+3.0V to +5.5V, 1µA, up to 250kbps, True

RS-232 Transceiver with AutoShutdown Plus™

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5

_______________Detailed Description

Dual Charge-Pump Voltage Converter

The MAX3238’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

over the 3.0V to 5.5V range. 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.

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 5.0V EIA/TIA-232 levels. The

MAX3238 transmitters 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 par-

alleled to drive multiple receivers. Figure 1 shows a

complete system connection.

LapLink is a trademark of Traveling Software.

When

FORCEOFF is driven to ground, the transmitters

and receivers are disabled and the outputs go high

impedance, except for R1OUTB. 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, but the receivers remain active.

When the power is off, the MAX3238 permits the out-

puts to be driven up to ±12V.

The transmitter inputs do not have pull-up resistors.

RS-232 Receivers

The receivers convert RS-232 signals to CMOS-logic

output levels. All receivers have inverting three-state

outputs and are inactive in shutdown (

FORCEOFF)

(Table 1). The MAX3238 also features an extra, always-

active noninverting output, R1OUTB. This extra output

monitors receiver activity while the other receivers are

high impedance, allowing Ring Indicator to be moni-

tored without forward biasing other devices connected

to the receiver outputs. This is ideal for systems where

als, such as UARTs (Figure 2).

Table 1. Output Control Truth Table

FORCEON

FORCEOFF

AUTOSHUTDOWN

PLUS*

OPERATION

STATUS

T_OUT

R_OUT

R1OUTB

X

0

X

Shutdown

(Forced Off)

High-Z

High-Z

Active

1

1

X

Normal Operation

(Forced On)

Active

Active

Active

0

1

<30sec*

Normal

Operation

(AutoShutdown

Plus)

Active

Active

Active

0

1

>30sec*

Shutdown

(AutoShutdown

Plus)

High-Z

Active

Active

X = Don’t Care

* Time since last receiver or transmitter input transition.