ISL3171E INTERSIL [Intersil Corporation], ISL3171E Datasheet - Page 11
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ISL3171E
Manufacturer Part Number
ISL3171E
Description
Manufacturer
INTERSIL [Intersil Corporation]
Datasheet
1.ISL3171E.pdf
(19 pages)
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Receivers easily meet the data rates supported by the
corresponding driver, and all receiver outputs - except on the
ISL3171E, ISL3174E and ISL3177E- are tri-statable via the
active low RE input.
Driver Features
The RS-485/422 driver is a differential output device that
delivers at least 1.5V across a 54Ω load (RS-485), and at
least 2V across a 100Ω load (RS-422). The drivers feature
low propagation delay skew to maximize bit width, and to
minimize EMI.
All drivers are tri-statable via the active high DE input, except
on the ISL3171E, ISL3174E and ISL3177E.
The 250kbps and 500kbps driver outputs are slew rate
limited to minimize EMI, and to reduce reflections in
unterminated or improperly terminated networks. Outputs of
the ISL3176E - ISL3178E drivers are not limited, so faster
output transition times allow data rates of at least 20Mbps.
Hot Plug Function
When a piece of equipment powers up, there is a period of
time where the processor or ASIC driving the RS-485 control
lines (DE, RE) is unable to ensure that the RS-485 Tx and
Rx outputs are kept disabled. If the equipment is connected
to the bus, a driver activating prematurely during power up
may crash the bus. To avoid this scenario, the ISL317XE
family incorporates a “Hot Plug” function. During power up,
circuitry monitoring V
remain disabled for a period of time, regardless of the state of
DE and RE. This gives the processor/ASIC a chance to
stabilize and drive the RS-485 control lines to the proper states.
ESD Protection
All pins on these devices include class 3 (>7kV) Human
Body Model (HBM) ESD protection structures, but the
RS-485 pins (driver outputs and receiver inputs)
incorporate advanced structures allowing them to survive
ESD events in excess of ±15kV HBM and ±15kV
IEC61000. The RS-485 pins are particularly vulnerable to
ESD damage because they typically connect to an exposed
port on the exterior of the finished product. Simply touching
the port pins, or connecting a cable, can cause an ESD
event that might destroy unprotected ICs. These new ESD
structures protect the device whether or not it is powered
up, and without degrading the RS-485 common mode
range of -7V to +12V. This built-in ESD protection
eliminates the need for board level protection structures
(e.g., transient suppression diodes), and the associated,
undesirable capacitive load they present.
IEC61000-4-2 Testing
The IEC61000 test method applies to finished equipment,
rather than to an individual IC. Therefore, the pins most likely
to suffer an ESD event are those that are exposed to the
outside world (the RS-485 pins in this case), and the IC is
tested in its typical application configuration (power applied)
rather than testing each pin-to-pin combination. The lower
ISL3170E, ISL3171E, ISL3172E, ISL3173E, ISL3174E, ISL3175E, ISL3176E, ISL3177E, ISL3178E
CC
ensures that the Tx and Rx outputs
11
current limiting resistor coupled with the larger charge
storage capacitor yields a test that is much more severe than
the HBM test. The extra ESD protection built into this
device’s RS-485 pins allows the design of equipment
meeting level 4 criteria without the need for additional board
level protection on the RS-485 port.
AIR-GAP DISCHARGE TEST METHOD
For this test method, a charged probe tip moves toward the
IC pin until the voltage arcs to it. The current waveform
delivered to the IC pin depends on approach speed,
humidity, temperature, etc., so it is difficult to obtain
repeatable results. The ISL317XE RS-485 pins withstand
±15kV air-gap discharges.
CONTACT DISCHARGE TEST METHOD
During the contact discharge test, the probe contacts the
tested pin before the probe tip is energized, thereby
eliminating the variables associated with the air-gap
discharge. The result is a more repeatable and predictable
test, but equipment limits prevent testing devices at voltages
higher than ±8kV. The ISL317XE survive ±8kV contact
discharges on the RS-485 pins.
Data Rate, Cables, and Terminations
RS-485/422 are intended for network lengths up to 4000’,
but the maximum system data rate decreases as the
transmission length increases. Devices operating at 20Mbps
are limited to lengths less than 100’, while the 250kbps
versions can operate at full data rates with lengths of several
thousand feet.
Twisted pair is the cable of choice for RS-485/422 networks.
Twisted pair cables tend to pick up noise and other
electromagnetically induced voltages as common mode
signals, which are effectively rejected by the differential
receivers in these ICs.
Proper termination is imperative, when using the 20Mbps
devices, to minimize reflections. Short networks using the
250kbps versions need not be terminated, but, terminations
are recommended unless power dissipation is an overriding
concern.
In point-to-point, or point-to-multipoint (single driver on bus)
networks, the main cable should be terminated in its
characteristic impedance (typically 120Ω) at the end farthest
from the driver. In multi-receiver applications, stubs
connecting receivers to the main cable should be kept as
short as possible. Multipoint (multi-driver) systems require
that the main cable be terminated in its characteristic
impedance at both ends. Stubs connecting a transceiver to
the main cable should be kept as short as possible.
Built-In Driver Overload Protection
As stated previously, the RS-485 spec requires that drivers
survive worst case bus contentions undamaged. These
devices meet this requirement via driver output short circuit
current limits, and on-chip thermal shutdown circuitry.
August 28, 2006
FN6307.2