Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP3070E-SP3078E_100_061109

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DETaIlED DESCRIPTION

The SP307XE family of high speed trans-

ceivers for RS-485/RS-422 communication

contain one driver and one receiver. These

devices feature fail-safe circuitry, which

guaranteesalogic-highreceiveroutputwhen

the receiver inputs are open or shorted, or

when they are connected to a terminated

transmissionlinewithalldriversdisabled.The

SP3070E, SP3072E, SP3073E, SP3075E,

SP3076E and SP3078E also feature a

hotswap capability allowing live insertion

without error data transfer. The SP3070E,

SP3071E and SP3072E feature reduced

slew-rate drivers that minimize EMI and

reduce reflections caused by improperly

terminated cables, allowing error-free data

transmission up to 250kbps. The SP3073E,

SP3074E and SP3075E also offer slew -rate

limitsallowingtransmitspeedsupto500kbps.

The SP3076E, SP3077E, SP3078E driver

slew rates are not limited, making transmit

speeds up to 16Mbps possible.

The SP3072E, SP3075E and SP3078E are

half-duplextransceivers,whiletheSP3070E,

SP3071E, SP3073E, SP3074E, SP3076E,

and SP3077E are full duplex transceivers.

All devices operate from a single 3.3V sup-

ply. Drivers are output short-circuit current

limited. Thermal-shutdown circuitry protects

drivers against excessive power dissipation.

When activated, the thermal-shutdown cir-

cuitry places the driver outputs into a high-

impedance state.

RECEIVER INPuT FIlTERING

SP3070E-SP3075E receivers incorporate

input filtering in addition to input hysteresis.

This filtering enhances noise immunity with

differential signals that have very slow rise

and fall times. Receiver propagation delay

increases by 25% due to this filtering.

FaIl SaFE

The SP3070E family guarantees a logic-high

receiver output when the receiver inputs are

shorted, open, or when they are connected

to a terminated transmission line with all

drivers disabled. If A - B is less than or equal

to -200mV, RO is logic low. In the case of a

terminated bus with all transmitters disabled

the receiver's differential input voltage is

pulled to ZeroV by the termination. With the

receiver thresholds of the SP3070E family,

this results in a logic high with a 50mV mini-

mum noise margin. In compliance with the

EIA/TIA-485 standard, the SP3070E family

has a 50mV - 200mV threshold.

HOT-SWaP CaPaBIlITy

When circuit boards are inserted into a hot

backplane, differential disturbances to the

data bus can lead to data errors. Upon initial

circuit board insertion, the data communica-

tion processor undergoes its own power-up

sequence.Duringthisperiod,theprocessor's

logic-output drivers are high impedance and

are unable to drive the DE and RE inputs

of these devices to a defined logic level.

Leakage currents up to 10µA from the high-

impedance state of the processor's logic

drivers could cause standard CMOS enable

inputs of a transceiver to drift to an incor-

rect logic level. Additionally, parasitic circuit

board capacitance could cause coupling of

VCC or GND to the enable inputs. Without

the hot-swap capability, these factors could

improperly enable the transceiver's driver

or receiver.

When VCC rises, an internal pulldown circuit

holds DE low and RE high for approximately

10 microseconds. After the initial power-up

sequence, the pulldown circuit becomes

transparent, resetting the hot-swap toler-

able input.

15KV ESD PROTECTION

As with all Exar devices, ESD-protection

structures are incorporated on all pins to

protect against electrostatic discharges

encountered during handling and assembly.

The driver output and receiver inputs of

the SP3070E family of devices have extra

protection against static electricity. Exar's

engineering team have developed state

of the art structures to protect these pins

against ESD of 15kV without damage. The

DETaIlED DESCRIPTION