© 2014 Exar Corporation

XR32430/XR32431

10 / 13

exar.com/XR32430_XR32431

Rev 1A

The charge pump operates with 0.1μF capacitors for 3.3V

operation. For other supply voltages, see the table for

required capacitor values. Do not use values smaller than

those listed. Increasing the capacitor values (e.g., by dou-

bling in value) reduces ripple on the transmitter outputs and

may slightly reduce power consumption. C2, C3, and C4

can be increased without changing C1’s value.

For best charge pump efficiency locate the charge pump

and bypass capacitors as close as possible to the IC. Sur-

face mount capacitors are best for this purpose. Using

capacitors with lower equivalent series resistance (ESR)

and self-inductance, along with minimizing parasitic PCB

trace inductance will optimize charge pump operation.

Designers are also advised to consider that capacitor val-

ues may shift over time and operating temperature.

AUTO ON-LINE® Circuitry

The XR32430/31 devices have a patent pending AUTO

ON-LINE® circuitry on board that saves power in applica-

tions such as laptop computers, palmtop (PDA) computers

and other portable systems.

The XR32430/31 devices incorporate an AUTO ON-LINE®

circuit that automatically enables itself when the external

transmitters are enabled and the cable is connected. Con-

versely, the AUTO ON-LINE® circuit also disables most of

the internal circuitry when the device is not being used and

goes into a standby mode where the device typically draws

1μA. This function is externally controlled by the ONLINE

pin. When this pin is tied to a logic LOW, the AUTO ON-

LINE® function is active. Once active, the device is enabled

until there is no activity on the receiver inputs. The receiver

input typically sees at least +3V, which are generated from

the transmitters at the other end of the cable with a +5V

minimum.

When the external transmitters are disabled or the cable is

disconnected, the receiver inputs will be pulled down by

their internal 5kΩ resistors to ground. When this occurs

over a period of time, the internal transmitters will be dis-

abled and the device goes into a shutdown or standby

mode. When ONLINE is HIGH, the AUTO ON-LINE® mode

is disabled.

Figure 11: Auto On-Line Timing

The STATUS pin goes to a logic LOW when the cable is dis-

connected or the external transmitters are disabled.

When the XR32430/31 drivers and internal charge pump

are disabled, the supply current is reduced to 1μA. This can

commonly occur in hand-held or portable applications

where the RS-232 cable is disconnected or the RS-232

drivers of the connected peripheral are turned off.

The AUTO ON-LINE® mode can be disabled by the SHUT-

DOWN pin. If this pin is a logic LOW, the AUTO ON-LINE®

function will not operate regardless of the logic state of the

ONLINE pin. The truth table logic of the XR32430/31 driver

and receiver outputs can be found in Table 3 on page 11.

The STATUS pin outputs a logic LOW signal if the no valid

RS-232 level is detected at either of the receiver inputs.

This pin goes to a logic HIGH when the external transmit-

ters are enabled and the cable is connected providing valid

RS-232 voltage levels to all the RxIN receiver input pins.

Table 1:

Minimum Recommended Charge Pump

Capacitor Values

Supply Voltage

Charge Pump Capacitor

Value for XR32430/31

3.0V to 3.6V

C1 - C4 = 0.1

F

4.5V to 5.5V

C1 = 0.047

F,

C2 - C4 = 0.33

F

3.0V to 5.5V

C1 - C4 = 0.22

F

Invalid

Region

RS-232

Receiver

Input

Voltage

RS-232

Driver

Output

Voltage

STATUS

+5V

-5V

Charge

Pump

V+

V-

0V