
R_IN
FORCEON
30µs
TIMER
R
INVALID
AUTOSHDN
-2.7V
INVALID DEASSERTED IF ANY RECEIVER INPUT HAS BEEN BETWEEN +2.7V AND -2.7V
FOR
LESS THAN
30μs.
* POWERDOWN IS ONLY AN INTERNAL SIGNAL.
IT CONTROLS THE OPERATIONAL STATUS OF
THE TRANSMITTERS AND THE POWER SUPPLIES.
Figure 3b.
INVALID
Functional Diagram,
INVALID
High
Figure 3d. Power-Down Logic
Table 2.
INVALID
Truth Table
RS-232 SIGNAL
PRESENT AT ANY
RECEIVER INPUT
Yes
No
INVALID HIGH
INVALID
OUTPUT
RECEIVER INPUT LEVELS
+2.7V
INDETERMINATE
+0.3V
0
-0.3V
INDETERMINATE
-2.7V
INVALID HIGH
High
Low
INVALID LOW
peripheral transmitters are turned off, and the UART dri-
ving the transmitter inputs is inactive. The system turns
on again when a valid transition is applied to any
RS-232 receiver or transmitter input. As a result, the sys-
tem saves power without changes to the existing BIOS
or operating system.
Figures 3a and 3b depict valid and invalid RS-232
receiver voltage levels.
INVALID
indicates the receiver
input’s condition, and is independent of FORCEON and
FORCEOFF
states. Figure 3 and Tables 1 and 2 sum-
marize the operating modes of the MAX3224–
MAX3227/MAX3244/MAX3245 devices. FORCEON and
FORCEOFF
override AutoShutdown Plus circuitry.
When neither control is asserted, the IC selects
Figure 4a. Receiver Positive/Negative Thresholds for
INVALID
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+ is pulled to V
CC
, V- is pulled to ground, the transmit-
ter outputs are high impedance, and READY
(MAX3224–MAX3227) is driven low. The time required
to exit shutdown is typically 100µs (Figure 8).
10
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