8

FN6045.2

April 13, 2007

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 100

μs.

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 30

μs (see Figure 7).

INVALID switches back high 1

μs 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

circuitry. When automatic powerdown is utilized, INVALID = 0

indicates that the ISL422XE is in powerdown mode.

Capacitor Selection

The charge pumps require 0.1

μF, 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 0.1

μF bypass capacitor is

adequate. In applications that are particularly sensitive to

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 3k

Ω in parallel with 2500pF.

Note that the transmitters enable only when the magnitude

of the supplies exceed approximately 3V.

FIGURE 6. DEFINITION OF VALID RS-232 RECEIVER LEVELS

0.3V

-0.3V

-2.7V

2.7V

INVALID LEVEL - POWERDOWN OCCURS AFTER 30

μs

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

RECEIVER

INPUTS

TRANSMITTER

OUTPUTS

INVALID

OUTPUT

V+

0

V-

0

INVALID

REGION

}

FIGURE 7. AUTOMATIC POWERDOWN AND INVALID

TIMING DIAGRAMS

AUTOPWDN

PWR UP

ISL4221E, ISL4223E