8

FN8037.7

June 10, 2010

ISL4241E and ISL4243E monitor receivers remain active

even during manual power-down, making them extremely

useful for Ring Indicator monitoring. Standard receivers

driving powered down peripherals must be disabled to

prevent current flow through the peripheral’s protection

diodes (see Figures 2 and 3). This renders them useless for

wake-up functions, but the corresponding monitor receiver

can be dedicated to this task as shown in Figure 3.

Low Power Operation

These 3V devices require a nominal supply current of

power-down mode). This is considerably less than the 5mA

to 11mA current required by comparable 5V RS-232 devices,

allowing users to reduce system power simply by switching

to this new family.

Power-down Functionality

The already low current requirement drops significantly

when the device enters power-down mode. In power-down,

supply current drops to 10nA, because the on-chip charge

and the transmitter outputs three-state. Inverting receiver

outputs disable only in manual power-down; refer to Table 2

for details. This micro-power mode makes the ISL4241E and

ISL4243E ideal for battery powered and portable

applications.

TABLE 2. POWER-DOWN AND ENABLE LOGIC TRUTH TABLE

RS-232

SIGNAL

PRESENT

AT

RECEIVER

INPUT?

SHDN OR

FORCEOFF

INPUT

FORCEON

INPUT

EN

INPUT

TRANSMITTER

OUTPUTS

RECEIVER

OUTPUTS

OUTPUTS

INVALID

OUTPUT

MODE OF OPERATION

ISL4241E

N.A.

L

N/A

L

High-Z

Active

Active

N/A

Manual Power-down

N.A.

L

N/A

H

High-Z

High-Z

Active

N/A

Manual Power-down w/Receiver

Disabled

N.A.

H

N/A

L

Active

Active

Active

N/A

Normal Operation

N.A.

H

N/A

H

Active

High-Z

Active

N/A

Normal Operation w/Receiver

Disabled

ISL4243E

NO

H

H

N/A

Active

Active

Active

L

Normal Operation

(Auto Power-down Disabled)

YES

H

L

N/A

Active

Active

Active

H

Normal Operation

(Auto Power-down Enabled)

NO

H

L

N/A

High-Z

Active

Active

L

Power-down Due to Auto Power-down

Logic

YES

L

X

N/A

High-Z

High-Z

Active

H

Manual Power-down

NO

L

X

N/A

High-Z

High-Z

Active

L

Manual Power-down

FIGURE 2. POWER DRAIN THROUGH POWERED DOWN

PERIPHERAL

OLD

POWERED

GND

SHDN = GND

Rx

Tx

CURRENT

FLOW

RS-232 CHIP

DOWN

UART

ISL4241E, ISL4243E