
8
FORCEOFF switches high. The rising edge of the Master 
Powerdown signal forces the device to power up, and the 
ICL3238E returns to enhanced automatic powerdown mode 
an RC time constant after this rising edge. The time constant 
isn’t critical, because the ICL3238E remains powered up for 
30 seconds after the FORCEON falling edge, even if there 
are no signal transitions. This gives slow-to-wake systems 
(e.g., a mouse) plenty of time to start transmitting, and as 
long as it starts transmitting within 30 seconds both systems 
remain enabled.
INVALID Output
The INVALID output always indicates (see Table 2) whether 
or not 30
μ
s 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).
Enhanced Automatic Powerdown
Even greater power savings is available by using this device 
which features 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 
10nA. The ICL3238E 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 ICL3238E enters powerdown (manually 
or automatically), the 30 second timer remains timed out 
(set), keeping the ICL3238E powered down until FORCEON 
transitions high, or until a transition occurs on a receiver or 
transmitter input.
The INVALID output signal switches low to indicate that 
invalid levels have persisted on all of the receiver inputs for 
more than 60
μ
s (see Figure 8), but this has no direct effect 
on the state of the ICL3238E (see the next sections for 
methods of utilizing INVALID to power down the device). 
INVALID switches high 1
μ
s 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 25
μ
s.
FIGURE 5. CIRCUIT TO ENSURE IMMEDIATE POWER UP 
WHEN EXITING FORCED POWERDOWN
FORCEOFF
FORCEON
POWER
MANAGEMENT
UNIT
MASTER POWERDOWN LINE
1M
0.1μF
ICL3238E
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
0.3V
-0.3V
-2.7V
2.7V
INVALID LEVEL - INVALID = 0
VALID RS-232 LEVEL - INVALID = 1
VALID RS-232 LEVEL - INVALID = 1
INDETERMINATE
INDETERMINATE
FIGURE 7. ENHANCED AUTOMATIC POWERDOWN LOGIC
30s
TIMER
S
R
FORCEOFF
AUTOSHDN
FORCEON
R_IN
T_IN
EDGE
DETECT
EDGE
DETECT
ICL3238E