MAX3320A Datasheet, PDF (6/12 Pages) Maxim Integrated Products – 3V to 5.5V, up to 250kbps True RS-232 Transceiver with 4μA AutoShutdown Plus and Power-On Reset

3V to 5.5V, up to 250kbps True RS-232 Transceiver
with 4µA AutoShutdown Plus and Power-On Reset
Table 1. MAX3320 Output Control Truth Table
FORCEON
FORCEOFF
AUTOSHUTDOWN
PLUS*
X
0
X
1
1
X
0
1
<30sec*
0
1
>30sec*
OPERATION
STATUS
Shutdown
(forced off)
Normal operation
(forced on)
Normal operation
(AutoShutdown Plus)
Shutdown
(AutoShutdown Plus)
T_OUT
High-Z
Active
Active
High-Z
R_OUT
Active
Active
Active
Active
X = Don’t Care
*Time since last receiver or transmitter input activity
______________ Detailed Description
Dual Charge-Pump Voltage Converter
The MAX3320’s internal power supply consists of a reg-
ulated dual charge pump that provides output voltages
of +5.5V (doubling charge pump) and -5.5V (inverting
charge pump) while V_CC remains in the 3V to 5.5V
range. The charge pumps operate in discontinuous
mode; they are enabled if the output voltages are less
than 5.5V, and disabled if output voltages exceed 5.5V.
Each charge pump requires a flying capacitor (C1, C2)
and a reservoir capacitor (C3, C4) to generate the V+
and V- supplies.
RS-232 Transmitters
The transmitters are inverting level translators that con-
vert CMOS-logic levels to 5V EIA/TIA-232 levels. The
MAX3320 transmitters guarantee a 250kbps data rate
with worst-case loads of 3kΩ in parallel with 1000pF,
providing compatibility with PC-to-PC communication
software (such as LapLink™). Transmitters can be par-
alleled to drive multiple receivers. Figure 1 shows a
complete system connection.
When FORCEOFF is driven to ground, the transmitter’s
outputs become high impedance. When the
AutoShutdown Plus circuitry senses that all receiver
and transmitter inputs are inactive for more than 30sec,
the transmitters turn off and the outputs go into a high-
impedance state, but the receivers remain active.
When the power is off or the MAX3320 is shut down,
outputs may be driven up to ±12V.
The transmitter inputs do not have pull-up resistors.
Connect unused inputs to GND or V_CC.
Laplink is a trademark of Traveling Software.
POWER-
MANAGEMENT
UNIT OR
KEYBOARD
CONTROLLER
FORCEOFF
FORCEON
INVALID
I/O
CHIP
WITH
UART
CPU
MAX3320
T1
T2
R1
R2
RS-232
Figure 1. Interface Under Control of PMU
RS-232 Receivers
The receivers convert RS-232 signals to CMOS-logic out-
put levels. All receivers have inverting outputs and are
active in shutdown FORCEOFF (Table 1).
AutoShutdown Plus Mode
Maxim’s AutoShutdown Plus feature, which operates
when FORCEOFF is high and FORCEON is low, achieves
a 4µA supply current. When the MAX3320 senses no
valid signal levels on all receiver and transmitter inputs for
30sec, the on-board power supply and drivers shut off,
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