+2.35V to +5.5V, 1µA, 2Tx/2Rx RS-232 Transceivers
with ±15kV ESD-Protected I/O and Logic Pins
MAX3380E/MAX3381E
Pin Description
PIN
1
2
3
4
5
6
7
8, 9
10, 11
12
13
14, 15
16, 17
18
19
20
NAME
C1+
V+
C1-
C2+
C2-
V-
INVALID
T_IN
R_OUT
FORCEON
V
L
R_IN
T_OUT
GND
FORCEOFF
V
CC
FUNCTION
Positive Terminal of Voltage-Doubler Charge-Pump Capacitor
+5.5V/+4.0V Generated by the Charge Pump
Negative Terminal of Voltage-Doubler Charge-Pump Capacitor
Positive Terminal of Inverting Charge-Pump Capacitor
Negative Terminal of Inverting Charge-Pump Capacitor
-5.5V/-4.0V Generated by the Charge Pump
INVALID
is asserted if any inputs of the receivers are in an invalid state;
-0.3V < V
R
_
IN
< +0.3V
TTL/CMOS Transmitter Inputs Referenced to V
L
(T1IN, T2IN)
TTL/CMOS Receiver Outputs Referenced to V
L
(R2OUT, R1OUT)
Force-On Input. Drive high to override automatic circuitry keeping transmitters on
(FORCEOFF must be high) (Table 1).
Logic Level Supply. +1.65V to +5.5V, sets CMOS logic thresholds and CMOS
outputs.
RS-232 Receiver Inputs (R2IN, R1IN)
RS-232 Transmitter Outputs (T2OUT, T1OUT)
Ground
Force-Off Input. Drive low to shut down transmitters and on-board power supply.
This overrides all automatic circuitry and FORCEON (Table 1).
+2.35V to +5.5V Supply Voltage
ESD
PROTECTED
—
—
—
—
—
—
—
✔
✔
✔
—
✔
✔
—
✔
—
Detailed Description
The MAX3380E/MAX3381E are RS-232 transceivers that
maximize battery life by reducing current consumption
at low battery levels. When the supply voltage is above
+3.7V, the RS-232 outputs are at ±5.5V, which is com-
pliant with the RS-232 standard. As the supply voltage
drops below the +3.1V set point, the RS-232 outputs
change to ±3.7V, which is compatible with the RS-232
standard. The outputs will remain at the compatible lev-
els until the supply voltage rises above +3.5V, where
they return to compliant levels. 400mV of hysteresis pro-
tects against power-supply bounce that may cause
numerous mode changes.
Most devices that use charge pumps to double and
invert voltages consume higher current when the supply
voltage is less than half of the required output voltage.
This is due to the fact that the charge pump is constant-
ly operating because the output voltage is below the
regulation voltage. This requires more supply current
because the output will never reach the regulation volt-
age and switch off. The MAX3380E/MAX3381E reduce
6
the output voltage requirement allowing the charge
pump to operate with supply voltages down to +2.35V.
Dual-Mode Regulated Charge-Pump
Voltage Converter
The MAX3380E/MAX3381Es’ internal power supply is a
dual-mode regulated charge pump. The output regula-
tion point depends on V
CC
and the direction in which
V
CC
moves through the switchover region of +2.95V <
V
CC
< +3.7V.
For supply voltages above +3.7V, the charge pump will
generate +5.5V at V+ and -5.5V at V-. The charge
pumps operate in a discontinuous mode. If the output
voltages are less than ±5.5V, the charge pumps are
enabled; if the output voltages exceed ±5.5V, the
charge pumps are disabled.
For supply voltages below +2.95V, the charge pump
will generate +4.0V at V+ and -4.0V at V-. The charge
pumps operate in a discontinuous mode.
Each charge pump requires a flying capacitor (C1, C2)
and a reservoir capacitor (C3, C4) to generate the V+
and V- supplies (see
Typical Operating Circuit).
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