ISL3332 Datasheet, PDF (19/26 Pages) Intersil Corporation – 3.3V, ±15kV ESD Protected, Two Port, Dua Protocol RS-232/RS-485 Transceivers

ISL3332, ISL3333
ON/OFF) being driven to the low input state. The worst case
I_L current occurs during SHDN (see Figure 21), due to the I_L
through the ON/OFF pin pull-up resistor when that pin is
driven low. I_IL through an input pull-up resistor is ~11µA (6µA
for DE1 and DE2), so the I_L in Figure 20 drops by about
22µA (at V_L = 3.3V) when the two SP inputs are high versus
low (next to bottom vs. top curve). SHDN I_L is lowest in the
RS-232 mode, because only the DEN pins and/or the
ON/OFF pin should be driven low. When all the inputs with
pull-downs are driven high, I_L drops to <<1µA (see Figure
20), so to minimize power dissipation drive these inputs high
when unneeded (e.g., SP inputs aren’t used in RS-232
mode, and DEN inputs aren’t used in RS-485 mode, so drive
them high in those modes).
QFN logic input pins that are externally tied high in an
application, should use the V_L supply for the high voltage
level.
RS-232 Mode Tx Enable/Disable (DEN)
The ISL3333 also adds an RS-232 mode Tx enable pin
(DENX) for each port. Driving one of these pins low disables
both drivers in the corresponding port. Because RS-232 is a
point-to-point (only one Tx allowed on the bus) standard, the
main use for this disable function is to reduce power by
eliminating the load current (approximately 1mA per Tx
output) through the 5kΩ resistor in the Rx at the cable’s far
end. The I_CC in this mode is still considerably higher than in
SHDN, but the enable time from Tx disable is much faster
(1.5µs vs. 25µs) than the enable time from SHDN due to the
charge pumps remaining on during Tx disable.
The DENX pin is ignored if the corresponding port is set for
RS-485 mode, and it is internally pulled high.
Active Low Rx Enable (RXEN)
In many RS-485 applications, especially half duplex
configurations, users like to accomplish “echo cancellation”
by disabling the corresponding receiver while its driver is
transmitting data. This function is available on the QFN
package via an active low RXEN pin for each port. The
active low function also simplifies direction control, by
allowing a single Tx/Rx direction control line. If an active high
RXEN were used, either two valuable I/O pins would be
used for direction control, or an external inverter is required
between DE and RXEN. Figure 13 details the advantage of
using the RXEN pin.
RS-485 Slew Rate Limited Data Rates
The SSOP version of this IC operates with Tx output
transitions optimized for a 20Mbps data rate. These fast
edges may increase EMI and reflection issues, even though
fast transitions aren’t required at the lower data rates used
by many applications. The ISL3333 (QFN version) solves
this problem by offering two additional, slew rate limited,
data rates that are optimized for speeds of 115kbps, and
460kbps.The slew limited edges permit longer unterminated
networks, or longer stubs off terminated busses, and help
Tx/Rx
+3.3V
ISL3330
RA
RXEN
V_CC
RB
A
DEN
DY
D
Y
Z
GND
+
0.1µF
ACTIVE HIGH RX ENABLE
+3.3V
ISL3333
RA
RXEN *
Tx/Rx
DE
DY
V_CC
RB
A
Y
D
Z
GND
* QFN ONLY
+
0.1µF
ACTIVE LOW RX ENABLE
FIGURE 13. USING ACTIVE LOW vs ACTIVE HIGH RX
ENABLE
minimize EMI and reflections. Nevertheless, for the best jitter
performance when driving long cables, the faster speed
options may be preferable, even at lower data rates. The
faster output transitions deliver less variability (jitter) when
loaded with the large capacitance associated with long
cables. Of course, faster transitions require more attention to
ensuring short stub lengths and quality terminations, so
there are trade-offs to be made. Assuming a jitter budget of
10%, it is likely better to go with the slow speed option for
data rates of 115kbps or less, to minimize fast edge effects.
Likewise, the medium speed option is a good choice for data
rates between 115kbps and 460kbps. For higher data rates,
or when the absolute best jitter is required, use the high
speed option. Speed selection is via the SPA and SPB pins
(see Table 3), and the selection pertains to each port
programmed for RS-485 mode.
Evaluation Board
An evaluation board, part number ISL3333EVAL1Z, is
available to assist in assessing the dual protocol IC’s
performance. The evaluation board contains a QFN
packaged device, but because the same die is used in all
packages, the board is also useful for evaluating the
functionality of the other versions. The board’s design allows
for evaluation of all standard features, plus the QFN specific
features. Refer to the eval board application note for details,
and contact your sales rep for ordering information.
19
FN6362.0
May 27, 2008