MAX3221 データシートの表示（PDF） - Maxim Integrated

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MAX3221

+3V to +5.5V RS-232 Transceivers with AutoShutdown

Maxim Integrated 

MAX3221/MAX3223/MAX3243

+3V to +5.5V RS-232 Transceivers

with AutoShutdown

Table 2. AutoShutdown Logic

RS-232 SIGNAL

PRESENT AT FORCEOFF INPUT

RECEIVER INPUT

Yes

H

No

H

No

H

Yes

L

No

L

FORCEON

INPUT

X

H

L

X

X

INVALID OUTPUT

TRANSCEIVER STATUS

H

Normal Operation

L

Normal Operation (Forced On)

L

Shutdown (AutoShutdown)

H

Shutdown (Forced Off)

L

Shutdown (Forced Off)

MAX3243 is awake. If the other system outputs 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 turned

off, V+ decays to VCC, V- decays to ground, and the

transmitter outputs are disabled (high impedance). The

time required to exit shutdown is typically 100µs (Figure

5b).

Software-Controlled Shutdown

If direct software control is desired, INVALID can be

used to indicate DTR or Ring Indicator signal. Tie

FORCEOFF and FORCEON together to bypass Auto-

Shutdown so the line acts like a SHDN input.

Applications Information

Capacitor Selection

The capacitor type used for C1–C4 is not critical for

proper operation; either polarized or nonpolarized

capacitors may be used. The charge pump requires

0.1µF capacitors for 3.3V operation. For other supply

voltages, refer to Table 3 for required capacitor values.

Do not use values smaller than those listed in Table 3.

Increasing the capacitor values (e.g., by a factor of 2)

reduces ripple on the transmitter outputs and slightly

reduces power consumption. C2, C3, and C4 can be

increased without changing C1’s value. However, do

not increase C1 without also increasing the values

of C2, C3, and C4 to maintain the proper ratios (C1

to the other capacitors).

When using the minimum required capacitor values,

make sure the capacitor value does not degrade

excessively with temperature. If in doubt, use capaci-

tors with a larger nominal value. The capacitor’s equiva-

lent series resistance (ESR) usually rises at

low temperatures and influences the amount of ripple

on V+ and V-.

+2.7V

+0.3V

0V

-0.3V

-2.7V

(a)

TRANSMITTERS ENABLED, INVALID HIGH

INDETERMINATE

AUTOSHUTDOWN, TRANSMITTERS DISABLED,

1μA SUPPLY CURRENT, INVALID LOW

INDETERMINATE

TRANSMITTERS ENABLED, INVALID HIGH

RECEIVER

INPUT

VOLTAGE

(V)

VCC

INVALID

OUTPUT

(V)

0

tINVL

V+

VCC

0

V-

(b)

Figure 5. AutoShutdown Trip Levels

INVALID

REGION

tINVH

tWU

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