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______________ Detailed Description

Dual Charge-Pump Voltage Converter

The MAX3320’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

range. The charge pumps operate in discontinuous

mode; they are enabled if the output voltages are less

than 5.5V, and disabled if output voltages exceed 5.5V.

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 5V EIA/TIA-232 levels. The

MAX3320 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.

When FORCEOFF is driven to ground, the transmitter’s

outputs become high impedance. When the

AutoShutdown Plus circuitry senses that all receiver

and transmitter inputs are inactive for more than 30sec,

the transmitters turn off and the outputs go into a high-

impedance state, but the receivers remain active.

When the power is off or the MAX3320 is shut down,

outputs may be driven up to ±12V.

The transmitter inputs do not have pull-up resistors.

Laplink is a trademark of Traveling Software.

RS-232 Receivers

The receivers convert RS-232 signals to CMOS-logic out-

put levels. All receivers have inverting outputs and are

active in shutdown FORCEOFF (Table 1).

AutoShutdown Plus Mode

Maxim’s AutoShutdown Plus feature, which operates

when FORCEOFF is high and FORCEON is low, achieves

a 4µA supply current. When the MAX3320 senses no

valid signal levels on all receiver and transmitter inputs for

30sec, the on-board power supply and drivers shut off,

MAX3320

I/O

CHIP

WITH

UART

CPU

RS-232

POWER-

MANAGEMENT

UNIT OR

KEYBOARD

CONTROLLER

FORCEOFF

FORCEON

INVALID

T1

T2

R1

R2

Figure 1. Interface Under Control of PMU

Table 1. MAX3320 Output Control Truth Table

FORCEOFF

0

High-Z

T_OUT

Active

R_OUT

1

Active

Active

Normal operation

(forced on)

X

1

1

Active

Active

Normal operation

(AutoShutdown Plus)

<30sec*

0

1

High-Z

Active

Shutdown

(AutoShutdown Plus)

>30sec*

0

FORCEON

Shutdown

(forced off)

X

X

OPERATION

STATUS

AUTOSHUTDOWN

PLUS*

X = Don’t Care

*

Time since last receiver or transmitter input activity

3V to 5.5V, up to 250kbps True RS-232 Transceiver

with 4µA AutoShutdown Plus and Power-On Reset