XR32220EBER-L-TR Datasheet, PDF (11/16 Pages) Exar Corporation – 2-Driver/2-Receiver Intelligent RS-232 Transceiver with 1.65V to 5.5V Interface

XR32220
Applications Information (Continued)
Table 1. Minimum Recommended Charge
Pump Capacitor Values
Supply Voltage
V_CC
3.0V to 3.6V
4.5V to 5.5V
3.0V to 5.5V
Charge Pump Capacitor
Value for XR32220
C1 - C4 = 0.1μF
C1 = 0.047μF, C2 - C4 = 0.33μF
C1 - C4 = 0.22μF
For best charge pump efficiency locate the charge pump
and bypass capacitors as close as possible to the IC.
Surface 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 values
may shift over time and operating temperature.
AUTO ON-LINE^® Circuitry
The XR32220 devices have a patent pending AUTO
ON-LINE^® circuitry on board that saves power in applications
such as laptop computers, palmtop (PDA) computers and
other portable systems.
The XR32220 devices incorporate an AUTO ON-LINE^®
circuit that automatically enables itself when the external
transmitters are enabled and the cable is connected.
Conversely, 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 disabled and
the device goes into a shutdown or standby mode. When
ONLINE is HIGH, the AUTO ON-LINE^® mode is disabled.
RS232
Receiver
Input
Voltage
Invalid
Region
STATUS
RS232
Driver
Output
Voltage
V+
Charge
Pump
V-
tSTSL
tOFF LIN E
tSTSH
tON LIN E
+5V
-5V
VCC
0V
Figure 14. Auto On-Line^® Timing
The STATUS pin goes to a logic LOW when the cable is
disconnected or the external transmitters are disabled.
When the XR32220 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
SHUTDOWN 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
XR32220 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 transmitters
are enabled and the cable is connected providing valid
RS-232 voltage levels to the receiver input pins R1IN and
R2IN.
When the XR32220 is shut down, the charge pumps
are turned off. V+ charge pump output decays to V_CC,
the V- output decays to GND. The decay time will depend
on the size of capacitors used for the charge pump. Once in
shutdown, the time required to exit the shut down state and
have valid V+ and V- levels is typically 30μs.
For easy programming, the STATUS can be used to
indicate DSR or a Ring Indicator signal. Tying ONLINE and
SHUTDOWN together will bypass the AUTO ON-LINE^®
circuitry so this connection acts like a shutdown input pin.
REV1B
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