ISL3333EVAL1Z Intersil, ISL3333EVAL1Z Datasheet - Page 17

• Manufacturer Part Number: ISL3333EVAL1Z
• Description: EVAL BOARD 1 FOR ISL3333
• Manufacturer: Intersil

Specifications of ISL3333EVAL1Z

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

below 115kbps can operate at the maximum length of 4000’
(1220m).
Higher data rates require faster edges, so both the ISL333X
versions offer an edge rate capable of 20Mbps data rates.
The ISL3333 also offers two slew rate limited edge rates to
minimize problems at slower data rates. Nevertheless, for
the best jitter performance when driving long cables, the
faster speed settings may be preferable, even at low data
rates. See the “RS-485 Slew Rate Limited Data Rates”
section for details.
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.
The preferred cable connection technique is “daisy-
chaining”, where the cable runs from the connector of one
device directly to the connector of the next device, such that
cable stub lengths are negligible. A “backbone” structure,
where stubs run from the main backbone cable to each
device’s connector, is the next best choice, but care must be
taken to ensure that each stub is electrically “short”. See
Table 4 for recommended maximum stub lengths for each
speed option.
Proper termination is imperative to minimize reflections
when using the 20Mbps speed option. Short networks using
the medium and slow speed options need not be terminated,
but terminations are recommended unless power dissipation
is an overriding concern. Note that the RS-485 spec allows a
maximum of two terminations on a network, otherwise the Tx
output voltage may not meet the required V
In point-to-point, or point-to-multipoint (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, but definitely shorter than the limits shown in Table
4. Multipoint (RS-485) systems require that the main cable
be terminated in its characteristic impedance at both ends.
Again, keep stubs connecting a transceiver to the main
cable as short as possible, and refer to Table 4. Avoid “star”,
and other configurations, where there are many “ends”
which would require more than the two allowed terminations
to prevent reflections.
SPEED OPTION
TABLE 4. RECOMMENDED STUB LENGTHS
SLOW
FAST
MED
17
MAXIMUM STUB LENGTH
100-150 (30.5 - 46)
350-500 (107-152)
1-3 (0.3 - 0.9)
ft (m)
OD
.
ISL3332, ISL3333
High ESD
All pins on the ISL333X include ESD protection structures
rated at ±2.5kV (HBM), which is good enough to survive
ESD events commonly seen during manufacturing. But the
bus pins (Tx outputs and Rx inputs) are particularly
vulnerable to ESD events because they connect to an
exposed port on the exterior of the finished product. Simply
touching the port pins, or connecting a cable, can destroy an
unprotected port. ISL333X bus pins are fitted with advanced
structures that deliver ESD protection in excess of ±15kV
(HBM), without interfering with any signal in the RS-485 or
the RS-232 range. This high level of protection may
eliminate the need for board level protection, or at the very
least will increase the robustness of any board level scheme.
Small Packages
Competing 3.3V dual protocol ICs are available only in a 28
Ld SSOP. The ISL3333’s tiny 6x6mm QFN footprint is 80%
smaller than the competing SSOP.
Flow Through Pinouts
Even the ISL333X pinouts are features, in that the true
flow-through design simplifies board layout. Having the bus
pins all on one side of the package for easy routing to a
cable connector, and the Rx outputs and Tx inputs (logic
pins) on the other side for easy connection to a UART,
avoids costly and problematic crossovers. Competing “flow
through” pinouts mix logic and bus pin inputs on one side of
the package, and logic and bus pin outputs on the other side.
This forces the designer to route four traces from the right
side of the IC around the IC to the cable connector. Figure 11
illustrates the flow-through nature of the ISL333X’s pinout.
Low Power Shutdown (SHDN) Mode
The ON/OFF pin is driven low to place the IC (both ports) in
the SHDN mode, and the already low supply current drops to
as low as 21μA. If this functionality isn’t desired, the pin can
be left disconnected (thanks to the internal pull-up), or it
should be connected to V
1kΩ resistor. SHDN disables the Tx and Rx outputs, and
disables the charge pumps if either port is in RS-232 mode,
so V+ collapses to V
FIGURE 11. ILLUSTRATION OF FLOW THROUGH PINOUT
A1
B1
B2
A2
Y1
Y2
Z1
Z2
CC
ISL3332
, and V- collapses to GND.
R
CC
D
(V
L
for the QFN), through a
RA1
DY1
DY2
RA2
µCONTROLLER
UART
ASIC
OR
OR
May 27, 2008
FN6362.0