ISL32173EIVZ-T Datasheet, PDF (13/22 Pages) Intersil Corporation – QUAD, 16.5kV ESD Protected, 3.0V to 5.5V, RS-485/RS-422 Receivers

ISL32173E, ISL32175E, ISL32177E, ISL32273E, ISL32275E, ISL32277E
TABLE 2. TYPICAL V_IH, V_IL AND DATA RATE vs. V_L FOR
V_CC = 3.3V OR 5V
ISL32177E
V_L
V_IH
V_IL
DATA RATE
(V)
(V)
(V)
(Mbps)
1.4
0.55
0.5
25
1.6
0.6
0.55
50
1.8
0.8
0.7
65
2.3
1
0.9
70
2.7
1.1
1
75
3.3
1.3
1.2
80
Neglecting the RO I_OH currents, the quiescent V_L supply
current (I_L) is typically less than 1µA for enable input
voltages at ground or V_L, as shown in Figure 6. Enable
pin pull-up resistors connect to V_CC, so the current due
to a low enable input adds to I_CC rather than to I_L.
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 (EN, EN, ENX) is unable to ensure that the
RS-485 Rx outputs are kept disabled. If the equipment is
connected to the bus, a receiver activating prematurely
during power up may generate RO transitions that could
cause interrupts. To avoid this scenario, this family
incorporates a “Hot Plug” function. During power up,
circuitry monitoring V_CC ensures that the Rx outputs
remain disabled for a period of time, regardless of the
state of the enables. 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 (receiver inputs) incorporate advanced
structures allowing them to survive ESD events in excess
of ±15kV HBM, and ±16.5kV IEC 61000-4-2. 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.
IEC 61000-4-2 Testing
The IEC 61000 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 A and B RS-485 pins
withstand ±16.5kV 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. These Quad receivers survive ±8kV contact
discharges on the RS-485 pins.
Data Rate, Cables, and Terminations
RS-485 and RS-422 are intended for network lengths up
to 4000’, but the maximum system data rate decreases
as the transmission length increases. Networks operating
at 80Mbps are limited to lengths much less than 100’
(30m), while a 20Mbps version can operate at full data
rates with lengths up to 200’ (60m).
Any of these ICs may be used at slower data rates over
longer cables, but there are some limitations for the
80Mbps versions. The 80Mbps Rx is optimized for high
speed operation, so its output may glitch if the Rx input
differential transition times are too slow. Keeping the
transition times below 500ns, which equates to a Tx
driving a 1000’ (305m) CAT 5 cable, yields excellent
performance over the full operating temperature range.
Twisted pair is the cable of choice for RS-485 and 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.
When using these receivers, proper termination is
imperative to minimize reflections. Short networks using
slew rate limited transmitters 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
a bus with multiple receivers) 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
13
FN7529.1
March 14, 2013