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XR32430/XR32431

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exar.com/XR32430_XR32431

Rev 1A

Application Information

General Description

The XR32430/31 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

XR32430/31 device features Exar's proprietary and pat-

ented (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 XR32430/31EU and XR32430/

31EH devices can operate at a data rate of 1000Kbps and

460Kbps fully loaded while the XR32430/31EB devices fea-

ture slew-rate limited outputs for reduced crosstalk and

EMI.

The XR32430/31 is a 3-driver/5-receiver device, ideal for

portable or hand-held applications.

The XR32430/31 is an ideal choice for power sensitive

designs. The XR32430/31 devices feature AUTO ON-

LINE® circuitry which reduces the power supply drain to a

1μA supply current. The XR32430/31 includes one comple-

mentary receiver that remains alert to monitor an external

device’s Ring Indicator while the XR32430/31 is shutdown.

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 on line. This feature allows design

engineers to address power saving concerns without major

design changes.

Theory of Operation

The XR32430/31 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

The drivers have a minimum data rate of 250kbps

(XR32430/31EB), 460Kbps (XR32430/31EH) or 1Mbps

(XR32430/31EU) 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 XR32430/31 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

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 — During this phase of the clock cycle,

the positive side of capacitors C1 and C2 are initially

charge in C1– is transferred to C2–. Since C2+ is con-