ISL43485 Datasheet, PDF (8/11 Pages) Intersil Corporation – 3.3V, Low Power, 30Mbps, RS-485/RS-422

ISL43485
Driver Features
The RS-485, RS-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) even with
V_CC = 3V. The driver features low propagation delay skew to
maximize bit width, and to minimize EMI, and it is tri-statable
via the active high DE input.
Outputs of the ISL43485 driver are not slew rate limited, so
faster output transition times allow data rates of at least
30Mbps.
Data Rate, Cables, and Terminations
Twisted pair is the cable of choice for RS-485, RS-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 this IC.
RS-485, RS-422 are intended for network lengths up to
4000’, but the maximum system data rate decreases as the
transmission length increases. Devices operating at 30Mbps
are often limited to lengths of less than one hundred feet.
Figure 6 details the ISL43485’s 30Mbps performance driving
200’ of “CAT5” cable terminated in 120Ω at both ends. Note
that the differential signal delivered to the receiver at the end
of the cable (A-B) still exceeds 1.5V peak. Longer cable
lengths are possible by reducing the data rate, as shown in
Figure 7 for a data rate of 20Mbps.
To minimize reflections, proper termination is imperative
when using this 30Mbps device. 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. The
ISL43485 meets 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 which ensures that the output current never
exceeds the RS-485 spec, even at the common mode
voltage range extremes. Additionally, it utilizes 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, this device
also includes 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 reenable after the die
temperature drops about 15 degrees. If the contention
persists, the thermal shutdown/reenable cycle repeats until
the fault is cleared. Receivers stay operational during
thermal shutdown.
Low Power Shutdown Mode
This BiCMOS transceiver uses a fraction of the power
required by its bipolar counterparts, nevertheless, the
ISL43485 includes a shutdown feature that reduces the
already low quiescent I_CC to a 15nA trickle. They enter
shutdown whenever the receiver and driver are
simultaneously disabled (RE = V_CC and DE = GND) for a
period of at least 300ns. Disabling both the driver and the
receiver for less than 80ns guarantees that shutdown is not
entered.
Note that receiver and driver enable times increase when
these devices enable from shutdown. Refer to Notes 5-9, at
the end of the Electrical Specification table, for more
information.
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