ISL31480E_10 Datasheet, PDF (16/24 Pages) Intersil Corporation – Fault Protected, Extended CMR, RS-485/RS-422 Transceivers with Cable Invert

ISL31480E, ISL31483E, ISL31485E, ISL31486E
utilizing the ISL3148xE in “star” and other
multi-terminated, nonstandard network topologies.
Figure 9, details the transmitter’s V_OD vs I_OUT
characteristic, and includes load lines for four (30Ω) and
six (20Ω) 120Ω terminations. The figure shows that the
driver typically delivers ±1.3V into six terminations, and
the “Electrical Specification” table guarantees a V_OD of
±0.8V at 21Ω over the full temperature range. The
RS-485 standard requires a minimum 1.5V V_OD into two
terminations, but the ISL3148xE deliver RS-485 voltage
levels with 2x to 3x the number of terminations.
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 (DE, RE) is unable to ensure that the
RS-485 Tx and Rx 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 ISL3148xE devices incorporate a
“Hot Plug” function. Circuitry monitoring V_CC ensures
that, during power-up and power-down, the Tx and Rx
outputs remain disabled, regardless of the state of DE
and RE, if V_CC is less than ≈3.5V. This gives the
processor/ASIC a chance to stabilize and drive the
RS-485 control lines to the proper states. Figure 8
illustrates the power-up and power-down performance of
the ISL3148xE compared to an RS-485 IC without the
Hot Plug feature.
VCC
5.0
2.5
A/Y
0
RO
3.5V
2.8V
DE, DI = V_CC
RE = GND 5.0
2.5
0
ISL3148XE
RL = 1kΩ
ISL3148XE
5.0
RL = 1kΩ
2.5
0
TIME (40µs/DIV)
FIGURE 8. HOT PLUG PERFORMANCE (ISL3148XE)
vs ISL83088E WITHOUT HOT PLUG
CIRCUITRY
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. These 1Mbps versions
can operate at full data rates with lengths up to 800’
(244m). Jitter is the limiting parameter at this data rate,
so employing encoded data streams (e.g., Manchester
coded or Return-to-Zero) may allow increased
transmission distances.
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 to minimize reflections, and
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 ±25V. 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 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 CMOS transceivers all use a fraction of the power
required by competitive devices, but they also include a
shutdown feature (except the ISL31485E) 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
11, 12, 13, 14 and 15, at the end of the “Electrical
Specification” table on page 11, for more information.
16
FN7638.0
June 25, 2010