XR

6/12

REV1A

Functional Block Diagram

Figure 2. Functional Block Diagram

Z

DI

DE

Y

XR3319x

Applications Information

The XR3319x RS-485/RS-422 devices are part of Exar’s high

performance serial interface product line. These standalone

drivers operate off a single 3.3V supply and meet RS-485 and

RS-422 standards for balanced RS-485 and RS-422 serial

communications networks.

Hot Swap Capability

driver enable inactive for approximately 10μs. During power

ramp-up, other system ICs may drive unpredictable values or

tristated lines may be influenced by stray capacitance. The hot

swap feature prevents the XR3319x family from driving any

output signal until power has stabilized. After the initial 10μs,

the driver enable pin is weakly pulled to the disabled state(low

for DE) until the first transition. After the first transition, the DE

pin operate as high impedance input.

If circuit boards are inserted into an energized backplane

(commonly called “live insertion” or “hot swap”) power

may suddenly be applied to all circuits. Without the hot

swap capability, this situation could improperly enable the

transceiver’s driver, driving invalid data onto shared buses

and possibly causing driver contention or device damage.

Driver Output Protection

Two mechanisms prevent excessive output current and power

dissipation caused by faults or by bus contention. First, a driver

current limit on the output stage provides immediate protection

against short circuits over the whole common-mode voltage

range. Second, a thermal shutdown circuit forces the driver

outputs into a high impedance state if junction temperature

becomes excessive.

Line Length

The RS-485/RS-422 standard covers line lengths up to

4000ft. Maximum achievable line length is a function of

signal attenuation and noise. Termination prevents signal

reflections by eliminating the impedance mismatches on

a transmission line. Line termination is generally used if

rise and fall times are shorter than the round trip signal

propagation time. Higher output drivers may allow longer

cables to be used.

±15kV HBM ESD Protection (Unpowered Part)

ESD protection structures are incorporated on all pins to

protect against electrostatic discharges encountered during

handling and assembly. The driver outputs of the XR3319x

family have extra protection against static electricity.

Exar uses state-of-the-art structures to protect these pins

against ESD damage:

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■

±15kV HBM for bus pins to GND

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■

±4kV HBM for all other pins

ESD Test Conditions

ESD performance depends on a variety of conditions.

Contact Exar for a reliability report that documents test

setup, methodology and results.

Low Power Shutdown Mode

The XR3319x has a low-power shutdown mode that is

initiated by bringing DE low (to disable the XR33193/94/95).

While in shutdown the XR3319x draws less than 2μA of

supply current.