SP3070E Datasheet, PDF (14/20 Pages) Exar Corporation – ±15kV ESD-Protected, 3.3V, 1/8 Load, Failsafe RS-485/RS422 Transceivers

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
guarantees a logic-high receiver output when
the receiver inputs are open or shorted, or
when they are connected to a terminated
transmission line with all drivers disabled. 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
limits allowing transmit speeds up to 500kbps.
The SP3076E, 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 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
DETAILED DESCRIPTION
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. 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.
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
Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com
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