6

hysteresis to increase noise immunity and decrease errors

due to slow input signal transitions.

Low Power Operation

This 3V device requires a nominal supply current of 0.3mA,

powerdown mode). This is considerably less than the 11mA

current required by comparable 5V RS-232 devices, allowing

users to reduce system power simply by replacing the old

style device with the ISL83385E in new designs.

Powerdown Functionality

The already low current requirement drops significantly

when the device enters powerdown mode. In powerdown,

supply current drops to 1

µA, because the on-chip charge

and the transmitter outputs tristate. This micro-power mode

makes these devices ideal for battery powered and portable

applications.

Software Controlled (Manual) Powerdown

The ISL83385E may be forced into its low power, standby

state via a simple shutdown (SHDN) pin (see Figure 2).

Driving this pin high enables normal operation, while driving

it low forces the IC into it’s powerdown state. The time

required to exit powerdown, and resume transmission is less

than 100

function isn’t needed.

Capacitor Selection

These charge pumps only require 0.1

µF 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) 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 3 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.

GND

5k

Ω

-25V

GND

FIGURE 1. INVERTING RECEIVER CONNECTIONS

FIGURE 2. CONNECTIONS FOR MANUAL POWERDOWN

PWR

SHDN

CPU

I/O

ISL83385E

MGT

LOGIC

UART

TABLE 3. REQUIRED CAPACITOR VALUES

(V)

(

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

0.1

0.47

TIME (20

µs/DIV.)

T1

T2

2V/DIV

5V/DIV

SHDN

FIGURE 3. TRANSMITTER OUTPUTS WHEN EXITING

POWERDOWN

C1 - C4 = 0.1

µF

ISL83385E