ISL32601EFBZ Datasheet, PDF (13/23 Pages) Intersil Corporation – 1.8V to 3.3V, Micro-Power, ±15kV ESD, +125°C, Slew Rate Limited, RS-485/RS-422 Transceivers

ISL32600E, ISL32601E, ISL32602E, ISL32603E
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
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 ISL3260XE 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 ISL3260XE 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’
(1220m), but the maximum system data rate decreases as the
transmission length increases. The ISL32600E and ISL32601E
operate at data rates up to 128kbps at the maximum (4000’)
distance, or at data rates of 256kbps for cable lengths less
than 3000’ (915m). The ISL32602E and ISL32603E, with
V_CC = 1.8V, are limited to 1000’ (305m) at 256kbps, or 2000’
(610m) at 128kbps. With V_CC = 3.3V, the ISL32602E and
ISL32603E deliver 460kbps over 2000’, 256kbps over 3000’,
or 128kbps over 4000’ cables.
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.
Short networks using these transceivers need not be
terminated, but terminations are recommended for 2.7V to
3.6V powered networks unless power dissipation is an
overriding concern. Terminations are not recommended for
1.8V applications, due to the low drive available from those
transmitters.
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.
Terminated networks using the ISL3260XE may require bus
biasing resistors (pull-up on noninverting input, pull-down on
inverting input) to preserve the bus idle state when the bus is
not actively driven. Without bus biasing, the termination
resistor collapses the undriven, differential bus voltage to 0V,
which is an undefined level to the ISL3260XE Rx. Bus biasing
forces a few hundred milli-volt positive differential voltage on
the undriven bus, which all RS-485/422 Rx interpret as a valid
logic high.
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.
The driver output stages incorporate short circuit current
limiting circuitry that ensures that the output current never
exceeds the RS-485 spec, even at the common mode voltage
range extremes. Additionally, these devices utilize a foldback
circuit which reduces the short circuit current, and thus the
power dissipation, whenever the contending voltage exceeds
either supply.
In the event of a major short circuit condition, these ICs also
include a thermal shutdown feature that disables the drivers
whenever the die temperature becomes excessive. This
eliminates the power dissipation, allowing the die to cool. The
drivers automatically re-enable after the die temperature
drops by about 20°C. If the condition persists, the thermal
shutdown / re-enable cycle repeats until the fault is cleared.
Receivers remain operational during thermal shutdown.
Low Power Shutdown Mode
These micro-power transceivers all use a fraction of the power
required by their counterparts, but they also include a
shutdown feature that reduces the already low quiescent I_CC to
a 10nA trickle. These devices enter shutdown whenever the
receiver and driver are simultaneously disabled (RE = V_CC and
DE = GND) for a period of at least 600ns (1200ns at
V_CC = 1.8V). Disabling both the driver and the receiver for less
than 50ns guarantees that the transceiver will not enter
shutdown.
Note that most receiver and driver enable times increase when
the transceiver enables from shutdown. Refer to Notes 9 through
13, at the end of the “Electrical Specification table” on page 7, for
more information.
13
FN7967.0
June 22, 2012