ISL32172E Datasheet, PDF (12/20 Pages) Intersil Corporation – QUAD, ±16.5kV ESD Protected, 3.0V to 5.5V, Low Power, RS-422 Transmitters

ISL32172E, ISL32272E, ISL32372E, ISL32174E, ISL32274E, ISL32374E, ISL32179E
discharge. The result is a more repeatable and predictable
test, but equipment limits prevent testing devices at voltages
higher than ±9kV. Devices in this family survive ±9kV contact
discharges on the RS-422 pins.
Data Rate, Cables, and Terminations
RS-422 is intended for network lengths up to 4000’, but the
maximum system data rate decreases as the transmission
length increases. Devices operating at 32Mbps handle
lengths up to 328’ (100m) in 5V systems, and lengths up to
200’ (62m) in 3.3V systems (see Figures 31 and 32). The
460kbps versions can operate at full data rates with lengths
of thousands of feet. Note that system jitter requirements
may limit a network to shorter distances.
Twisted pair is the cable of choice for RS-422 networks.
Twisted pair cables tend to pick up noise and other
electromagnetically induced voltages as common mode
signals, which are effectively rejected by the differential
receivers in RS-422 ICs.
Proper termination is imperative, when using the 10Mbps or
32Mbps devices, to minimize reflections. Short networks
using the 460kbps versions need not be terminated, but,
terminations are recommended unless power dissipation is
an overriding concern.
In point-to-point, or point-to-multipoint (multiple receivers on
bus) 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.
Built-In Driver Overload Protection
The driver output stages incorporate short circuit current
limiting circuitry which ensures that the output current never
exceeds the RS-422 specification. A novel design sets the
short circuit current limit depending on the V_CC value, so
unlike some competing devices, the V_CC = 5V short circuit
current is only slightly higher than the corresponding
V_CC = 3.3V level (see Figure 12).
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 20°. If the fault persists, the thermal
shutdown/re-enable cycle repeats until the fault is cleared.
High Temperature Operation
With T_A = +125°C and V_CC = 5.5V, four 100Ω differentially
terminated drivers in the TSSOP package put the IC at the
edge of its maximum allowed junction temperature. Using
larger termination resistors, a lower maximum supply
voltage, or one of the packages with a lower thermal
resistance (θ_JA) provides more safety margin. When
designing for +125°C operation, be sure to measure the
application’s switching current, and include this in the
thermal calculations.
Low Power Shutdown Mode (ISL32179E Only)
These BiCMOS transmitters all use a fraction of the power
required by their bipolar counterparts, but the QFN version
includes a shutdown feature that reduces the already low
quiescent I_CC by 90%. The ISL32179E enters shutdown
(SHDN) whenever the SHDNEN pin is high and all four
drivers are disabled (see “Pin Descriptions” on page 4). Note
that the enable times from SHDN are longer than the enable
times when the IC isn’t in SHDN.
Typical Performance Curves V_CC = V_L = 3.3V or 5V, T_A = +25°C; Unless Otherwise Specified. V_L notes apply to the ISL32179E
only.
110
100
+85°C
90
+25°C
80 _+85°C
70
60
+25°C
50
+125°C
40
+125°C
30
20
10
V_CC = 3.3V
V_CC = 5V
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
DIFFERENTIAL OUTPUT VOLTAGE (V)
FIGURE 7. DRIVER OUTPUT CURRENT vs DIFFERENTIAL
OUTPUT VOLTAGE
4.3
R_DIFF = 100Ω
4.1
V_CC = 5V
3.9
3.7
3.5
3.3
3.1
2.9
V_CC = 3.3V
2.7
2.5
-40 -25 -10
5 20 35 50 65 80
TEMPERATURE (°C)
95 110 125
FIGURE 8. DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs
TEMPERATURE
12
FN6824.0
December 16, 2008