XR32220EBER-L-TR Datasheet, PDF (9/16 Pages) Exar Corporation – 2-Driver/2-Receiver Intelligent RS-232 Transceiver with 1.65V to 5.5V Interface

XR32220
Applications Information
General Description
The XR32220 transceiver meets the EIA/TIA-232 and
ITU-T V.28/V.24 communication protocols and can be
implemented in battery-powered, portable, or hand-
held applications such as notebook or laptop computers.
The XR32220 device features Exar’s proprietary and
patented (U.S. 5,306,954) on-board charge pump circuitry
that generates ±5.5V RS-232 voltage levels from a single
+3.0V to +5.5V power supply. The XR32220EUER devices
can operate at a data rate of 1000Kbps fully loaded.
The XR32220 is a 2-driver/2-receiver device, ideal for
portable or hand-held applications.
The XR32220 is an ideal choice for power sensitive
designs. The XR32220 devices feature AUTO ON-LINE^®
circuitry which reduces the power supply drain to a 1μA
supply current.
In many portable or hand-held applications, an RS-232
cable can be disconnected or a connected peripheral can
be turned off. Under these conditions, the internal charge
pump and the drivers will be shut down. Otherwise, the
system automatically comes online. This feature allows
design engineers to address power saving concerns without
major design changes.
Theory of Operation
The XR32220 series is made up of four basic circuit blocks:
1. Drivers
2. Receivers
3. The Exar proprietary charge pump, and
4. AUTO ON-LINE^® circuitry.
Drivers
The drivers are inverting level transmitters that convert
TTL or CMOS logic levels to 5.0V EIA/TIA-232 levels
with an inverted sense relative to the input logic levels.
Typically, the RS-232 output voltage swing is ±5.5V with
no load and ±5V minimum fully loaded. The driver outputs
are protected against infinite short-circuits to ground
without degradation in reliability. These drivers comply
with the EIA-TIA-232-F and all previous RS-232 versions.
Unused drivers inputs should be connected to GND or V_L.
The drivers have a minimum data rate of 250kbps
(XR32220EB) or 1000kbps (XR32220EU) fully loaded.
Receivers
The receivers convert +5.0V EIA/TIA-232 levels to TTL or
CMOS logic output levels. Receivers are High-Z when the
AUTO ON-LINE^® circuitry is enabled and activated or when
in shutdown. The truth table logic of the XR32220 driver and
receiver outputs can be found in Table 2 on page 11.
Since receiver input is usually from a transmission line
where long cable lengths and system interference can
degrade the signal, the inputs have a typical hysteresis
margin of 300mV. This ensures that the receiver is virtually
immune to noisy transmission lines. Should an input be left
unconnected, an internal 5KΩ pull-down resistor to ground
will commit the output of the receiver to a HIGH state.
Charge Pump
The charge pump is a Exar–patented design (U.S.
5,306,954) and uses a unique approach compared to older
less–efficient designs. The charge pump still requires four
external capacitors, but uses a four–phase voltage shifting
technique to attain symmetrical ±5.5V power supplies.
The internal power supply consists of a regulated dual charge
pump that provides output voltages of ±5.5V regardless
of the input voltage (V_CC) over the +3.0V to +5.5V range.
This is important to maintain compliant RS-232 levels
regardless of power supply fluctuations.
The charge pump operates in a discontinuous mode using
an internal oscillator. If the output voltages are less than
a magnitude of ±5.5V, the charge pump is enabled. If the
output voltages exceed a magnitude of 5.5V, the charge
pump is disabled. This oscillator controls the four phases
of the voltage shifting. A description of each phase follows.
Phase 1
V- charge storage — During this phase of the clock cycle,
the positive side of capacitors C1 and C2 are initially charged
to V_CC. C1+ is then switched to GND and the charge in C1–
is transferred to C2–. Since C2+ is connected to V_CC, the
voltage potential across capacitor C2 is now 2 times V_CC.
VCC
C1
-VCC
+VCC
C2
-VCC
C4
V+ Storage Capacitor
V- Storage Capacitor
C3
Figure 10. Charge Pump - Phase 1
REV1B
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