ISL4270E Datasheet, PDF (9/13 Pages) Intersil Corporation – QFN Packaged, ±15kV ESD Protected, +3V to +5.5V, 300nA, 250kbps, RS-232 Transceivers with Enhanced Automatic Powerdown and a Separate Logic Supply

ISL4270E
“INVALID DRIVING FORCEON AND FORCEOFF” section
of Table 2 for an operational summary. This operational
mode is perfect for handheld devices that communicate with
another computer via a detachable cable. Detaching the
cable allows the internal receiver pull-down resistors to pull
the inputs to GND (an invalid RS-232 level), causing the
30μs timer to time-out and drive the IC into powerdown.
Reconnecting the cable restores valid levels, causing the IC
to power back up.
reduces ripple on the transmitter outputs and slightly
reduces power consumption.
TABLE 3. REQUIRED CAPACITOR VALUES
V_CC
(V)
C₁
C₂, C₃, C₄
(μF)
(μF)
3.0 to 3.6
0.1
0.1
4.5 to 5.5
0.047
0.33
3.0 to 5.5
0.22
1
Hybrid Automatic Powerdown Options
For devices which communicate only through a detachable
cable, connecting INVALID to FORCEOFF (with
FORCEON = 0) may be a desirable configuration. While the
cable is attached INVALID and FORCEOFF remain high, so
the enhanced automatic powerdown logic powers down the
RS-232 device whenever there is 30 seconds of inactivity on
the 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 ISL4270E remains powered down until the
cable is reconnected (INVALID = FORCEOFF = 1) and a
transition occurs on a receiver or transmitter input (see
Figure 5). For immediate power up when the cable is
reattached, connect FORCEON to FORCEOFF through a
network similar to that shown in Figure 3.
Capacitor Selection
The ISL4270E charge pumps require only 0.1μF capacitors
for the full operational voltage range. Table 3 lists other
acceptable capacitor values for various supply voltage
ranges. Do not use values smaller than those listed in
Table 3. Increasing the capacitor values (by a factor of 2)
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 C₁.
Connect the bypass capacitor as close as possible to the IC.
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
transmitter is loaded with 3kΩ in parallel with 2500pF. Note
that the transmitters enable only when the magnitude of the
supplies exceed approximately 3V.
RECEIVER
INPUTS
TRANSMITTER
INPUTS
TRANSMITTER
OUTPUTS
INVALID
OUTPUT
t_INVL
V+
V_CC
0
V-
t_INVH
t_AUTOPWDN
} INVALID
REGION
t_WU
t_AUTOPWDN
t_WU
FIGURE 7. ENHANCED AUTOMATIC POWERDOWN AND INVALID TIMING DIAGRAMS
9
FN6041.2
June 16, 2010