XR33152HDEVB datasheet(17/21 Pages) EXAR

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33152/XR33155/XR33156/XR33158

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REV2B

Applications Information (Continued)

The XR3315x RS-485/RS-422 devices are part of

Exar’s high performance serial interface product line.

The analog bus pins can survive direct shorts up to ±60V

and are protected against ESD events up to ±15kV.

Enhanced Failsafe

Ordinary RS-485 differential receivers will be in an

indeterminate state whenever the data bus is not being

actively driven. The enhanced failsafe feature of the

XR3315x family guarantees a logic-high receiver output

when the receiver inputs are open, shorted or when they

are connected to a terminated transmission line with all

drivers disabled. In a terminated bus with all transmitters

disabled, the receivers’ differential input voltage is pulled

to 0V by the termination. The XR3315x family interprets

0V differential as a logic high with a minimum 50mV noise

margin while maintaining compliance with the RS-485

standard of ±200mV. Although the XR3315x family does

not need failsafe biasing resistors, it can operate without

issue if biasing is used.

Hot Swap Capability

When VCC is first applied the XR3315x family holds the

driver enable and receiver 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

XR3315x family from driving any output signal until power

has stabilized. After the initial 10μs, the driver and receiver

enable pins are weakly pulled to their disabled states (low

for DE and high for RE) until the first transition. After the first

transition, the DE and RE pins operate as high impedance

inputs.

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 or receiver, 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 and receiver

inputs of the XR3315x family have extra protection against

static electricity. Exar uses state-of-the-art structures to

protect these pins against ESD damage:

■

±15kV HBM for bus pins to GND

■

±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.

Maximum Number of Transceivers on the Bus

The standard RS-485 receiver input impedance is 12kΩ (1

unit load). A standard driver can drive up to 32 unit loads.

The XR33152 transceiver has a 1/10th unit load receiver

input impedance of 120kΩ, allowing up to 320 transceivers

to be connected in parallel on a communication line. The

XR33156/58 transceivers have a 1/2.5 unit load receiver

input impedance of 30kΩ, allowing up to 80 transceivers

to be connected in parallel on a communication line. Any

combination of these devices and other RS-485 transceivers

up to a total of 32 unit loads may be connected to the line.

Low Power Shutdown Mode

The XR33156 has a low-power shutdown mode that is

initiated by bringing both RE high and DE low simultaneously.

While in shutdown the XR33156 draws less than 1μA of

supply current. DE and RE may be tied together and driven

by a single control signal. Devices are guaranteed not to

enter shutdown if RE is high and DE is low for less than

50ns. If the inputs are in this state for at least 600ns, the

parts will enter shutdown.

XR33156 enable times, tZH and tZL, apply when the part is

not in low power shutdown state. Enable times, tZH(SHDN)

and tZL(SHDN) apply when the part is shutdown. The driver

and receiver take longer to become enabled from low

power shutdown tZH(SHDN) and tZL(SHDN) than from driver

or receiver disable mode (tZH and tZL).

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