ISL3330_14 Datasheet, PDF (16/25 Pages) Intersil Corporation – 3.3V, ±15kV ESD Protected, Dual Protocol (RS-232/RS-485) Transceivers

ISL3330, ISL3331
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 specification
allows a maximum of two terminations on a network,
otherwise the Tx output voltage may not meet the required
V_OD.
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 (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.
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
20 Ld or 24 Ld SSOP. The ISL3331’s tiny 6mmx6mm QFN
footprint is 36% to 44% smaller than the competing SSOPs.
Flow-Through Pinouts
Even the ISL333x pinouts are features, in that the
“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 on the
other side for easy connection to a UART, avoids costly and
problematic crossovers. Figure 10 illustrates the
flow-through nature of the pinout.
Low Power Shutdown (SHDN) Mode
The ISL333x enter the SHDN mode when ON = 0, and the
Tx and Rx are disabled (DEN = 0, RXEN = 0, and
RXEN = 1) and the already low supply current drops to as
low as 10µA. SHDN disables the Tx and Rx outputs, and
disables the charge pumps if the port is in RS-232 mode, so
V+ collapses to V_CC, and V- collapses to GND.
All but 10µA of SHDN I_CC current is due to control input
(SPB, SLEW) pull-up resistors (~10µA/resistor), so SHDN
I_CC varies depending on the ISL333x configuration. The
specification tables indicate the worst case values, but
careful selection of the configuration yields lower currents.
For example, in RS-232 mode the SPB pin isn’t used, so
floating it or tying it high minimizes SHDN I_CC.
ISL3330
A
B
R
RA
UART
OR
Y
D
DY
Z
ASIC
OR
µCONTROLLER
FIGURE 10. ILLUSTRATION OF FLOW-THROUGH PINOUT
When enabling from SHDN in RS-232 mode, allow at least
25µs for the charge pumps to stabilize before transmitting
data. If fast enables are required, and I_CC isn’t the greatest
concern, disable the drivers with the DEN pin to keep the
charge pumps active. The charge pumps aren’t used in
RS-485 mode, thus the transceiver is ready to send or
receive data in less than 2µs, which is much faster than
competing devices that require the charge pump for all
modes of operation.
Internal Loopback Mode
Setting ON = 0, DEN = 1, and RXEN = 1 or RXEN = 0 (QFN
only), places the port in the loopback mode, a mode that
facilitates implementing board level self test functions. In
loopback, internal switches disconnect the Rx inputs from
the Rx outputs, and feed back the Tx outputs to the
appropriate Rx output. This way the data driven at the Tx
input appears at the corresponding Rx output (refer to
“Typical Operating Circuits” on page 6). The Tx outputs
remain connected to their terminals, so the external loads
are reflected in the loopback performance. This allows the
loopback function to potentially detect some common bus
faults such as one or both driver outputs shorted to GND, or
outputs shorted together.
Note that the loopback mode uses an additional set of
receivers, as shown in the “Typical Operating Circuits” on
page 6. These loopback receivers are not standards
compliant, so the loopback mode can’t be used to implement
a half-duplex RS-485 transceiver. See Application Note
AN1401 for specific details on implementing a 3-pin, half
duplex, dual protocol port.
16
FN6361.0
May 20, 2008