12
FN4876.10
March 1, 2006
transition occurs on a receiver or transmitter input (see
Figure 7). For immediate power up when the cable is
reattached, connect FORCEON to FORCEOFF
through a
network similar to that shown in Figure 5.
Ready Output (ICL3224 and ICL3226 only)
The Ready output indicates that the ICL322X is ready to
transmit. Ready switches low whenever the device enters
powerdown, and switches back high during power-up when
V- reaches -4V or lower.
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.
Transmitter Outputs when Exiting
Powerdown
Figure 10 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
transmitter is loaded with 3kΩ in parallel with 2500pF. Note
that the transmitters enable only when the magnitude of the
supplies exceed approximately 3V.
Operation Down to 2.7V
ICL32XX transmitter outputs meet RS-562 levels (±3.7V), at
the full data rate, with V
CC
as low as 2.7V. RS-562 levels
typically ensure interoperability with RS-232 devices.
Mouse Driveability
The ICL3244 is specifically designed to power a serial
mouse while operating from low voltage supplies. Figure 11
shows the transmitter output voltages under increasing load
current. The on-chip switching regulator ensures the
transmitters will supply at least
±5V during worst case
conditions (15mA for paralleled V+ transmitters, 7.3mA for
single V- transmitter).
High Data Rates
The ICL32XX maintain the RS-232 ±5V minimum transmitter
output voltages even at high data rates. Figure 12 details a
transmitter loopback test circuit, and Figure 13 illustrates the
loopback test result at 120kbps. For this test, all transmitters
were simultaneously driving RS-232 loads in parallel with
TABLE 3. REQUIRED CAPACITOR VALUES (Note 8)
V
CC
(V) C
1
(µF) C
2
, C
3
, C
4
(µF)
3.0 to 3.6 (3.3V ±10%) 0.1 (0.22) 0.1 (0.22)
3.15 to 3.6 (3.3V ±5%) (0.1) (0.1)
4.5 to 5.5 0.047 0.33
3.0 to 5.5 0.1 (0.22) 0.47 (1.0)
NOTE:
7. Parenthesized values apply only to the ICL3238
TIME (20µs/DIV.)
T1
T2
2V/DIV
5V/DIV
V
CC
= +3.3V
FORCEOFF
FIGURE 10. TRANSMITTER OUTPUTS WHEN EXITING
POWERDOWN
C1 - C4 = 0.1µF
5V/DIV
READY
FIGURE 11. TRANSMITTER OUTPUT VOLTAGE vs LOAD
CURRENT (PER TRANSMITTER, i.e., DOUBLE
CURRENT AXIS FOR TOTAL V
OUT+
CURRENT)
TRANSMITTER OUTPUT VOLTAGE (V)
LOAD CURRENT PER TRANSMITTER (mA)
0246810
-6
-4
-2
0
2
4
6
-5
-3
-1
1
3
5
13579
V
OUT
+
V
OUT
-
V
CC
V
OUT
+
V
OUT
-
T1
T2
T3
V
CC
= 3.0V
ICL3244
ICL3224, ICL3226, ICL3238, ICL3244
