ICL3221EM, ICL3221EF

4. Application Information

FN7552 Rev.2.00

Page 13 of 22

May.29.19

With any of the control schemes, the time required to exit power-down and resume transmission is only 100µs. A

mouse or other application may need more time to wake up from shutdown. If automatic power-down is being

used, the RS-232 device reenters powerdown if valid receiver levels are not reestablished within 30µs of the

ICL3221EM and ICL3221EF powering up. Figure 9 illustrates a circuit that prevents the ICL3221EM and

ICL3221EF from initiating automatic powerdown for 100ms after powering up. The delay gives the slow-to-wake

peripheral circuit time to reestablish valid RS-232 output levels.

4.5.3 Automatic Powerdown

Even greater power savings are available by using the ICL3221EM and ICL3221EF’s automatic powerdown

function. When no valid RS-232 voltages (see Figure 10) are sensed on any receiver input for 30µs, the

charge-pump and transmitters powerdown and reduce supply current to 1µA. 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 ICL3221EM and ICL3221EF power back up whenever they detect a valid RS-232 voltage level

on any receiver input, which provides additional system power savings without changes to the existing operating

system.

Figure 8. Connections for Manual Powerdown when No Valid Receiver Signals are Present

Figure 9. Circuit to Prevent Auto Powerdown for 100ms After Forced Power-Up

Power

FORCEOFF

INVALID

CPU

I/O

FORCEON

ICL3221EM

Management

Logic

UART

ICL3221EF

ICL3221EM

FORCEOFF

FORCEON

Power

Master Powerdown Line

1MΩ

0.1µF

Management

Unit

ICL3221EF