+2.35V to +5.5V, 1µA, 2Tx/2Rx RS-232 Transceivers

with ±15kV ESD-Protected I/O and Logic Pins

6

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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

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-

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).

Pin Description

PIN

NAME

FUNCTION

ESD

PROTECTED

1

C1+

Positive Terminal of Voltage-Doubler Charge-Pump Capacitor

—

2

V+

+5.5V/+4.0V Generated by the Charge Pump

—

3

C1-

Negative Terminal of Voltage-Doubler Charge-Pump Capacitor

—

4

C2+

Positive Terminal of Inverting Charge-Pump Capacitor

—

5

C2-

Negative Terminal of Inverting Charge-Pump Capacitor

—

6

V-

-5.5V/-4.0V Generated by the Charge Pump

—

7

INVALID

INVALID is asserted if any inputs of the receivers are in an invalid state;

—

8, 9

T_IN

✔

10, 11

R_OUT

✔

12

FORCEON

Force-On Input. Drive high to override automatic circuitry keeping transmitters on

(

FORCEOFF must be high) (Table 1).

✔

13

Logic Level Supply. +1.65V to +5.5V, sets CMOS logic thresholds and CMOS

outputs.

—

14, 15

R_IN

RS-232 Receiver Inputs (R2IN, R1IN)

✔

16, 17

T_OUT

RS-232 Transmitter Outputs (T2OUT, T1OUT)

✔

18

GND

Ground

—

19

FORCEOFF

Force-Off Input. Drive low to shut down transmitters and on-board power supply.

This overrides all automatic circuitry and FORCEON (Table 1).

✔

—

20

+2.35V to +5.5V Supply Voltage