MAX3228E/AE and MAX3229E/AE maintain the outputs

in regulation until the battery voltage drops below +3.1V.

Then the output regulation points change to ±4.0V

age of +3.5V, the charge pump switches modes to

generate an output of ±5.5V.

Table 1 shows different supply schemes and their oper-

ating voltage ranges.

RS-232 Transmitters

The transmitters are inverting level translators that con-

vert CMOS-logic levels to RS-232 levels. The

MAX3228E/AE and MAX3229E/AE will automatically

reduce the RS-232 compliant levels (±5.5V) to RS-232

approximately +3.1V. The reduced levels also reduce

supply current requirements, extending battery life.

ensures that the RS-232 output levels do not change if

supply voltage to drop slightly. The outputs will return to

approximately +3.5V.

The MAX3228E/AE and MAX3229E/AE transmitters

guarantee a 250kbps data rate with worst-case loads of

3kΩ in parallel with 1000pF.

When

FORCEOFF is driven to ground, the transmitters

and receivers are disabled and the outputs become

high impedance. When the AutoShutdown circuitry

senses that all receiver and transmitter inputs are inac-

tive for more than 30µs, the transmitters are disabled

and the outputs go to a high-impedance state. When

the power is off, the MAX3228E/AE and MAX3229E/AE

permit the transmitter outputs to be driven up to ±12V.

The transmitter inputs do not have pullup resistors.

RS-232 Receivers

The MAX3228E/AE and MAX3229E/AE receivers con-

vert RS-232 signals to logic output levels. All receivers

have inverting three-state outputs and can be active or

inactive. In shutdown (

FORCEOFF = low) or in

AutoShutdown, the MAX3228E/AE and MAX3229E/AE

receivers are in a high-impedance state (Table 3).

The MAX3228E/AE and MAX3229E/AE feature an

INVALID output that is enabled low when no valid

RS-232 signal levels have been detected on any

receiver inputs.

INVALID is functional in any mode

(Figures 2 and 3).

Figure 2c. MAX322_E AutoShutdown Logic

FORCEOFF

POWER DOWN

INVALID

FORCEON

INVALID IS AN INTERNALLY GENERATED SIGNAL

THAT IS USED BY THE AUTOSHUTDOWN LOGIC

AND APPEARS AS AN OUTPUT OF THE DEVICE.

POWER DOWN IS ONLY AN INTERNAL SIGNAL.

IT CONTROLS THE OPERATIONAL STATUS OF

THE TRANSMITTERS AND THE POWER SUPPLIES.

Figure 2a. MAX322_E Entering 1µA Supply Mode via

AutoShutdown

+0.3V

-0.3V

INVALID

TO MAX322 _E

POWER SUPPLY

AND TRANSMITTERS

R_IN

*TRANSMITTERS ARE DISABLED, REDUCING SUPPLY CURRENT TO 1

μA IF

ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR AT LEAST 30

μs.

30

μs

COUNTER

R

Figure 2b. MAX322_E with Transmitters Enabled Using

AutoShutdown

+2.7V

-2.7V

INVALID

TO MAX322 _E

POWER SUPPLY

R_IN

*TRANSMITTERS ARE ENABLED IF:

ANY RECEIVER INPUT IS GREATER THAN +2.7V OR LESS THAN -2.7V.

ANY RECEIVER INPUT HAS BEEN BETWEEN +0.3V AND -0.3V FOR LESS THAN 30

μs.

30

μs

COUNTER

R

±15kV ESD-Protected +2.5V to +5.5V

RS-232 Transceivers in UCSP and WLP

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