ISL3333EVAL1Z Intersil, ISL3333EVAL1Z Datasheet - Page 16

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

Specifications of ISL3333EVAL1Z

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

nominal values), the charge pump contribution to RS-232
mode I
that require the charge pump in RS-485 mode, disabling the
charge pump saves power, and minimizes noise. If the
application keeps both ports in RS-485 mode (e.g., a
dedicated dual channel RS-485 interface), then the charge
pump capacitors aren’t even required.
Data Rates and Cabling
Drivers operate at data rates up to 400kbps, and are
guaranteed for data rates up to 250kbps. The charge pumps
and drivers are designed such that one driver in each port
can be operated at the rated load, and at 250kbps (see
Figure 34). Figure 34 also shows that drivers can easily drive
two to three thousand picofarads at data rates up to
250kbps, while still delivering compliant ±5V output levels.
Receivers operate at data rates up to 2Mbps. They are
designed for a higher data rate to facilitate faster factory
downloading of software into the final product, thereby
improving the user’s manufacturing throughput.
Figures 37 and 38 illustrate driver and receiver waveforms at
250kbps, and 500kbps, respectively. For these graphs, one
driver of each port drives the specified capacitive load, and a
receiver in the port.
RS-232 doesn’t require anything special for cabling; just a
single bus wire per transmitter and receiver, and another
wire for GND. So an ISL333X RS-232 port uses a five
conductor cable for interconnection. Bus terminations are
not required, nor allowed, by the RS-232 standard.
RS-485 Mode
Rx Features
RS-485 receivers convert differential input signals as small
as 200mV, as required by the RS-485 and RS-422
standards, to TTL/CMOS output levels. The differential Rx
provides maximum sensitivity, noise immunity, and common
mode rejection. Per the RS-485 standard, receiver inputs
function with common mode voltages as great as +12V and
-7V, regardless of supply voltage, making them ideal for long
networks where induced voltages are a realistic concern.
Each RS-485/RS-422 port includes a single receiver (RA),
and the unused Rx output (RB) is disabled but pulled high by
an internal current source. The internal current source turns
off in SHDN.
Worst case receiver input currents are 20% lower than the 1
“unit load” (1mA) RS-485 limit, which translates to a 15kΩ
minimum input resistance.
These receivers include a “full fail-safe” function that
guarantees a high level receiver output if the receiver inputs
are unconnected (floating), shorted together, or if the bus is
terminated but undriven (i.e., differential voltage collapses to
near zero due to termination). Failsafe with shorted, or
terminated and undriven inputs is accomplished by setting
CC
is reduced significantly. Unlike competing devices
16
ISL3332, ISL3333
the Rx upper switching point at -40mV, thereby ensuring that
the Rx recognizes a 0V differential as a high level.
All the Rx outputs are short circuit protected, and are tri-state
when the IC is forced into SHDN, but ISL3332 (SSOP)
receiver outputs are not independently tri-statable. ISL3333
(QFN) receiver outputs are tri-statable via an active low
RXEN input for each port (see “ISL3333 Special Features”
for more details).
Tx Features
The RS-485/RS-422 driver is a differential output device that
delivers at least 1.5V across a 54Ω load (RS-485), and at
least 2V across a 100Ω load (RS-422). The drivers feature
low propagation delay skew to maximize bit widths, and to
minimize EMI.
To allow multiple drivers on a bus, the RS-485 spec requires
that drivers survive worst case bus contentions undamaged.
The ISL333X drivers meet this requirement via driver output
short circuit current limits, and on-chip thermal shutdown
circuitry. The output stages incorporate current limiting
circuitry that ensures that the output current never exceeds
the RS-485 spec, even at the common mode voltage range
extremes of 12V and -7V. 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 degrees. If the
contention persists, the thermal shutdown/re-enable cycle
repeats until the fault is cleared. Receivers stay operational
during thermal shutdown.
RS-485 multi-driver operation also requires drivers to include
tri-state functionality, so each port has a DE pin to control
this function. If the driver is used in an RS-422 network, such
that driver tri-state isn’t required, then the DE pin can be left
unconnected and an internal pull-up keeps it in the enabled
state. Drivers are also tri-stated when the IC is in SHDN, or
when the 3.3V power supply is off.
Speed Options
The ISL3332 (SSOP) has fixed, high slew rate driver outputs
optimized for 20Mbps data rates. The ISL3333 (QFN) offers
three user selectable data rate options: “Fast” for high slew
rate and 20Mbps; “Medium” with slew rate limiting set for
460kbps; “Slow” with even more slew rate limiting for
115kbps operation. See the “Data Rate“ and “Slew Rate
Limited Data Rates” sections for more information.
Receiver performance is the same for all three speed
options.
Data Rate, Cables, and Terminations
RS-485/RS-422 are intended for network lengths up to 4000’
(1220m), but the maximum system data rate decreases as
the transmission length increases. Devices operating at the
maximum data rate of 20Mbps are limited to maximum
lengths of 20-100’ (6-31m), while devices operating at or
May 27, 2008
FN6362.0