7

mode. For always enabled operation, FORCEON and

FORCEOFF are both strapped high. To switch between

active and powerdown modes, under logic or software

control, only the FORCEOFF input need be driven. The

FORCEON state isn’t critical, as FORCEOFF dominates

over FORCEON. Nevertheless, if strictly manual control over

powerdown is desired, the user must strap FORCEON high

to disable the automatic powerdown circuitry.

Connecting FORCEOFF and FORCEON together disables

the automatic powerdown feature, enabling them to function

as a manual SHUTDOWN input (see Figure 4).

The time to recover from automatic powerdown mode is

typically 100

µs.

Automatic Powerdown

Even greater power savings is available by using the

automatic powerdown function. When no valid RS-232

voltages (see Figure 6) are sensed on any receiver input for

30

µs, the charge pump and transmitters powerdown, thereby

reducing supply current to 10nA. Invalid receiver levels occur

whenever the driving peripheral’s outputs are shut off

(powered down) or when the RS-232 interface cable is

disconnected. The ISL422XE powers back up whenever it

detects a valid RS-232 voltage level on any receiver input.

This automatic powerdown feature provides additional

system power savings without changes to the existing

operating system.

Automatic powerdown operates when the FORCEON input

is low, and the FORCEOFF input is high. Tying FORCEON

high disables automatic powerdown, but manual powerdown

is always available via the overriding FORCEOFF input.

Table 2 summarizes the automatic powerdown functionality.

Some applications may need more time to wake up from

shutdown. If automatic powerdown is being utilized, the RS-

232 device will reenter powerdown if valid receiver levels

aren’t reestablished within 30

µs of the ISL422XE powering

up. Figure 5 illustrates a circuit that keeps the ISL422XE

TABLE 2. POWERDOWN AND ENABLE LOGIC TRUTH TABLE

RS-232 SIGNAL

PRESENT AT

RECEIVER INPUT?

FORCEOFF

INPUT

FORCEON

INPUT

EN

INPUT

TRANSMITTER

OUTPUTS

RECEIVER

OUTPUTS

INVALID

OUTPUT

MODE OF OPERATION

NO

H

H

L

Active

Active

L

Normal Operation

(Auto Powerdown Disabled)

NO

H

H

H

Active

High-Z

L

YES

H

L

L

Active

Active

H

Normal Operation

(Auto Powerdown Enabled)

YES

H

L

H

Active

High-Z

H

NO

H

L

L

High-Z

Active

L

Powerdown Due to Auto Powerdown

Logic

NO

H

L

H

High-Z

High-Z

L

YES

L

X

L

High-Z

Active

H

Manual Powerdown

YES

L

X

H

High-Z

High-Z

H

Manual Powerdown w/Rcvr. Disabled

NO

L

X

L

High-Z

Active

L

Manual Powerdown

NO

L

X

H

High-Z

High-Z

L

Manual Powerdown w/Rcvr. Disabled

FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN

WHEN NO VALID RECEIVER SIGNALS ARE

PRESENT

PWR

FORCEOFF

INVALID

CPU

I/O

FORCEON

ISL422XE

MGT

LOGIC

UART

FIGURE 5. CIRCUIT TO PREVENT AUTO POWERDOWN FOR

100ms AFTER FORCED POWERUP

ISL422XE

FORCEOFF

FORCEON

POWER

MASTER POWERDOWN LINE

1M

Ω

0.1

µF

MANAGEMENT

UNIT

ISL4221E, ISL4223E