ISL4221E Intersil Corporation, ISL4221E Datasheet - Page 8

• Manufacturer Part Number: ISL4221E
• Description: QFN Packaged, +/-15kV Esd Protected, +2.7V to +5.5V, 150Nanoamp, 250kBps, RS-232 Transmitters/receivers
• Manufacturer: Intersil Corporation

from initiating automatic powerdown for 100ms after
powering up. This gives the slow-to-wake peripheral circuit
time to reestablish valid RS-232 output levels.
The time to recover from automatic powerdown mode is
typically 100µs.
INVALID Output
The INVALID output always indicates whether or not a valid
RS-232 signal (see Figure 6) is present at any of the
receiver inputs (see Table 2), giving the user an easy way to
determine when the interface block should power down.
Invalid receiver levels occur whenever the driving
peripheral’s outputs are shut off (powered down) or when
the RS-232 interface cable is disconnected. In the case of a
disconnected interface cable where all the receiver inputs
are floating (but pulled to GND by the internal receiver pull
down resistors), the INVALID logic detects the invalid levels
and drives the output low. The power management logic
then uses this indicator to power down the interface block.
Reconnecting the cable restores valid levels at the receiver
inputs, INVALID switches high, and the power management
logic wakes up the interface block. INVALID can also be
used to indicate the DTR or RING INDICATOR signal, as
long as the other receiver inputs are floating, or driven to
GND (as in the case of a powered down driver).
INVALID switches low after invalid levels have persisted on
all of the receiver inputs for more than 30µs (see Figure 7).
INVALID switches back 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
FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS
-0.3V
-2.7V
2.7V
0.3V
INVALID LEVEL - POWERDOWN OCCURS AFTER 30µs
VALID RS-232 LEVEL - ISL422XE IS ACTIVE
VALID RS-232 LEVEL - ISL422XE IS ACTIVE
INDETERMINATE - POWERDOWN MAY OR
INDETERMINATE - POWERDOWN MAY OR
MAY NOT OCCUR
MAY NOT OCCUR
8
ISL4221E, ISL4223E
circuitry. When automatic powerdown is utilized, INVALID =
0 indicates that the ISL422XE is in powerdown mode.
Capacitor Selection
The charge pumps require 0.1µF, or greater, capacitors for
proper operation. Increasing the capacitor values (by a
factor of 2) reduces ripple on the transmitter outputs and
slightly reduces power consumption.
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
capacitor of the same value as the charge-pump capacitor C
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.
TRANSMITTER
FIGURE 7. AUTOMATIC POWERDOWN AND INVALID
OUTPUT
INVALID
RECEIVER
OUTPUTS
INPUTS
V
V
CC
CC
V+
V-
0
TIMING DIAGRAMS
0
AUTOPWDN
t
INVL
.
CC
to ground with a
t
INVH
PWR UP
}
INVALID
REGION
1
.