10
FN8037.7
June 10, 2010
then uses this indicator to power down the interface block.
Reconnecting the cable restores valid levels at the receiver
inputs, INVALID
switches high, and the power management
logic wakes up the interface block. INVALID
can also be
used to indicate the DTR or RING INDICATOR signal, as
long as the other receiver inputs are floating, or driven to
GND (as in the case of a powered down driver).
INVALID
switches low after invalid levels have persisted on
all of the receiver inputs for more than 30µs (see Figure 7).
INVALID
switches back high 1µs after detecting a valid
RS-232 level on a receiver input. INVALID
operates in all
modes (forced or automatic power-down, or forced on), so it
is also useful for systems employing manual power-down
circuitry. When automatic power-down is utilized, INVALID
= 0
indicates that the ISL4243E is in power-down mode.
Automatic Power-down (ISL4243E Only)
Even greater power savings is available by using the
ISL4243E which features an automatic power-down
function. When no valid RS-232 voltages (see Figure 5) are
sensed on any receiver input for 30µs, the charge pump and
transmitters power-down, 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 power-down
feature provides additional system power savings without
changes to the existing operating system.
Automatic power-down operates when the FORCEON input
is low, and the FORCEOFF
input is high. Tying FORCEON
high disables automatic power-down, but manual
power-down is always available via the overriding
FORCEOFF
input. Table 2 summarizes the automatic
power-down functionality.
The time to recover from automatic power-down 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
power supply noise, decouple V
CC
to ground with a
capacitor of the same value as the charge-pump capacitor C
1
.
Connect the bypass capacitor as close as possible to the IC.
Transmitter Outputs when Exiting
Power-down
Figure 8 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.
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
0.3V
-0.3V
-2.7V
2.7V
INVALID LEVEL - POWER-DOWN OCCURS AFTER 30µs
VALID RS-232 LEVEL - ISL4243E IS ACTIVE
VALID RS-232 LEVEL - ISL4243E IS ACTIVE
INDETERMINATE - POWER-DOWN MAY OR
INDETERMINATE - POWER-DOWN MAY OR
MAY NOT OCCUR
MAY NOT OCCUR
RECEIVER
INPUTS
TRANSMITTER
OUTPUTS
INVALID
OUTPUT
V+
V
CC
0
V-
V
CC
0
t
INVL
t
INVH
INVALID
REGION
}
FIGURE 7. AUTOMATIC POWER-DOWN AND INVALID
TIMING DIAGRAMS
AUTOPWDN
PWR UP
ISL4241E, ISL4243E
