ISL4238E Intersil Corporation, ISL4238E Datasheet - Page 12

• Manufacturer Part Number: ISL4238E
• Description: QFN Packaged, 15kV Esd Protected, +2.7V to +5.5V, 10Nanoamp, 250kbps/ 1Mbps, RS-232 Transceivers With Enhanced Automatic Powerdown
• Manufacturer: Intersil Corporation

cable is attached INVALID and FORCEOFF remain high, so
the enhanced automatic powerdown logic powers down the
RS-232 device whenever there is 30 seconds of inactivity on
the receiver and transmitter inputs. Detaching the cable
allows the receiver inputs to drop to an invalid level (GND),
so INVALID switches low and forces the RS-232 device to
power down. The ISL4238E/4XE remains powered down
until the cable is reconnected (INVALID = FORCEOFF = 1)
and a transition occurs on a receiver or transmitter input (see
Figure 7). For immediate power up when the cable is
reattached, connect FORCEON to FORCEOFF through a
network similar to that shown in Figure 5.
Capacitor Selection
The ISL4238E charge pumps require 0.1µF, or greater,
capacitors for 3.3V (5% tolerance) operation. For other
supply voltages refer to Table 3 for capacitor values. Do not
use values smaller than those listed in Table 3.
The ISL4244E/45E charge pumps require 0.1µF capacitors
for proper operation. Increasing the capacitor values (by a
factor of 2) reduces ripple on the transmitter outputs and
slightly reduces power consumption. C
increased without increasing C
increase C
maintain the proper ratios (C
3.0 to 3.6 (3.3V ±10%)
3.15 to 3.6 (3.3V ±5%)
TRANSMITTER
TRANSMITTER
TABLE 3. REQUIRED CAPACITOR VALUES (ISL4238E)
RECEIVER
OUTPUTS
4.5 to 5.5
3.0 to 5.5
OUTPUT
OUTPUT
V
INVALID
INPUTS
INPUTS
CC
1
V
(V)
without also increasing C
V+
CC
V-
0
t
INVL
FIGURE 9. ENHANCED AUTOMATIC POWERDOWN AND INVALID TIMING DIAGRAMS
C
0.047
0.22
0.22
1
12
1
0.1
(µF)
to the other capacitors).
1
’s value, however, do not
t
INVH
2
2
, C
, C
3
C
3
, and C
ISL4238E, ISL4244E, ISL4245E
, and C
2
, C
t
AUTOPWDN
0.22
0.33
3
0.1
, C
1
4
4
4
to
(µF)
can be
When using minimum required capacitor values, make sure
that capacitor values do not degrade excessively with
temperature. If in doubt, use capacitors with a larger nominal
value. The capacitor’s equivalent series resistance (ESR)
usually rises at low temperatures and it influences the
amount of ripple on V+ and V-.
Power Supply Decoupling
In most circumstances a 0.1µF bypass capacitor is
adequate. In applications that are particularly sensitive to
power supply noise, decouple V
capacitor of the same value as the charge-pump capacitor C
Connect the bypass capacitor as close as possible to the IC.
Transmitter Outputs when Exiting
Powerdown
Figure 10 shows the response of two transmitter outputs
when exiting powerdown mode. As they activate, the two
transmitter outputs properly go to opposite RS-232 levels,
with no glitching, ringing, nor undesirable transients. Each
transmitter is loaded with 3kΩ in parallel with 2500pF. Note
that the transmitters enable only when the magnitude of the
supplies exceed approximately 3V.
Operation Down to 2.7V
ISL4238E/4XE transmitter outputs meet RS-562 levels
(
RS-562 levels typically ensure interoperability with RS-232
devices.
±
t
3.7V), at the full data rate, with V
WU
t
AUTOPWDN
CC
CC
t
to ground with a
WU
as low as 2.7V.
}
INVALID
REGION
1
.