AutoShutdown

The MAX3228E/AE and MAX3229E/AE achieve a 1µA

supply current with Maxim’s AutoShutdown feature,

which operates when FORCEON is low and

FORCEOFF

is high. When these devices sense no valid signal lev-

els on all receiver inputs for 30µs, the on-board charge

current to 1µA. This occurs if the RS-232 cable is dis-

connected or the connected peripheral transmitters are

turned off. The device turns on again when a valid level

is applied to any RS-232 receiver input. As a result, the

system saves power without changes to the existing

BIOS or operating system.

Table 3 and Figure 2c summarize the MAX3228E/AE

and MAX3229E/AE operating modes. FORCEON and

FORCEOFF override AutoShutdown. When neither con-

trol is asserted, the IC selects between these states

automatically, based on receiver input levels. Figures

2a, 2b, and 3a depict valid and invalid RS-232 receiver

levels. Figures 3a and 3b show the input levels and tim-

ing diagram for AutoShutdown operation.

A system with AutoShutdown may need time to wake

up. Figure 4 shows a circuit that forces the transmitters

on for 100ms, allowing enough time for the other

system to realize that the MAX3228E/AE and

MAX3229E/AE are active. If the other system transmits

valid RS-232 signals within that time, the RS-232 ports

on both systems remain enabled.

When shut down, the device’s charge pumps are off,

transmitter outputs are high-impedance. The time

required to exit shutdown is typically 100µs (Figure 3b).

FORCEON and FORCEOFF

In case FORCEON and

FORCEOFF are inaccessible,

these pins have 60kΩ (typ) pullup resistors connected to

not connected, the MAX3228E/AE and MAX3229E/AE

will always be active. Pulling these pins to ground will

(typ) pullup resistor.

Unlike other RS-232 interface devices, where the

±15kV ESD-Protected +2.5V to +5.5V

RS-232 Transceivers in UCSP and WLP

8

_______________________________________________________________________________________

Figure 3. AutoShutdown Trip Levels

V-

0

V+

0

INVALID

REGION

RECEIVER

INPUT

VOLTAGE

(V)

INVALID

OUTPUT

(V)

TRANSMITTERS ENABLED, INVALID HIGH

AUTOSHUTDOWN, TRANSMITTERS DISABLED,

1

μA SUPPLY CURRENT, INVALID LOW

TRANSMITTERS ENABLED, INVALID HIGH

a)

b)

-2.7V

-0.3V

+2.7V

+0.3V

0

INDETERMINATE

INDETERMINATE

Figure 4. AutoShutdown with Initial Turn-On to Wake Up a

Mouse or Another System

FORCEON

MASTER SHDN LINE

0.1

μF1MΩ

FORCEOFF

MAX3228E/AE

MAX3229E/AE

POWER-

MANAGEMENT

UNIT

PIN NAME

POWER-ON DEFAULT

MECHANISM

FORCEON

High

Internal pullup

FORCEOFF

High

Internal pullup

Table 2. Power-On Default States