ISL4241E Intersil Corporation, ISL4241E Datasheet - Page 9

• Manufacturer Part Number: ISL4241E
• Description: QFN Packaged, +/-15kV Esd Protected, +2.7V to +5.5V, 10Nanoamp, 250kbps, RS-232 Transmitters/receivers
• Manufacturer: Intersil Corporation

Automatic Powerdown (ISL4243E Only)
Even greater power savings is available by using the
ISL4243E which features an automatic powerdown function.
When no valid RS-232 voltages (see Figure 5) are sensed
on any receiver input for 30µs, the charge pump and
transmitters powerdown, thereby reducing supply current to
10nA. Invalid receiver levels occur whenever the driving
peripheral’s outputs are shut off (powered down) or when
the RS-232 interface cable is disconnected. The ISL4243E
powers back up whenever it detects a valid RS-232 voltage
level on any receiver input. This automatic powerdown
feature provides additional system power savings without
changes to the existing operating system.
Automatic powerdown operates when the FORCEON input
is low, and the FORCEOFF input is high. Tying FORCEON
high disables automatic powerdown, but manual powerdown
is always available via the overriding FORCEOFF input.
Table 2 summarizes the automatic powerdown functionality.
The time to recover from automatic powerdown mode is
typically 100µs.
Capacitor Selection
The charge pumps require 0.1µF, or greater, capacitors for
proper operation. Increasing the capacitor values (by a
factor of 2) reduces ripple on the transmitter outputs and
slightly reduces power consumption.
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
TRANSMITTER
FIGURE 7. AUTOMATIC POWERDOWN AND INVALID
OUTPUT
INVALID
RECEIVER
OUTPUTS
INPUTS
V
V
CC
CC
V+
V-
0
TIMING DIAGRAMS
0
AUTOPWDN
t
INVL
9
.
t
INVH
PWR UP
ISL4241E, ISL4243E
}
INVALID
REGION
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 8 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
ISL424XE transmitter outputs meet RS-562 levels (
at the full data rate, with V
typically ensure inter operability with RS-232 devices.
High Data Rates
The ISL424XE maintain the RS-232
transmitter output voltages even at high data rates. Figure 9
details a transmitter loopback test circuit, and Figure 10
illustrates the loopback test result at 120kbps. For this test,
all transmitters were simultaneously driving RS-232 loads in
parallel with 1000pF, at 120kbps. Figure 11 shows the
loopback results for a single transmitter driving 1000pF and
an RS-232 load at 250kbps. The static transmitters were
also loaded with an RS-232 receiver.
5V/DIV.
2V/DIV.
FIGURE 8. TRANSMITTER OUTPUTS WHEN EXITING
V
C1 - C4 = 0.1µF
CC
= +3.3V
POWERDOWN
FORCEOFF
CC
TIME (20µs/DIV.)
as low as 2.7V. RS-562 levels
CC
to ground with a
±
5V minimum
T1
T2
±
3.7V),
1
.