Date: 03/

31/05

SP3070E - SP3078E Family, ± 15kV ESD-Protected, Failsafe RS-485/RS422 Transceivers

© Copyright 2005 Sipex Corporation

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DETAILED DESCRIPTION

The SP307XE family of high speed transceiv-

ers for RS-485/RS-422 communication con-

tain one driver and one receiver.

These

devices feature fail-safe circuitry, which guar-

antees a logic-high receiver output when the

receiver inputs are open or shorted, or when

they are connected to a terminated transmis-

sion line with all drivers disabled. The

SP3070E, SP3072E, SP3073E, SP3075E,

SP3076E and SP3078E also feature a

hotswap capability allowing live insertion with-

out error data transfer. The SP3070E,

SP3071EandSP3072Efeaturereducedslew-

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 limits allowing

transmitspeedsupto500kbps. TheSP3076E,

SP3077E, SP3078E driver slew rates are not

limited, making transmit speeds up to 16Mbps

possible.

The SP3072E, SP3075E and SP3078E are

half-duplex transceivers, while the SP3070E,

SP3071E, SP3073E, SP3074E, SP3076E,

and SP3077E are full duplex transceivers.

All devices operate from a single 3.3V supply.

Drivers are output short-circuit current limited.

Thermal-shutdown circuitry protects drivers

against excessive power dissipation. When

activated, the thermal-shutdown circuitry

places the driver outputs into a high-imped-

ance state.

RECEIVER INPUT FILTERING

SP3070-SP3075 receivers incorporate input

filtering in addition to input hysteresis. This

filtering enhances noise immunity with differ-

ential signals that have very slow rise and fall

times. Receiver propagation delay increases

by 25% due to this filtering.

FAIL SAFE

The SP3070 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

thereceiver'sdifferentialinputvoltageispulled

to 0V by the termination. With the receiver

thresholds of the SP3070E family, this results

in a logic high with a 50mV minimum 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. During this period, the processor'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. Leak-

age currents up to 10µA from the high-imped-

ance state of the processor's logic drivers

could cause standard CMOS enable inputs of

a transceiver to drift to an incorrect logic level.

Additionally, parasitic circuit board capaci-

tance could cause coupling of VCC or GND to

the enable inputs. Without the hot-swap capa-

bility, 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,thepulldowncircuitbecomestrans-

parent, resetting the hot-swap tolerable input.

15KV ESD PROTECTION

As with all Sipex devices, ESD-protection

structures are incorporated on all pins to

protect against electrostatic discharges en-

countered during handling and assembly. The

driver output and receiver inputs of the

SP3070E family of devices have extra protec-

tion against static electricity. Sipex's engi-

neering team have developed state of the art

structures to protect these pins against ESD

of 15kV without damage. The ESD structures

withstand high ESD in all states: normal op-

DETAILED DESCRIPTION