+3.0V to +5.5V, 10nA, 250kbps RS-232 Transceivers

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

_______________________________________________________________________________________

7

AutoShutdown Plus Mode

A 10nA supply current is achieved with Maxim’s

AutoShutdown Plus feature, which operates when

FORCEOFF is low and FORCEON is high. When the

MAX3238E/MAX3248E sense no valid signal transitions

on all receiver and transmitter inputs for 30s, the on-

board power supply and drivers are shut off, reducing

supply current to 1µA. If the receiver inputs are in the

invalid range (-0.3V < R_IN < +0.3V) and the transmit-

reduced to 10nA. This occurs if the RS-232 cable is

disconnected or if the connected peripheral transmit-

ters are turned off. The system turns on again when a

valid transition is applied to any RS-232 receiver or

transmitter input. As a result, the system saves power

without changes to the existing BIOS or operating sys-

tem. The INVALID output is high when the receivers are

active. Since INVALID indicates the receiver inputs’

condition, it can be used in any mode (Figure 3).

MAX3238E/

MAX3248E

I/O

CHIP

WITH

UART

CPU

RS-232

POWER

MANAGEMENT

UNIT OR

KEYBOARD

CONTROLLER

FORCEOFF

FORCEON

INVALID

T1

T2

T3

T4

T5

R1

R2

R3

Figure 1. Interface Under Control of PMU

MAX3238E

MAX3248E

T1OUT

R1OUTB

Tx

5k

Ω

UART

T1IN

THREE-STATED

LOGIC

TRANSITION

DETECTOR

R1IN

PROTECTION

DIODE

R1OUT

FORCEOFF = GND

TO

µP

Rx

PREVIOUS

RS-232

Tx

UART

SHDN = GND

GND

Rx

5k

Ω

a) OLDER RS-232: POWERED-DOWN UART DRAWS CURRENT FROM ACTIVE

RECEIVER OUTPUT IN SHUTDOWN.

b) NEW MAX3238E/MAX3248E: IN SHUTDOWN, R1OUTB IS USED TO MONITOR

EXTERNAL DEVICES AND R1OUT IS THREE STATED, ELIMINATING A CURRENT

PATH THROUGH THE UART'S PROTECTION DIODE.

GND

PROTECTION

DIODE

I

I

Figure 2. MAX3238E/MAX3248E detect RS-232 activity when

the UART and interface are shut down.