ICL3225 Datasheet, PDF (11/18 Pages) Intersil Corporation – 1 Microamp, +3V to +5.5V, 1Mbps, RS-232 Transceivers with Enhanced Automatic Powerdown

ICL3225, ICL3245
receiver and transmitter inputs. Detaching the cable allows
the receiver inputs to drop to an invalid level (GND), so
INVALID switches low and forces the RS-232 device to
power down. The ICL32XX remains powered down until the
cable is reconnected (INVALID = FORCEOFF = 1) and a
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 (ICL3225 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. C₂, C₃, and C₄ can be
increased without increasing C₁’s value, however, do not
increase C₁ without also increasing C₂, C₃, and C₄ to
maintain the proper ratios (C₁ 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-.
TABLE 3. REQUIRED CAPACITOR VALUES
V_CC (V)
3.0 to 3.6
C₁ (µF)
0.1
C₂, C₃, C₄ (µF)
0.1
4.5 to 5.5
0.047
0.33
3.0 to 5.5
0.1
0.47
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 C₁.
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 inter operability with RS-232 devices.
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.
5V/DIV.
FORCEOFF
2V/DIV
T1
V_CC = +3.3V
C1 - C4 = 0.1µF
T2
5V/DIV.
READY
TIME (20µs/DIV.)
FIGURE 10. TRANSMITTER OUTPUTS WHEN EXITING
POWERDOWN
Mouse Driveability
The ICL3245 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).
6
5
4
3
V_CC = 3.0V
V_OUT+
2
T1
1
0
-1
T2
-2
-3 ^{VCC T3}
V_OUT+
ICL3245
V_OUT -
-4
V_OUT -
-5
-6
0 1 2 3 4 5 6 7 8 9 10
LOAD CURRENT PER TRANSMITTER (mA)
FIGURE 11. TRANSMITTER OUTPUT VOLTAGE vs LOAD
CURRENT (PER TRANSMITTER, i.e., DOUBLE
CURRENT AXIS FOR TOTAL V_OUT+ CURRENT)
11