XR32330 Datasheet, PDF (8/13 Pages) Exar Corporation – 3-Driver/3-Receiver Intelligent RS-232 Transceiver

XR32330
Application Information
General Description
The XR32330 transceiver meets the EIA/TIA-232 and ITU-
T V.28/V.24 communication protocols and can be imple-
mented in portable or hand-held applications such as bar
code scanners as well as point-of-sales terminals, security
and industrial control applications. The XR32330 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 sup-
ply.
The XR32330 is a 3-driver/3-receiver device, ideal for por-
table or hand-held applications.
The XR32330 is an ideal choice for power sensitive
designs. The XR32330 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 sys-
tem automatically comes online. This feature allows design
engineers to address power saving concerns without major
design changes.
Theory of Operation
The XR32330 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 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 XR32330 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 out-
put 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, see Figure 5 — 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 con-
nected 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 5: Charge Pump - Phase 1
June 16, 2016
8 / 13
exar.com/XR32330
Rev 1B