3V to 5.5V, up to 250kbps True RS-232 Transceiver

with 4µA AutoShutdown Plus and Power-On Reset

8

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__________Applications Information

Ensuring a Valid Reset Output

rent: it becomes an open circuit. Therefore, high-

impedance CMOS logic inputs connected to RESET

can drift to undefined voltages. This presents no prob-

lem in most applications, since most µPs and other cir-

applications where RESET must be valid down to 0V,

add a pull-down resistor to ground, holding RESET low

(Figure 4). R1’s value is not critical; 100k

Ω is large

enough not to load RESET, and small enough to pull it

to ground.

Capacitor Selection

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

proper operation; use either polarized or nonpolarized

capacitors. The charge pump requires 0.1µF capaci-

tors for 3.3V operation. For other supply voltages, refer

to Table 3 for required capacitor values. Do not use val-

ues 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, C4,

the other capacitors).

-2.7V

-0.3V

2.7V

0.3V

0V

INDETERMINATE

INVALID HIGH

INVALID LOW

INVALID HIGH

INDETERMINATE

Figure 3b. Receiver Positive/Negative Thresholds for INVALID

Figure 3a. AutoShutdown Plus and INVALID Timing Diagram

0

V+

V-

0

INVALID

OUTPUT

* ALL RECEIVER/TRANSMITTER INPUTS INACTIVE

** ANY ONE RECEIVER/TRANSMITTER INPUT BECOMES ACTIVE

TRANSMITTER

INPUTS

VOLTAGE

RECEIVER

INPUTS

VOLTAGE

TRANSMITTER

OUTPUTS

VOLTAGE

INVALID

REGION

*

**