ICL3221EM, ICL3221EF
8
FN7552.1
October 13, 2016
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With any of the control schemes, the time required to exit
power-down, and resume transmission is only 100µs. A mouse,
or other application, may need more time to wake up from
shutdown. If automatic power-down is being utilized, the RS-232
device will re-enter power-down if valid receiver levels aren’t
reestablished within 30µs of the ICL3221EM, ICL3221EF
powering up. Figure 6
illustrates a circuit that keeps the
ICL3221EM, ICL3221EF from initiating automatic power-down
for 100ms after powering up. This gives the slow-to-wake
peripheral circuit time to reestablish valid RS-232 output levels.
Automatic Power-Down
Even greater power savings is available by using the devices
which feature an automatic power-down function. When no valid
RS-232 voltages (see Figure 7
) are sensed on any receiver input
for 30µs, the charge pump and transmitters power-down, thereby
reducing supply current to 1µA. 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
ICL3221EM, ICL3221EF 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 on page 7
summarizes the automatic power-down functionality.
Devices with the automatic power-down feature include an
INVALID
output signal, which switches low to indicate that invalid
levels have persisted on all of the receiver inputs for more than
30µs (see Figure 8
). INVALID switches 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 ICL3221EM, ICL3221EF is in power-down mode.
The time to recover from automatic power-down mode is typically
100µs.
Receiver ENABLE Control
Several devices also feature an EN input to control the receiver
outputs. Driving EN
high disables all the inverting (standard)
receiver outputs placing them in a high impedance state. This is
useful to eliminate supply current, due to a receiver output
forward biasing the protection diode, when driving the input of a
powered down (V
CC
= GND) peripheral (see Figure 3 on page 7).
FIGURE 5. CONNECTIONS FOR MANUAL POWER-DOWN WHEN NO
VALID RECEIVER SIGNALS ARE PRESENT
PWR
FORCEOFF
INVALID
CPU
I/O
FORCEON
ICL3221EM
MGT
LOGIC
UART
ICL3221EF
FIGURE 6. CIRCUIT TO PREVENT AUTO POWER-DOWN FOR 100ms
AFTER FORCED POWER-UP
ICL3221EM
FORCEOFF
FORCEON
POWER
MASTER POWER-DOWN LINE
1MΩ0.1µF
MANAGEMENT
UNIT
ICL3221EF
FIGURE 7. DEFINITION OF VALID RS-232 RECEIVER LEVELS
0.3V
-0.3V
-2.7V
2.7V
INVALID LEVEL - POWER-DOWN OCCURS
VALID RS-232 LEVEL - ICL3221EM, ICL3221EF IS ACTIVE
VALID RS-232 LEVEL - ICL3221EM, ICL3221EF IS ACTIVE
INDETERMINATE - POWER-DOWN MAY OR
INDETERMINATE - POWER-DOWN MAY OR
MAY NOT OCCUR
MAY NOT OCCUR
AFTER 30µs
RECEIVER
INPUTS
TRANSMITTER
OUTPUTS
INVALID
OUTPUT
V+
V
CC
0
V-
V
CC
0
t
INVL
t
INVH
INVALID
REGION
}
FIGURE 8. AUTOMATIC POWER-DOWN AND INVALID TIMING
DIAGRAMS
AUTOPWDN
PWR UP
