ISL31470E_10 Datasheet, PDF (13/21 Pages) Intersil Corporation – Fault Protected, Extended Common Mode Range, RS-485/RS-422 Transceivers

ISL31470E, ISL31472E, ISL31475E, ISL31478E
High V_OD Improves Noise Immunity and
Flexibility
The ISL31470E, ISL31472E, ISL31475E, ISL31478E
driver design delivers larger differential output voltages
(V_OD) than the RS-485 standard requires, or than most
RS-485 transmitters can deliver. The typical ±2.5V V_OD
provides more noise immunity than networks built using
many other transceivers.
Another advantage of the large V_OD is the ability to drive
more than two bus terminations, which allows for utilizing
the ISL31470E, ISL31472E, ISL31475E, ISL31478E in
“star” and other multi-terminated, nonstandard network
topologies. Figure 8, details the transmitter’s V_OD vs I_OUT
characteristic, and includes load lines for four (30Ω) and
six (20Ω) 120Ω terminations. Figure 8 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 ISL31470E, ISL31472E,
ISL31475E, ISL31478E 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 ISL31470E, ISL31472E,
ISL31475E, ISL31478E 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 7 illustrates the power-up and
power-down performance of the ISL31470E, ISL31472E,
ISL31475E, ISL31478E compared to an RS-485 IC
without the Hot Plug feature.
V_CC
3.5V
2.8V
5.0
2.5
A/Y
0
ISL83088E
ISL3147xE
RO
ISL3147xE
DE, DI = V_CC
RE = GND 5.0
2.5
0
R_L = 1kΩ
5.0
R_L = 1kΩ
2.5
0
TIME (40μs/ DIV)
FIGURE 7. HOT PLUG PERFORMANCE (ISL3147xE) 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. Devices operating at
15Mbps may be used at lengths up to 150’ (46m), but
the distance can be increased to 328’ (100m) by
operating at 10Mbps. The 1Mbps versions can operate at
full data rates with lengths up to 800’ (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’ (1220m) distance, or
at 250kbps for lengths up to 3000’ (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 fold-back
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 fold-back
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 fold-back
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,
13
FN7639.0
June 17, 2010