7
This automatic powerdown feature provides additional
system power savings without changes to the existing
operating system or hardware.
Utilizing power management circuitry, to power down the
rest of the communication circuitry (e.g., the UART) when
the ICL3217 powers down, produces even greater power
savings. Connecting a transition detector to the V- pin (see
Figure 3) is an easy way for the power management logic to
determine when the ICL3217 enters and exits powerdown.
Capacitor Selection
The charge pumps require 0.1F, or greater, capacitors for
3.3V operation. With 0.1F capacitors, five percent tolerance
supplies (e.g., 3.14V minimum) deliver greater than 5V
transmitter swings at full data rate, while ten percent
tolerance supplies (e.g., 2.97V minimum) deliver 4.95V
transmitter swings. If greater than 5V transmitter swings
are required with a ten percent tolerance 3.3V supply,
0.22F capacitors are recommended (see Table 3). Existing
5V applications typically utilize either 0.1F or 1F
capacitors, and the ICL32X7E works well with either value.
New 5V designs should use 0.22F capacitors for the best
results. 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 0.1F 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.
Transmitter Outputs when Exiting
Powerdown
Figure 6 shows the response of two ICL3217 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 transmitter is loaded with 3kin parallel with 2500pF.
Note that the transmitters enable only when the magnitude
of the supplies exceed approximately 3V.
.
FIGURE 4. DEFINITION OF VALID RS-232 RECEIVER
LEVELS
0.3V
-0.3V
-2.7V
2.7V
INVALID LEVEL - POWERDOWN OCCURS AFTER 30s
VALID RS-232 LEVEL - ICL3217 IS ACTIVE
VALID RS-232 LEVEL - ICL3217 IS ACTIVE
INDETERMINATE - POWERDOWN MAY OR
MAY NOT OCCUR
INDETERMINATE - POWERDOWN MAY OR
MAY NOT OCCUR
RECEIVER
INPUTS
TRANSMITTER
OUTPUTS
V+
V
CC
0
V-
INVALID
REGION
}
FIGURE 5. AUTOMATIC POWERDOWN TIMING DIAGRAM
AUTOPWDN
PWR UP (t
WU
)
TABLE 3. REQUIRED CAPACITOR VALUES
V
CC
(V) C
1
(F) C
2
, C
3
, C
4
(F)
3.15 to 3.6 0.1 0.1
3.0 to 3.6 0.22 0.22
4.5 to 5.5 0.1 to 1.0 0.1 to 1.0
3.0 to 5.5 0.22 0.22
TIME (20s/DIV.)
T1
T2
2V/DIV
5V/DIV
V
CC
= +3.3V
C1 - C4 = 0.1F
FIGURE 6. TRANSMITTER OUTPUTS WHEN EXITING
POWERDOWN (ICL3217ONLY)
RX
IN
ICL3207, ICL3217
