ISL3293E Datasheet, PDF (11/17 Pages) Intersil Corporation – ±16.5kV ESD Protected, +125°C, 3.0V to 5.5V, SOT-23/TDFN Packaged, Low Power, RS-485/RS-422 Transmitters

ISL3293E, ISL3294E, ISL3295E, ISL3296E, ISL3297E, ISL3298E
TABLE 2. V_IH AND V_IL vs V_L FOR V_CC = 3.3V OR 5V
V_L (V)
1.35
V_IH (V)
0.7
V_IL (V)
0.4
1.5
0.8
0.5
1.8
0.9
0.7
2.3
1.1
1.0
2.7
1.3
1.1
3.3
1.5
1.4
5.0 (i.e., V_CC)
2.7
2.3
The V_L supply current (I_L) is typically much less than 20µA,
as shown in Figure 9, when DE and DI are above/below
V_IH/V_IL.
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
line (DE) is unable to ensure that the RS-485 Tx 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 ISL329xE family
incorporates a “Hot Plug” function. During power-up, circuitry
monitoring V_CC ensures that the Tx outputs remain disabled for
a period of time, regardless of the state of DE. 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 (8kV) Human
Body Model (HBM) ESD protection structures, but the
RS-485 pins (driver outputs) incorporate advanced
structures allowing them to survive ESD events in excess
of ±16.5kV HBM and ±7kV to the IEC61000 contact test
method. 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.
Data Rate, Cables, and Terminations
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 20Mbps
are limited to lengths less than 100’, while the 250kbps
versions can operate at full data rates with lengths of several
1000’.
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 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 transmitter or
receiver to the main cable should be kept as short as
possible.
Driver Overload Protection
As stated previously, the RS-485 specification requires that
drivers survive worst case bus contentions undamaged.
These drivers meet this requirement, for V_CC ≤ 3.6V, 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 specification, for V_CC ≤ 3.6V, 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 V_CC or
GND.
In the event of a major short circuit condition, devices 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 about +20°C. If the contention persists, the thermal
shutdown/re-enable cycle repeats until the fault is cleared.
At V_CC > 3.6V, the instantaneous short circuit current is high
enough that output stage damage may occur during short
circuit conditions to voltages outside of GND to V_CC, before
the short circuit limiting and thermal shutdown activate. For
V_CC = 5V operation, if output short circuits are a possibility
(e.g., due to bus contention), it is recommended that a 5Ω
resistor be inserted in series with each output. This resistor
limits the instantaneous current below levels that can cause
damage. The driver V_OD at V_CC = 5V is so large that this
small added resistance has little impact.
11
FN6544.0
September 19, 2007