13

Propagation Delay

When the HCPL-47XX optocoup-

ler is operated under very low

input and output current condi-

tions, the propagation delay times

will lengthen. When lower input

drive current level is used to

switch the high-efficiency AlGaAs

LED, the slower the charge and

discharge time will be for the

LED. Correspondingly, the propa-

gation delay times will become

longer as a result. In addition, the

split-Darlington (open-collector)

output amplifier needs a larger,

pull-up load resistance to ensure

the output current is within a

controllable range. Applications

that are not sensitive to longer

propagation delay times and that

are easily served by this HCPL-

47XX optocoupler, typically 65

µs

or greater, are those of status

monitoring of a telephone line,

power line, battery condition of a

portable unit, etc. For faster

HCPL-47XX propagation delay

times, approximately 30

µs, this

optocoupler needs to operate at

(

≥ 1 mA) levels.

Applications

Battery-Operated Equipment

Common applications for the

HCPL-47XX optocoupler are

within battery-operated, portable

equipment, such as test or

medical instruments, computer

peripherals and accessories where

energy conservation is required to

maximize battery life. In these

applications, the optocoupler

would monitor the battery voltage

and provide an isolated output to

another electrical system to

indicate battery status or the need

to switch to a backup supply or

begin a safe shutdown of the

equipment via a communication

port. In addition, the HCPL-47XX

optocouplers are specified to

operate with 3 Vdc CMOS logic

family of devices to provide logic-

signal isolation between similar or

different logic circuit families.

Telephone Line Interfaces

Applications where the HCPL-

47XX optocoupler would be best

used are in telephone line inter-

face circuitry for functions of ring

detection, on-off hook detection,

line polarity, line presence and

supplied-power sensing. In

particular, Integrated Services

Digital Network (ISDN) applica-

tions, as illustrated in Figure 10,

can severely restrict the input

power that an optocoupler inter-

face circuit can use (approxi-

mately 3 mW). Figure 10 shows

three isolated signals that can be

served by the small input LED

current of the HCPL-47XX dual-

and single-channel optocouplers.

Very low, total power dissipation

occurs with these series of

devices.

Switched-Mode Power

Supplies

Within Switched-Mode Power

Supplies (SMPS) the less power

consumed the better. Isolation for

monitoring line power, regulation

status, for use within a feedback

path between primary and

secondary circuits or to external

circuits are common applications

for optocouplers. Low-power

HCPL-47XX optocoupler can help

keep higher energy conversion

efficiency for the SMPS. The block

diagram of Figure 11 shows where

low-power isolation can be used.

Table 1. Typical HCPL-4701 Power Dissipation for 3 V and 5 V Applications

(

50

625

50

625

65

330

100

500

20

10

25

20

135

µW

965

µW

175

µW

1,145

µW

Notes:

(1,025

µW for 500 µA) LED current-limiting resistor, and 960 µW for the 11 kΩ pull-up resistor power dissipations. Similarly, for 5

µW for the 11 kΩ pull-up resistor power dissipations.

Power Dissipation