ISL41334 INTERSIL [Intersil Corporation], ISL41334 Datasheet - Page 16

• Manufacturer Part Number: ISL41334
• Manufacturer: INTERSIL [Intersil Corporation]

small 0.1µF capacitors for the voltage doubler and inverter
functions. By operating discontinuously (i.e., turning off as
soon as V+ and V- pump up to the nominal values), the
charge pump contribution to RS-232 mode I
significantly. Unlike competing devices 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 650kbps, and are
guaranteed for data rates up to 460kbps. The charge pumps
and drivers are designed such that one driver in each port
can be operated at the rated load, and at 460kbps (see
Figure 34). Figure 34 also shows that drivers can easily drive
several thousands of 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 ISLX1334 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 ±7V
outside the power supplies (i.e., +12V and -7V), making
them ideal for long networks where induced voltages are a
realistic concern. Each RS-485/422 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
16
CC
is reduced
ISL81334, ISL41334
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
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 ISL81334 (SOIC and
SSOP) receiver outputs are not independently tri-statable.
ISL41334 (QFN) receiver outputs are tri-statable via an
active low RXEN input for each port (see “ISL41334 Special
Features” for more details).
Tx Features
The RS-485/422 driver is a differential output device that
delivers at least 2.2V across a 54Ω load (RS-485), and at
least 2.5V across a 100Ω load (RS-422). Both levels
significantly exceed the standards requirements, and these
exceptional output voltages increase system noise immunity,
and/or allow for transmission over longer distances. 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 ISLX1334 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. 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 5V power supply is off.
Speed Options
The ISL81334 (SOIC/SSOP) has fixed, high slew rate driver
outputs optimized for 20Mbps data rates. The ISL41334
(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.
December 20, 2005
FN6202.1