
13
FN4910.18
March 8, 2005
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.
Devices with the automatic powerdown 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 7). 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. When automatic 
powerdown is utilized, INVALID = 0 indicates that the 
ICL32XXE is in powerdown mode.
The time to recover from automatic powerdown mode is 
typically 100μs.
Receiver ENABLE Control
(ICL3221E, ICL3222E, ICL3223E, ICL3241E Only)
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 2). The enable input has no effect on 
transmitter nor monitor (R
OUTB
) outputs.
Capacitor Selection
The charge pumps require 0.1μF capacitors for 3.3V 
operation. For other supply voltages refer to Table 3 for 
capacitor values. Do not use values smaller than those listed 
in Table 3. Increasing the capacitor values (by a factor of 2) 
reduces ripple on the transmitter outputs and slightly 
reduces power consumption. C
2
, C
3
, and C
4
 can be 
increased without increasing C
1
’s value, however, do not 
increase C
1
 without also increasing C
2
, C
3
, and C
4
 to 
maintain the proper ratios (C
1
 to the other capacitors).
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.
Operation Down to 2.7V
ICL32XXE transmitter outputs meet RS-562 levels (
±
3.7V), 
at full data rate, with V
CC
 as low as 2.7V. RS-562 levels 
typically ensure interoperability with RS-232 devices.
Transmitter Outputs when Exiting 
Powerdown
Figure 8 shows the response of two transmitter outputs 
when exiting powerdown mode. As they activate, the two 
transmitter outputs properly go to opposite RS-232 levels, 
with no glitching, ringing, nor undesirable transients. Each 
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
0.3V
-0.3V
-2.7V
2.7V
INVALID LEVEL - POWERDOWN OCCURS AFTER 30μs
VALID RS-232 LEVEL - ICL32XXE IS ACTIVE
VALID RS-232 LEVEL - ICL32XXE IS ACTIVE
INDETERMINATE - POWERDOWN MAY OR
MAY NOT OCCUR
INDETERMINATE - POWERDOWN MAY OR
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 POWERDOWN AND INVALID 
TIMING DIAGRAMS
AUTOPWDN
PWR UP
TABLE 3. REQUIRED CAPACITOR VALUES
V
CC
(V)
C
1
(μF)
C
2
, C
3
, C
4
(μF)
3.0 to 3.6
0.1
0.1
4.5 to 5.5
0.047
0.33
3.0 to 5.5
0.1
0.47
ICL3221E, ICL3222E, ICL3223E, ICL3232E, ICL3241E, ICL3243E