Applications Information

Using

INVALID

INVALID indicates when an RS-232 signal is present at

the receiver input, and therefore when the port is in use.

INVALID can be used in alternative shutdown control

schemes where it relieves the processor from constantly

polling the port for activity.

Capacitor Selection

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

proper operation; either polarized or nonpolarized

capacitors are acceptable. If polarized capacitors are

used, connect polarity as shown in the

Typical Operating

Circuit. The charge pump requires 0.1µF capacitors.

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

reduces ripple on the transmitter output and slightly

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

increased without changing C1’s value. However, do not

increase C1’s value without also increasing the values of

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

other capacitors).

When using the minimum 0.1µF capacitors, make sure

the capacitance does not degrade excessively with

temperature. If in doubt, use capacitors with a larger

nominal value. The capacitor’s equivalent series resis-

tance (ESR) usually rises at low temperatures and influ-

ences the amount of ripple on V+ and V-.

Power-Supply Decoupling

is adequate. In applications that are sensitive to power-

supply noise, use a capacitor of the same value as the

charge-pump capacitor C1. Connect bypass capaci-

tors as close to the IC as possible.

Transmitter Output when

Exiting Shutdown

Figure 5 shows the transmitter output when exiting shut-

down mode. The transmitter is loaded with 3k

Ω in paral-

lel with 2500pF. The transmitter output displays no

ringing or undesirable transients as the MAX221 comes

out of shutdown. Note that the transmitter is enabled only

when the magnitude of V- exceeds approximately 3V.

High Data Rates

The MAX221 maintains the RS-232 ±5.0V minimum

transmitter output voltage even at high data rates.

Figure 6 shows a transmitter loopback test circuit.

Figure 7 shows the loopback test result at 120kbps,

and Figure 8 shows the same test at 250kbps.

+5V, 1µA, Single RS-232 Transceiver

with AutoShutdown

_______________________________________________________________________________________

7

FORCEON

MASTER SHDN LINE

0.1

µF1MΩ

FORCEOFF

MAX221

POWER-

MANAGEMENT

UNIT

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

Mouse or Another System

V-

0

V+

0

INVALID

REGION

RECEIVER

INPUT

VOLTAGE

(V)

INVALID

OUTPUT

(V)

TRANSMITTER ENABLED, INVALID HIGH

AUTOSHUTDOWN, TRANSMITTER DISABLED,

1

µA SUPPLY CURRENT, INVALID LOW

TRANSMITTER ENABLED, INVALID HIGH

-2.7V

-0.3V

+2.7V

+0.3V

0

INDETERMINATE

INDETERMINATE

b)

a)

Figure 3. AutoShutdown Trip Levels