_______________Detailed Description

Dual Charge-Pump Voltage Converter

The MAX3233E/MAX3235E’s internal power supply

consists of a regulated dual charge pump that provides

output voltages of +5.5V (doubling charge pump) and

-5.5V (inverting charge pump), with no external capaci-

tors. The charge pump operates in discontinuous

mode: if the output voltages are less than 5.5V, the

charge pump is enabled; if the output voltages exceed

5.5V, the charge pump is disabled.

RS-232 Transmitters

The transmitters are inverting level translators that

convert CMOS-logic levels to 5.0V EIA/TIA-232 levels.

The devices guarantee a 250kbps data rate with worst-

case loads of 3k

Ω in parallel with 1000pF, providing

compatibility with PC-to-PC communication software

(such as LapLink™). Transmitters can be paralleled to

drive multiple receivers. Figure 1 shows a complete

system connection.

When FORCEOFF is driven to ground or when the Auto-

Shutdown Plus circuitry senses that all receiver and

transmitter inputs are inactive for more than 30sec, the

transmitters are disabled and the outputs go into a high-

impedance state. When powered off or shut down, the

outputs can be driven to ±12V. The transmitter inputs

do not have pull-up resistors. Connect unused inputs to

RS-232 Receivers

The receivers convert RS-232 signals to CMOS-logic

output levels. They feature inverting outputs that always

remain active (Table 1).

The MAX3233E/MAX3235E feature an INVALID output

that is enabled low when no valid RS-232 voltage levels

have been detected on all receiver inputs. Because

INVALID indicates the receiver input’s condition, it is

independent of FORCEON and FORCEOFF states

(Figures 2 and 3).

AutoShutdown Plus Mode

The devices achieve a 1µA supply current with Maxim’s

AutoShutdown Plus feature, which operates when

FORCEOFF is high and a FORCEON is low. When these

devices do not sense a valid signal transition on any

receiver or transmitter input for 30sec, the on-board

charge pumps are shut down, reducing supply current

to 1µA. This occurs if the RS-232 cable is disconnected,

or if the connected peripheral transmitters are turned off

and the UART driving the transmitter inputs is inactive.

The system turns on again when a valid transition is

±15kV ESD-Protected, 1µA, 250kbps, 3.3V/5V, Dual

RS-232 Transceivers with Internal Capacitors

6

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MAX3233E

MAX3235E

I/O

CHIP

WITH

UART

CPU

RS-232

POWER-

MANAGEMENT

UNIT OR

KEYBOARD

CONTROLLER

FORCEOFF

FORCEON

INVALID

Figure 1. Interface Under Control of PMU

LapLink is a trademark of Traveling Software.

+0.3V

-0.3V

INVALID

R_IN

INVALID ASSERTED IF ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR

AT LEAST 80

µs.

80µs

TIMER

R

+2.7V

-2.7V

INVALID

R_IN

INVALID DEASSERTED IF ANY RECEIVER INPUT HAS BEEN ABOVE +2.7V OR BELOW

-2.7V FOR 1

µs.

80µs

TIMER

R

Figure 2a. INVALID Functional Diagram, INVALID Low

Figure 2b. INVALID Functional Diagram, INVALID High