ISL32470E Datasheet, PDF (13/19 Pages) Intersil Corporation – Fault Protected, Extended Common Mode Range, RS-485/RS-422 Transceivers

ISL32470E, ISL32472E, ISL32475E, ISL32478E
and without interfering with the exceptional ±15V CMR. This built-in
ESD protection minimizes 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 feet,
but the maximum system data rate decreases as the transmission
length increases. Devices operating at 15Mbps may be used at
lengths up to 150 feet (46m), but the distance can be increased to
328 feet (100m) by operating at 10Mbps. The 1Mbps versions can
operate at full data rates with lengths up to 800 feet (244m). Jitter is
the limiting parameter at these faster data rates, so employing
encoded data streams (e.g., Manchester coded or Return-to-Zero)
may allow increased transmission distances. The slow versions can
operate at 115kbps or less at the full 4000-foot (1220m) distance,
or at 250kbps for lengths up to 3000 feet (915m). DC cable
attenuation is the limiting parameter, so using better-quality cables
(e.g., 22 AWG) may allow increased transmission distance.
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 15Mbps devices, to
minimize reflections. Short networks using the 250kbps versions
need not be terminated; however, terminations are recommended
unless power dissipation is an overriding concern.
In point-to-point or point-to-multipoint (single driver on bus like
RS-422) 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 specification requires that drivers
survive worst-case bus contentions undamaged. These transceivers
meet this requirement via driver output short circuit current limits
and on-chip thermal shutdown circuitry.
The driver output stages incorporate a double foldback short circuit
current limiting scheme, which ensures that the output current never
exceeds the RS-485 specification, even at the common mode and
fault condition voltage range extremes. The first foldback current
level (≈70mA) is set to ensure that the driver never folds back when
driving loads with common mode voltages up to ±15V. The very low
second foldback current setting (≈9mA) minimizes power dissipation
if the Tx is enabled when a fault occurs.
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 by about 15°C. If the
contention persists, the thermal shutdown/re-enable cycle repeats
until the fault is cleared. Receivers stay operational during thermal
shutdown.
Low Power Shutdown Mode
These BiCMOS transceivers all use a fraction of the power required
by competitive devices, but they also include a shutdown feature that
reduces the already low quiescent I_CC to a 10µA 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. Disabling both the driver and the receiver for less than
60ns guarantees that the transceiver will not enter shutdown.
Note that receiver and driver enable times increase when the
transceiver enables from shutdown. Refer to Notes 9, 10, 11, 12 and
13 on page 9, at the end of the “Electrical Specifications” table, for
more information.
Typical Performance Curves V_CC = 5V, T_A = +25°C; Unless Otherwise Specified.
90
3.6
R_D = 20Ω
R_D = 30Ω
80
+25°C
3.4
70
+85°C
60
R_D = 54Ω
3.2
R_D = 100Ω
50
3.0
40
R_D = 100Ω
2.8
30
2.6
20
R_D = 54Ω
10
2.4
0
0
1
2
3
4
5
DIFFERENTIAL OUTPUT VOLTAGE (V)
FIGURE 10. DRIVER OUTPUT CURRENT vs DIFFERENTIAL OUTPUT
VOLTAGE
2.2
-40 -25
0
25
50
TEMPERATURE (°C)
75 85
FIGURE 11. DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs
TEMPERATURE
13
FN7784.0
January 21, 2011