ICL3224E_14 Datasheet, PDF (11/20 Pages) Intersil Corporation – +/-15kV ESD Protected, +3V to +5.5V, 1Microamp, 250kbps, RS-232 Transceivers with Enhanced Automatic Powerdown

ICL3224E, ICL3226E, ICL3244E
POWER
MANAGEMENT
UNIT
MASTER POWERDOWN LINE
0.1µF
1MΩ
FORCEOFF FORCEON
ICL32XXE
FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP
WHEN EXITING FORCED POWERDOWN
INVALID Output
The INVALID output always indicates (see Table 2) whether
or not 30µs have elapsed with invalid RS-232 signals (see
Figures 6 and 8) persisting on all of the receiver inputs,
giving the user an easy way to determine when the interface
block should power down. Invalid receiver levels occur
whenever the driving peripheral’s outputs are shut off
(powered down) or when the RS-232 interface cable is
disconnected. In the case of a disconnected interface cable
where all the receiver inputs are floating (but pulled to GND
by the internal receiver pull down resistors), the INVALID
logic detects the invalid levels and drives the output low. The
power management logic 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).
2.7V
VALID RS-232 LEVEL - INVALID = 1
INDETERMINATE
0.3V
-0.3V
INVALID LEVEL - INVALID = 0
INDETERMINATE
-2.7V
VALID RS-232 LEVEL - INVALID = 1
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
Enhanced Automatic Powerdown
Even greater power savings is available by using these
devices which feature an enhanced automatic powerdown
function. When the enhanced powerdown logic determines
that no transitions have occurred on any of the transmitter
nor receiver inputs for 30 seconds, the charge pump and
transmitters powerdown, thereby reducing supply current to
1µA. The ICL32XXE automatically powers back up
whenever it detects a transition on one of these inputs. This
automatic powerdown feature provides additional system
power savings without changes to the existing operating
system.
Enhanced 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 enhanced
automatic powerdown functionality.
Figure 7 illustrates the enhanced powerdown control logic.
Note that once the ICL32XXE enters powerdown (manually
or automatically), the 30 second timer remains timed out
(set), keeping the ICL32XXE powered down until FORCEON
transitions high, or until a transition occurs on a receiver or
transmitter input.
T_IN
EDGE
DETECT
R_IN
EDGE
DETECT
FORCEOFF
S
30s
TIMER
R
AUTOSHDN
FORCEON
FIGURE 7. ENHANCED AUTOMATIC POWERDOWN LOGIC
The INVALID output signal switches low to indicate that
invalid levels have persisted on all of the receiver inputs for
more than 30µs (see Figure 8), but this has no direct effect
on the state of the ICL32XXE (see the next sections for
methods of utilizing INVALID to power down the device).
INVALID switches high 1µs after detecting a valid RS-232
level on a receiver input. INVALID operates in all modes
(forced or automatic powerdown, or forced on), so it is also
useful for systems employing manual powerdown circuitry.
The time to recover from automatic powerdown mode is
typically 100µs.
Emulating Standard Automatic Powerdown
If enhanced automatic powerdown isn’t desired, the user can
implement the standard automatic powerdown feature
(mimics the function on the ICL3221E/23E/43E) by
connecting the INVALID output to the FORCEON and
FORCEOFF inputs, as shown in Figure 9. After 30µs of
invalid receiver levels, INVALID switches low and drives the
ICL32XXE into a forced powerdown condition. INVALID
switches high as soon as a receiver input senses a valid
RS-232 level, forcing the ICL32XXE to power on. See the
“INVALID DRIVING FORCEON AND FORCEOFF” section
of Table 2 for an operational summary. This operational
mode is perfect for handheld devices that communicate with
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