isl3330 Intersil Corporation, isl3330 Datasheet - Page 17

• Manufacturer Part Number: isl3330
• Description: 3.3v, ?15kv Esd Protected, Dual Protocol Rs-232/rs-485 Transceivers
• Manufacturer: Intersil Corporation

ISL3331 (QFN Package) Special Features
Logic Supply (V
The ISL3331 (QFN) includes a V
inputs (Tx inputs and control pins) and Rx outputs. These
pins interface with “logic” devices such as UARTs, ASICs,
and µcontrollers, and today many of these devices use
power supplies significantly lower than 3.3V. Thus, a 3.3V
output level from a 3.3V powered dual protocol IC might
seriously overdrive and damage the logic device input.
Similarly, the logic device’s low V
V
V
Figure 11) limits the ISL3331’s Rx output V
Figure 14) and reduces the Tx and control input switching
points to values compatible with the logic device output
levels. Tailoring the logic pin input switching points and
output levels to the supply voltage of the UART, ASIC, or
µcontroller eliminates the need for a level shifter/translator
between the two ICs.
V
switching points may not provide enough noise margin when
V
various V
a particular V
IH
L
L
L
pin to the power supply of the logic device (as shown in
can be anywhere from V
< 1.5V. Table 5 indicates typical V
FIGURE 11. USING V
of a 3.3V powered dual protocol input. Connecting the
V
V
GND
GND
L
CC
CC
ISL3330
ISL3331
voltages so the user can ascertain whether or not
= +3.3V
= +3.3V
L
voltage meets his needs.
R
R
D
D
V
L
A
Y
A
Y
L
Pin)
V
V
V
V
OH
IH
OH
IH
L
= 1.4V
≥ 2
PIN TO ADJUST LOGIC LEVELS
= 3.3V
= 2V
CC
17
V
V
OH
OH
down to 1.2V, but the input
L
OH
≤ 2
≤ 2
pin that powers the logic
UART/PROCESSOR
UART/PROCESSOR
might not exceed the
IH
R
T
R
T
XD
XD
XD
XD
V
V
and V
CC
CC
OH
= +2V
= +2V
DIODE
DIODE
ESD
GND
ESD
GND
IL
to V
values for
L
(see
ISL3330, ISL3331
.
Note: With V
data rate unless the logic signal V
the typical value listed in Table 5.
The V
shown in Figures 19 and 20. All of the DC V
to inputs with internal pull-up resistors (SPB, SLEW, RXEN)
being driven to the low input state. The worst case I
occurs when all three of the inputs are low (see Figure 19),
due to the I
input pull-up resistor is ~10µA, so the I
by about 18µA (at V
mode disables the SLEW pin pull-up (middle vs top curve).
When all three inputs are driven high, I
Thus, to minimize power dissipation, drive these inputs high
when unneeded (e.g., SPB isn’t used in RS-232 mode, so
drive it high).
QFN logic input pins that are externally tied high in an
application, should use the V
level.
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. The active low function
also simplifies direction control by allowing a single Tx/Rx
direction control line. If the active high RXEN were used,
either two valuable I/O pins would be used for direction
control, or an external inverter is required between DEN and
RXEN. Figure 12 details the advantage of using the RXEN
pin. When using RXEN, ensure that RXEN is tied to GND.
RS-485 Slew Rate Limited Data Rates
The ISL333x FAST speed option (SLEW = High) utilizes 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. With the SLEW pin low, both
product types switch to a moderately slew rate limited output
transition targeted for 460kbps (MED) data rates. The
ISL3331 (QFN version), offers an additional slew rate limited
data rate that is optimized for 115kbps (SLOW), and is
selected when SLEW = 0 and SPB = 0 (see Table 3). The
L
supply current (I
V
TABLE 5. V
L
1.2
1.5
1.8
2.3
2.7
3.3
L
(V)
L
through the pull-up resistors. I
≤ 1.6V, the ISL3331 may not operate at the full
L
IH
= 3.3V) when the SPB is high and 232
AND V
L
) is typically less than 35µA, as
IL
L
V
supply for the high voltage
vs V
IH
0.85
0.9
0.9
1.2
1.4
1.8
(V)
IL
L
is at least 0.2V below
FOR V
L
L
in Figure 19 drops
drops to ~10nA.
CC
IL
L
through an
current is due
= 3.3V
V
0.26
0.73
IL
0.5
1.0
1.3
1.7
May 20, 2008
L
(V)
current
FN6361.0