ISL4221E Datasheet, PDF (7/13 Pages) Intersil Corporation – QFN Packaged, +/-15kV ESD Protected, +2.7V to +5.5V, 150Nanoamp, 250kBps, RS-232 Transmitters/Receivers

ISL4221E, ISL4223E
RS-232 SIGNAL
PRESENT AT
RECEIVER INPUT?
NO
NO
YES
YES
NO
NO
YES
YES
NO
NO
TABLE 2. POWERDOWN AND ENABLE LOGIC TRUTH TABLE
FORCEOFF FORCEON EN TRANSMITTER RECEIVER INVALID
INPUT
INPUT INPUT OUTPUTS OUTPUTS OUTPUT
MODE OF OPERATION
H
H
L
Active
Active
L
Normal Operation
(Auto Powerdown Disabled)
H
H
H
Active
High-Z
L
H
L
L
Active
Active
H
Normal Operation
(Auto Powerdown Enabled)
H
L
H
Active
High-Z
H
H
L
L
High-Z
Active
L
Powerdown Due to Auto Powerdown
Logic
H
L
H
High-Z
High-Z
L
L
X
L
High-Z
Active
H
Manual Powerdown
L
X
H
High-Z
High-Z
H
Manual Powerdown w/Rcvr. Disabled
L
X
L
High-Z
Active
L
Manual Powerdown
L
X
H
High-Z
High-Z
L
Manual Powerdown w/Rcvr. Disabled
mode. For always enabled operation, FORCEON and
FORCEOFF are both strapped high. To switch between
active and powerdown modes, under logic or software
control, only the FORCEOFF input need be driven. The
FORCEON state isn’t critical, as FORCEOFF dominates
over FORCEON. Nevertheless, if strictly manual control over
powerdown is desired, the user must strap FORCEON high
to disable the automatic powerdown circuitry.
Connecting FORCEOFF and FORCEON together disables
the automatic powerdown feature, enabling them to function
as a manual SHUTDOWN input (see Figure 4).
PWR
MGT
LOGIC
FORCEOFF
FORCEON
INVALID
ISL422XE
CPU
I/O
UART
FIGURE 4. CONNECTIONS FOR MANUAL POWERDOWN
WHEN NO VALID RECEIVER SIGNALS ARE
PRESENT
The time to recover from automatic powerdown mode is
typically 100µs.
POWER
MANAGEMENT
UNIT
MASTER POWERDOWN LINE
0.1µF
1MΩ
FORCEOFF FORCEON
ISL422XE
FIGURE 5. CIRCUIT TO PREVENT AUTO POWERDOWN FOR
100ms AFTER FORCED POWERUP
Automatic Powerdown
Even greater power savings is available by using the
automatic powerdown function. When no valid RS-232
voltages (see Figure 6) 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 ISL422XE 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.
Some applications may need more time to wake up from
shutdown. If automatic powerdown is being utilized, the RS-
232 device will reenter powerdown if valid receiver levels
aren’t reestablished within 30µs of the ISL422XE powering
up. Figure 5 illustrates a circuit that keeps the ISL422XE
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