ISL4238E_14 Datasheet, PDF (12/17 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +2.7V to +5.5V, 10Nanoamp, 250kbps/1Mbps, RS-232 Transceivers with Enhanced Automatic Power-down

ISL4238E, ISL4244E, ISL4245E
RECEIVER
INPUTS
TRANSMITTER
INPUTS
TRANSMITTER
OUTPUTS
INVALID
OUTPUT
t_INVL
OUTPUT
V+
t_INVH
t_AUTOPWDN
INVALID
} _REGION
t_WU
t_AUTOPWDN
t_WU
V_CC
0
V-
FIGURE 9. ENHANCED AUTOMATIC POWER-DOWN AND INVALID TIMING DIAGRAMS
Hybrid Automatic Power-down Options
For devices which communicate only through a detachable
cable, connecting INVALID to FORCEOFF (with
FORCEON = 0) may be a desirable configuration. While the
cable is attached INVALID and FORCEOFF remain high, so the
enhanced automatic power-down 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, ISL4244E, ISL4245 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.
TABLE 3. REQUIRED CAPACITOR VALUES (ISL4238E)
V_CC (V)
3.0 to 3.6 (3.3V ±10%)
C₁ (µF)
0.22
C₂, C₃, C₄ (µF)
0.22
3.15 to 3.6 (3.3V ±5%)
0.1
0.1
4.5 to 5.5
0.047
0.33
3.0 to 5.5
0.22
1
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. C₂, C₃, and C₄ can be
increased without increasing C₁’s value, however, do not
increase C₁ without also increasing C₂, C₃, and C₄ to
maintain the proper ratios (C₁ to the other capacitors).
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_CC to ground with a
capacitor of the same value as the charge-pump capacitor C₁.
Connect the bypass capacitor as close as possible to the IC.
Transmitter Outputs when Exiting
Power-down
Figure 10 shows the response of two transmitter outputs
when exiting power-down 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, ISL4244E, ISL4245 transmitter outputs meet
RS-562 levels (±3.7V), at the full data rate, with V_CC as low
as 2.7V. RS-562 levels typically ensure interoperability with
RS-232 devices.
12
FN8038.5
May 13, 2010