14
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
higher I
F
(≥ 500 µA) and I
o
(≥ 1mA) 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
V
CC
= 3.3 Vdc V
CC
= 5 Vdc
(µW) I
F
= 40 µAI
F
= 500 µAI
F
= 40 µAI
F
= 500 µA
P
LED
50 625 50 625
P
Vcc
65 330 100 500
P
O-C
[1]
20 10 25 20
P
TOTAL
[2]
135 µW 965 µW 175 µW 1,145 µW
Notes:
1. R
L
of 11 kΩ open-collector (o-c) pull-up resistor was used for both 3.3 Vdc and 5 Vdc calculations.
2. For typical total interface circuit power consumption in 3.3 Vdc application, add to P
TOTAL
approximately 80 µW for 40 µA
(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
Vdc applications, add to P
TOTAL
approximately 150 µW for 40 µA (1,875 µW for 500 µA) LED current-limiting resistor and 2,230
µW for the 11 kΩ pull-up resistor power dissipations.
Power Dissipation
