MAX3228/MAX3229 feature a separate logic supply

outputs. The transmitter inputs (T_IN), FORCEON and

maximum flexibility in interfacing to different systems

ply voltage (+1.65V to +5.5V), and bypass it with a

0.1µF capacitor to GND. If the logic supply is the same

Software-Controlled Shutdown

If direct software control is desired, connect FORCEOFF

and FORCEON together to disable AutoShutdown. The

microcontroller then drives FORCEOFF and FORCEON

like a SHDN input, INVALID can be used to alert the

microcontroller to indicate serial data activity.

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. However, ceramic chip

capacitors with an X7R or X5R dielectric work best. The

charge pump requires 0.1µF capacitors for 3.3V opera-

tion. For other supply voltages, refer to Table 4 for

required capacitor values. Do not use values smaller

than those listed in Table 4. Increasing the capacitor

values (e.g., by a factor of 2) reduces ripple on the

transmitter outputs and slightly reduces power con-

sumption. 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 capac-

itors).

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 equiv-

alent series resistance (ESR) usually rises at low

temperatures and influences 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.

+2.5V to +5.5V RS-232 Transceivers

in UCSP

_______________________________________________________________________________________

9

Table 3. Output Control Truth Table

TRANSCEIVER STATUS

FORCEON

FORCEOFF

RECEIVER STATUS

INVALID

Shutdown (AutoShutdown)

Low

High

High-Z

L

Shutdown (Forced Off)

X

Low

High-Z

†

Normal Operation (Forced On)

High

High

Active

†

Normal Operation (AutoShutdown)

Low

High

Active

H

X = Don’t care.

† = INVALID output state is determined by R_IN input levels.

µF)

C2, C3, C4 (

µF)

2.5 to 3.0

0.22

0.22

3.0 to 3.6

0.1

0.1

4.5 to 5.5

0.047

0.33

3.0 to 5.5

0.22

1

Table 4. Required Capacitor Values

Figure 5. Transmitter Outputs Exiting Shutdown or Powering Up

5V/div

FORCEON =

FORCEOFF

0

2V/div

4

µs/div

0